Common Adsense Violations on Mobile: On April 21, Google has launched a significant update in the search algorithm, the most important one, according to experts. It has changed the algorithm to favor websites that display well on mobile phones and tablets leaving behind sites with interfaces not apt for mobile screens.
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 This development favors mobile users greatly while, at the same time, pushes site owners and developers to build mobile-responsive sites. Site admins may evaluate if their site is mobile-friendly by using the Mobile-Friendly Test tool:https://goo.gl/REJQPb
Now that we’re on the mobile-first generation, Adsense has also updated its policies regarding ad placements on mobile sites. This is a fairly new update so if you would like to know if your mobile site is compliant with Adsense, check out these Common Adsense Violations on Mobile to avoid:

1) Showing more than 8 Adsense units on a mobile page.

Policy: Currently, on each page AdSense publishers may place:

  • Up to three AdSense For Content (AFC) ad units
  • Up to three link units
  • Up to two search boxes

Reference:https://support.google.com/adsense/answer/1346295#Ad_limit_per_page

GOOGLE SAYS: THE TOTAL NUMBER OF ADS YOU CAN SHOW ON A WEB PAGE VIEWED ON A DESKTOP AND A HIGH-END MOBILE DEVICE HAS NO DIFFERENCE. THERE ARE, HOWEVER, ADDITIONAL RESTRICTIONS FOR HOW MANY ADS CAN BE SHOWN ON A SINGLE SCREEN PAGE FOR HIGH-END MOBILE OPTIMIZED SITES THAT YOU SHOULD BE MINDFUL OF.

On a mobile web page, the number of ads on a single screen should not exceed one if the ad is fixed to the screen top or screen bottom. If the page scrolls, only one ad should be visible on screen at a time, and, according to the AdSense program policies, publishers may place no more than 3 AFC ad units on one entire page.
Reference: https://support.google.com/admob/answer/2753860?hl=en

2) Improper implementation of hiding ad units

There are cases wherein you may not want an ad to show, particularly on smaller mobile devices. It is possible to hide an ad unit by setting a parameter in CSS media queries to hide ads for specific screen sizes.
Please see example below on how to modify the ad code using CSS3 media queries to hide ads for a specific screen size:
GOOGLE SAYS: TO IMPROVE THE USER EXPERIENCE, PUBLISHERS ARE ALLOWED TO HIDE RESPONSIVE AD UNITS FOR PARTICULAR MOBILE SCREEN SIZES. HOWEVER, PROPER IMPLEMENTATION SHOULD BE ENSURED TO PREVENT UNNECESSARY AD REQUESTS, SO IT IS IMPERATIVE THAT INSTRUCTIONS SHOULD BE FOLLOWED ACCORDINGLY. NOTE THAT IMPROPER IMPLEMENTATION MAY RESULT IN LOWER PERFORMANCE AND VIOLATES OUR POLICIES. ONLY LIMITED MODIFICATIONS TO THE AD CODE IS ALLOWED BY THE ADSENSE PROGRAM POLICIES.

Reference: https://support.google.com/adsense/answer/3543893#hide

3) Placing a 300 x 250 ad on the first view screen of a smartphone

Policy: Publishers should avoid implementations in which the ads push content below the fold. These layouts make it hard for users to distinguish between content and ads.

Google says: Placing a 300×250 ad on the first part of the mobile page would be considered a policy violation. This placement allows the site content to be pushed below the fold as it takes too much space on a mobile view, thus provides a poor user experience. We aim that users will always go back to your site and continue to visit. You may want to consider implementing a 320×100 instead to keep the main content of your site above the fold.