
Acuity Scheduling - Detailed Review
Collaboration Tools

Acuity Scheduling - Product Overview
Introduction to Acuity Scheduling
Acuity Scheduling is a versatile and automated scheduling platform that simplifies the process of managing appointments, client bookings, and calendar management for various types of businesses.
Primary Function
The primary function of Acuity Scheduling is to automate the client booking process. It allows clients to view real-time availability and self-book appointments, reschedule, and make payments online, significantly reducing the administrative workload for businesses.
Target Audience
Acuity Scheduling is designed for a wide range of businesses, particularly those that rely on appointments. This includes health and wellness services, personal trainers, salons, dog groomers, car detailing shops, dance studios, and many other appointment-based businesses. The platform is suitable for solo entrepreneurs, multi-staff businesses, multi-location businesses, and even enterprise-level organizations.
Key Features
Automated Booking and Scheduling
Clients can book, reschedule, and cancel appointments online at their convenience, 24/7. The system updates availability in real-time, ensuring that the calendar is always current.
Client Intake Forms and Payments
Acuity allows clients to fill out intake forms and enter payment information when booking, streamlining the process and reducing administrative tasks. It also supports various payment options, including subscriptions, gift certificates, and service packages.
Multi-Service Support
The platform can manage different types of appointments, including in-person and virtual consultations, classes, workshops, and webinars. This flexibility makes it suitable for a broad range of services.
Integration and Accessibility
Acuity integrates with various platforms such as Squarespace, Facebook, Google, Instagram, and most email providers. This allows businesses to manage their website, schedule, and client communication seamlessly. The mobile app for iOS and Android ensures that businesses can manage their schedules from anywhere.
Time Management and Efficiency
Features like setting limits on when clients can book and cancel appointments, capping daily appointments, and scheduling different appointment types within specific time slots help in managing time effectively. Automated notifications and reminders also help in reducing no-shows.
Performance Metrics and Reporting
The platform provides insightful performance metrics, allowing businesses to make data-driven decisions and strategize their growth. This includes advanced reporting features to track what is working and what areas need improvement.
Overall, Acuity Scheduling is a comprehensive tool that helps businesses streamline their scheduling processes, enhance client experiences, and boost revenue through efficient and automated management of appointments and client interactions.

Acuity Scheduling - User Interface and Experience
User Interface Overview
The user interface of Acuity Scheduling is generally praised for its ease of use and simplicity, although it has some areas that could be improved.Ease of Use
Acuity Scheduling boasts a user-friendly interface that simplifies the process of booking, calendar management, and payment acceptance. The platform is easy to use, even for those who are not tech-savvy. It offers a lightweight and friendly user interface that makes it accessible from multiple devices, which is convenient for users who are often on the go.Key Interface Features
- Upon logging in, users are presented with a dashboard where their calendar is immediately available in the left-hand sidebar. This allows for quick access to all scheduled meetings for any given day.
- The scheduling link is easily accessible on the right side of the page, making it simple to share with clients.
- Users can manage their calendar, edit their availability, schedule new appointments, and share direct scheduling links with clients directly from the app or web interface.
Customization
While Acuity Scheduling offers several customization options, some users find the overall appearance to be quite plain. Users can customize their business name and logo, write custom scheduling instructions, and change the font and button colors. For more advanced users, there is even the option to further customize the scheduling page using CSS.User Experience
The user experience is generally positive, with many users appreciating the flexibility and versatility of the platform. Here are a few points to note:- Calendar Management: Acuity allows for one-way or two-way sync with external calendars like Google, iCloud, or Outlook, which helps in managing multiple calendars efficiently.
- Client Management: Users can keep track of appointments with push notification alerts and reminders, manage client lists, and update client notes.
- Integrations: The platform integrates well with various third-party applications, including accounting, email marketing, payment processing, and video conferencing tools.
Areas for Improvement
Despite its ease of use, some users have reported a few issues:- Intuitiveness: Some users find the interface to lack intuitiveness, particularly when trying to return to the main dashboard from the calendar view. This can be cumbersome and may require refreshing the page.
- Customer Service: There have been reports of a decline in customer service quality since Acuity merged with Squarespace, with slow responses and limited direct contact options.
- Time Zone Settings: The time zone setting can be small and easily overlooked by international clients, leading to potential scheduling confusion.

Acuity Scheduling - Key Features and Functionality
Acuity Scheduling Overview
Acuity Scheduling is a comprehensive appointment scheduling software that offers a wide range of features to streamline business operations and enhance customer interactions. Here are the main features and how they work, including any AI integrations where applicable:Online Booking and Calendar Synchronization
Acuity Scheduling allows clients to view real-time availability and book their own appointments online. The system automatically syncs with popular calendar services such as Google Calendar, Outlook, and iCloud, ensuring both business owners and clients are always up-to-date with their schedules.Customization and Branding
The software provides extensive customization options, enabling businesses to personalize the booking interface, appointment confirmation emails, and other touchpoints to align with their brand identity. This enhances the client experience, making it feel more cohesive and professional.Automated Reminders and Follow-ups
To reduce no-shows, Acuity Scheduling automates reminders and follow-up messages via email or SMS. These timely notifications help ensure clients remember their appointments and arrive prepared, which is crucial for maintaining operational efficiency.Staff Management
For businesses with multiple locations or a team of professionals, Acuity Scheduling offers robust staff management tools. Each team member can manage their own schedules, and businesses can allocate resources effectively across different locations. This feature is particularly useful for industries like healthcare, beauty, and wellness.Payment Processing
Acuity Scheduling integrates with several payment processing platforms, including Stripe, PayPal, and Square. This allows businesses to take payments at the time of booking, streamlining the customer experience and reducing administrative overhead.Video Conferencing Integration
The software integrates with video conferencing tools like Zoom and GoToMeeting, enabling businesses to schedule and manage virtual appointments effortlessly. This is particularly beneficial for businesses offering remote services.Custom Forms and Client Information
Acuity Scheduling allows businesses to collect important client information through custom forms during or before the booking process. This helps tailor services to individual client needs and ensures all necessary data is captured efficiently.AI Integration
While Acuity Scheduling itself does not inherently include AI features, it can be integrated with AI-driven tools to enhance its functionality. For example:Integrating Acuity Scheduling with AI
Integrating with platforms like Latenode can automate scheduling tasks, reduce human error, and provide AI-driven insights to personalize the scheduling experience. This integration can also automate notifications, analyze trends in scheduling data, and offer data-driven recommendations for optimizing scheduling.Integrations with Marketing Tools
Integrations with tools like Factors.ai through platforms like Integrately can help analyze customer journeys, measure campaign impact, and generate automated reports, although these integrations are more focused on marketing and sales rather than direct scheduling enhancements.Mobile Accessibility
Acuity Scheduling is available on both iOS and Android, allowing businesses to manage their schedules and client interactions on the go.Conclusion
In summary, Acuity Scheduling is a versatile tool that streamlines appointment booking, enhances customer experience, and integrates well with various other services, including AI-driven tools, to optimize business operations.
Acuity Scheduling - Performance and Accuracy
Evaluating Acuity Scheduling
Evaluating the performance and accuracy of Acuity Scheduling in the collaboration tools and AI-driven product category reveals several key strengths and some areas for improvement.
Performance
Acuity Scheduling is highly regarded for its performance in managing appointments and streamlining scheduling processes. Here are some notable aspects:
Automation and Time-Saving
Acuity Scheduling excels in automating various tasks such as appointment booking, reminders, and follow-ups. This automation significantly reduces the time spent on administrative work, allowing businesses to focus on their core activities.
Customization
The platform offers high customizability, enabling users to set up different appointment types, define specific rules, and customize email reminders and booking pages according to their business needs.
Integration Capabilities
Acuity Scheduling seamlessly integrates with various tools and platforms, including Google Calendar, iCalendar, Outlook, payment gateways like Stripe and PayPal, and even social media platforms like Facebook and Instagram. These integrations enhance the overall efficiency and user experience.
Scalability
For enterprise users, Acuity Scheduling provides unlimited calendars and scheduling pages, making it suitable for large organizations with multiple locations and staff members.
Accuracy
In terms of accuracy, Acuity Scheduling generally performs well:
Scheduling Accuracy
The platform ensures accurate scheduling by syncing appointments with your calendar, preventing double-booking, and allowing you to block off specific times when you are unavailable.
Data Management
Integrations with tools like Google Sheets enable accurate and efficient tracking of appointments, client information, and booking data, which helps in analyzing trends and performance.
Payment Processing
Acuity Scheduling accurately processes payments through integrated payment gateways, reducing errors and ensuring smooth financial transactions.
Limitations and Areas for Improvement
While Acuity Scheduling is highly effective, there are some areas where it could be improved:
Initial Setup
Some users have reported that setting up sessions and combining multiple calendars can be a bit confusing, although these issues are generally resolved with some effort.
User Interface for Recurring Sessions
Users with recurring sessions every day of the week have noted that viewing and managing these sessions can be cumbersome, as the sessions appear as a long list rather than a more visual chart format.
Client Booking Experience
There have been some feedback suggesting that clients may find it difficult to book multiple time slots or understand the need to ‘Add A Time’ until they have selected the correct number of slots.
Customization of Appearance
Some users have mentioned that tweaking the appearance of the booking page requires knowledge of CSS, which can be a barrier for those without technical expertise.
In summary, Acuity Scheduling is a powerful tool that excels in automating and customizing the scheduling process, integrating well with various platforms, and ensuring accurate data management. However, it may require some learning curve for initial setup and has some minor limitations in user interface and customization.

Acuity Scheduling - Pricing and Plans
Acuity Scheduling Pricing Overview
Acuity Scheduling offers a structured pricing model with several plans to cater to different business needs. Here’s a breakdown of the various tiers and their features:
Free Plan (Freebie Plan)
- This plan is free and suitable for individuals or small businesses with minimal scheduling needs.
- Features include:
- One calendar
- Client self-scheduling
- Unlimited services and appointments
- Appointment tracking
- Customizable scheduling page
Emerging Plan
- This plan is priced at $16 per month if paid annually, or $20 per month if paid monthly.
- Key features include:
- One calendar
- Advanced features such as automated email reminders and intake forms
- Synchronization with other calendars (e.g., Outlook, Google Calendar)
- Online payment processing through integrations with PayPal and Stripe
- Logo display, company colors, and customizable app text
- Option for clients to leave a tip
Growing Plan
- This plan costs $27 per month if paid annually, or $34 per month if paid monthly.
- Features include:
- Up to six users
- Up to 50 appointments per month
- Multiple calendars (up to six)
- Text messaging
- Subscriptions/memberships
- Appointment packages and gift certificates
- Automatic time zone conversion
- Support for more staff and locations
Powerhouse Plan
- This plan is priced at $49 per month if paid annually, or $61 per month if paid monthly.
- It includes all the features from the Growing Plan plus:
- Support for up to 36 calendars
- Multiple time zones
- Advanced reporting
- Custom API and CSS
- Business Associate Agreement
- No “Powered by Acuity” footer
- Features like checkout add-ons for upselling, coupons, and vouchers
Additional Information
- All plans come with a 7-day free trial.
- You can save 20% by opting for annual billing.
- The platform also offers a quote-based Enterprise plan for larger businesses, though details on this plan are not publicly disclosed.
Each plan is designed to meet the specific needs of different-sized businesses, from solo entrepreneurs to larger enterprises, ensuring that you can choose the one that best fits your requirements and budget.

Acuity Scheduling - Integration and Compatibility
Acuity Scheduling Overview
Acuity Scheduling is a versatile scheduling tool that offers extensive integration capabilities with various other tools and platforms, ensuring seamless synchronization and enhanced workflow efficiency.Calendar Integrations
Acuity Scheduling integrates seamlessly with popular calendar services such as Google Calendar, Outlook (including Office 365, Exchange, and Outlook.com), and iCloud. This allows users to sync their appointments and events across these calendars, preventing overlapping appointments and keeping their schedules up-to-date. To set this up, users simply log in to their Acuity account, go to the “Sync with other calendars” panel, select the desired calendar, and follow the prompts to complete the connection.Video Conferencing Integrations
Acuity Scheduling also integrates with leading video conferencing platforms like Zoom and Google Meet. This enables users to schedule virtual meetings and automatically generate video conference links for their appointments. To connect a video conferencing provider, users log in to their Acuity account, go to the “Integrations” panel, and set up the desired video conferencing service.Integration with Other Tools and Platforms
Acuity Scheduling boasts a wide array of integrations beyond calendar and video conferencing services. It integrates with over 500 apps via Zapier, including email marketing tools, CRM systems, and accounting software. This allows for automating various parts of the workflow, such as adding clients to mailing lists or CRM platforms upon booking. Additionally, Acuity integrates directly with payment processors like PayPal and Stripe, facilitating in-appointment payments.Compatibility Across Devices and Operating Systems
Acuity Scheduling is a web-based platform, making it accessible from any device with an internet connection and a web browser, including PCs, Macs, tablets, and smartphones. It also offers dedicated mobile apps for iOS and Android, ensuring users can manage appointments on the go with an optimized user experience.Custom Integrations
For more specialized needs, users can leverage tools like Integromat or Zapier to create custom integrations. For example, integrating Acuity Scheduling with Asana can be achieved using these automation platforms, allowing for a two-way sync between appointments and tasks.Conclusion
In summary, Acuity Scheduling’s integration capabilities are extensive and flexible, making it a powerful tool for managing schedules, appointments, and client interactions across various platforms and devices.
Acuity Scheduling - Customer Support and Resources
Customer Support Options
Phone Support
Acuity Scheduling provides a 24/7 phone support service. You can reach their customer service team at 35 357 868 4757. This line is available for addressing various issues, including technical problems, membership queries, payment and charge concerns, and privacy questions.Email Support
Users can also contact Acuity Scheduling via email at support@acuityscheduling.com. This is a reliable way to get written responses to your questions, submit complaints, or report issues. Make sure to include your contact details so the support team can follow up with you.Live Chat
For immediate assistance, Acuity Scheduling offers a live chat feature available on their website from Monday to Friday, 4 a.m. to 8 p.m. ET. You can access the live chat by clicking the dark circle at the bottom right side of the page during these hours.Online Request Form
If you prefer to submit your inquiry through a form, you can do so by visiting the Acuity Scheduling website, clicking the “Help” button at the top right, scrolling down to the “Contact Us” button, and filling out the form with your question or issue. You can also upload files such as screenshots or photos to support your query.Additional Resources
Help Center
Acuity Scheduling has a detailed Help Center that provides guides, articles, and video tutorials to help users get started and make the most out of the platform. This includes instructions on managing availability, setting up calendars, and customizing appointment notifications.Webinars
Acuity Scheduling offers webinar series, including “The Fundamentals” and “Next Steps,” which cover topics such as customizing the scheduler, using multiple staff members, and syncing third-party calendars. These webinars are a great resource for both new and experienced users.FAQ Section
The FAQ section on the Acuity Scheduling website addresses common questions about using the platform, including information on payment processing, subscription plans, and integration with other tools like Squarespace and various payment processors.Calendar and Scheduling Guides
Users can find detailed guides on managing availability and calendars, including setting regular weekly hours, override hours, and adding multiple calendars. These guides help in ensuring that clients can book appointments seamlessly according to the user’s availability. By leveraging these support options and resources, users of Acuity Scheduling can efficiently manage their appointment scheduling, resolve issues promptly, and optimize their use of the platform.
Acuity Scheduling - Pros and Cons
Advantages
Comprehensive Scheduling Solution
Acuity Scheduling offers a versatile and extensive set of features, making it suitable for various needs such as personal appointments, life coaching, nonprofit activities, and business scheduling. It allows for the customization of availability on a granular level, handling individual appointments, group classes, and events.
Flexibility and Customization
Users can customize their scheduling links, add buffer times between meetings, and prevent last-minute scheduling. This flexibility helps in managing multiple calendars and ensuring ample preparation time.
Integration with Other Platforms
Acuity Scheduling integrates well with platforms like Squarespace, Google Calendar, and other tools through services like Zapier. This integration simplifies the scheduling process and ensures seamless operation.
User-Friendly Interface
Despite its extensive features, Acuity Scheduling is generally easy to use. It provides a simple interface for managing appointments and client information, making it accessible for a wide range of users.
Automated Reminders and Payment Processing
The tool offers automated reminders to clients and integrates payment processing, which streamlines the appointment booking and payment collection process.
Contributor Access
Acuity allows you to invite contributors with different access levels (view only, view and edit, admin), which helps in sharing scheduling tasks with your team and increasing efficiency.
Disadvantages
Limited Customization Options
While Acuity Scheduling is customizable, some users find the appearance to be plain and lacking in customization options, particularly without coding knowledge.
Poor Customer Service
There have been reports of a decline in customer service quality since Acuity merged with Squarespace, including slow responses and limited direct contact options.
Inadequate Time Zone Settings
The time zone setting can be small and easily overlooked, which may lead to scheduling confusion, especially for international clients.
Specific Feature Limitations
Some users have noted that Acuity Scheduling lacks specific features they need, such as saving client addresses for businesses that require travel or supporting one-off meetings.
Integration Issues
Some users have experienced difficulties when integrating Acuity Scheduling with other platforms, which can hinder its functionality and efficiency.
By weighing these pros and cons, you can make an informed decision about whether Acuity Scheduling aligns with your specific needs and operational requirements.

Acuity Scheduling - Comparison with Competitors
When comparing Acuity Scheduling with its competitors in the category of scheduling and collaboration tools, several key points and alternatives stand out.
Unique Features of Acuity Scheduling
- Extensive Customization: Acuity Scheduling is known for its high degree of flexibility and customization in scheduling and booking appointments. It allows for setting up different types of appointments, managing multiple locations and employees, and adjusting time zones automatically for international clients.
- Payment Integration: It offers direct payment integration for services, which is a significant feature for businesses that need to process payments as part of their scheduling process.
- Comprehensive Mobile App: The mobile app of Acuity Scheduling is feature-rich, providing users with powerful functionality on the go.
Potential Alternatives
Calendly
- Simplicity and Efficiency: Calendly focuses on a streamlined user experience, making the scheduling process as effortless as possible. It is ideal for individuals and teams that value ease of use and quick setup over extensive customization. Calendly’s round-robin feature is particularly useful for team scheduling, ensuring meetings are evenly distributed among team members.
- Integration Capabilities: Calendly integrates well with productivity and collaboration tools like Slack, Zapier, HubSpot, and Intercom, and supports video conferencing through Zoom and Microsoft Teams.
Square Appointments
- Business-Oriented Features: Square Appointments is strong in its business-oriented features, including web-based booking, POS system integration, and the ability to request prior payments for scheduled appointments. It also allows staff to manage their own calendars and track customer transactions.
- Automated Reminders: Square Appointments sends automated SMS or email reminders to customers, which helps in maintaining appointment retention.
Setmore
- Affordability and Ease of Use: Setmore is one of the more affordable alternatives, offering a free version for up to 4 users. It is praised for its ease of use and setup, with a 9.1 rating score from users. Setmore supports virtual meetings through Zoom or Teleport and allows advance payments via Square or Stripe.
- Customization and Integration: Setmore offers native integration with sales, marketing, and business apps and allows enterprises to customize their solutions using its API.
YouCanBook.me
- Affordability and Flexibility: YouCanBook.me is another affordable option, offering flexible scheduling and the ability to set appointment types and payment options. It automatically detects customers’ time zones and supports integrations with Google Calendar, Zoom, and Stripe.
- Detailed Reporting: YouCanBook.me allows for detailed reporting and analysis by exporting booking details into CSV files.
10to8
- Comprehensive Appointment Features: 10to8 is used by over 50,000 businesses and provides a comprehensive list of appointment features, including integration with popular calendars, online payment requests via PayPal or Stripe, and advanced reminder features such as SMS, email, and voice call reminders.
- Customization and Branding: 10to8 allows for creating customized booking pages with logos and other branding elements, and it supports multiple time zones and virtual meetings through Zoom.
Key Considerations
- Ease of Use vs. Customization: The choice between Acuity Scheduling and its alternatives often comes down to whether you prioritize ease of use and simplicity (Calendly, Setmore) or extensive customization and advanced features (Acuity Scheduling, Square Appointments).
- Integration Capabilities: Consider the integrations that are most critical to your workflow. For example, if you use Microsoft products, Calendly or Square Appointments might be more suitable due to their strong integration capabilities.
- Cost and Scalability: Evaluate the pricing models of each tool, as they vary significantly. For instance, Calendly offers a free plan for a single user, while Setmore and 10to8 provide free versions with limited features.
By considering these factors, you can choose the scheduling tool that best fits your specific needs and workflow requirements.

Acuity Scheduling - Frequently Asked Questions
Frequently Asked Questions About Acuity Scheduling
What is Acuity Scheduling?
Acuity Scheduling is an online appointment-booking tool that helps businesses automate their scheduling workflows, payments, and bookings. It allows clients to book, reschedule, or cancel appointments independently, while giving you full control over your calendar.How much does Acuity Scheduling cost?
Acuity Scheduling offers several pricing plans. The Free Plan is free and suitable for individuals with limited scheduling needs. The Emerging Plan starts at $16 per month (if paid annually), the Growing Plan at $27 per month (if paid annually), and the Powerhouse Plan at $49 per month (if paid annually). All plans come with a 7-day free trial, and you can save 20% by opting for annual billing.Can I connect Acuity Scheduling to my personal calendar?
Yes, Acuity Scheduling can sync with Google, iCloud, and online Outlook calendars. This integration helps keep your schedules up-to-date across different platforms.Does Acuity Scheduling send email and text notifications?
Yes, Acuity Scheduling can send email and text messages to remind clients of upcoming appointments. It also sends emails for appointment confirmations, follow-ups, and other notifications. You can customize which notifications you receive.How many calendars do I need?
The number of calendars you need depends on your business requirements. Each calendar represents a set of times that clients can book with you. You might need one calendar for each employee, location, or even specific resources like rooms or stylist chairs. If you’re unsure, you can contact Acuity Scheduling for guidance.Can clients reschedule their own appointments?
Yes, clients can reschedule their own appointments by clicking the “Change/Cancel Appointment” link in their confirmation emails. This feature allows for greater flexibility and convenience for both you and your clients.Does Acuity Scheduling support payment processing?
Yes, Acuity Scheduling allows you to accept payments from clients using Stripe, Square, or PayPal. Clients can pay when they book online, or you can enter payments from the admin end.Is there a free trial available for Acuity Scheduling?
Yes, Acuity Scheduling offers a 7-day free trial for all its plans. This allows you to test the features and see if the service meets your needs before committing to a paid plan.Can I use Acuity Scheduling with any website provider?
Yes, you can use Acuity Scheduling with any website provider, not just Squarespace, even though Acuity Scheduling is a Squarespace company. It integrates seamlessly with various websites.Does Acuity Scheduling offer an enterprise solution?
Yes, Acuity Scheduling offers an Enterprise Plan for businesses that need more than 36 calendars, dedicated account manager support, or have other special requirements. You can contact them directly for more details on this plan.How do I edit a booked appointment?
You can edit a client’s existing appointment through the Acuity Scheduling interface. For detailed steps, you can visit the help section on booking appointments for clients with Acuity Scheduling.
Acuity Scheduling - Conclusion and Recommendation
Final Assessment of Acuity Scheduling
Acuity Scheduling is a comprehensive and versatile appointment scheduling tool that caters to a wide range of businesses, from individual service providers to large enterprises. Here’s a breakdown of its key features and who would benefit most from using it:Key Features
- Multi-Calendar Sync: Acuity Scheduling allows seamless integration with various calendar systems like Google, iCloud, Office365, and Outlook, ensuring real-time updates and eliminating double-booking issues.
- Automated Reminders and Notifications: The platform sends instant notifications via email and SMS, helping both businesses and clients stay on track with appointments.
- Customization and Branding: Users can customize the scheduling page to match their brand, including colors, pictures, and specific functionality like hiding prices or requiring client phone numbers.
- Integrated Payment Processing: Acuity supports major payment gateways such as Stripe, Square, and PayPal, making transactions convenient and secure.
- Client Management: The software includes features like client intake forms, activity tracking, and the ability to manage staff permissions and resource availability.
- Promotional Tools: Businesses can offer subscriptions, discounts, group classes, and gift certificates to attract and retain clients.
Who Would Benefit Most
Acuity Scheduling is particularly beneficial for:- Appointment-Based Businesses: Salons, personal trainers, dog groomers, car detailing shops, and any other service providers that rely heavily on scheduling appointments will find Acuity invaluable.
- Health and Wellness Industries: Given its HIPAA compliance, it is well-suited for health, wellness, and fitness businesses, as well as hospital and healthcare providers.
- Small to Medium-Sized Businesses: The majority of Acuity’s users are small businesses with fewer than 50 employees, but it also supports larger enterprises with over 1000 employees.
- Businesses with Multiple Locations or Time Zones: The automatic time zone adjustments and multi-location support make it ideal for businesses operating across different regions.