I am trying to make an interactive periodic table of elements. I need to change the background color of more `<td>` with the `classname "nemetale"` when a button is clicked. It's not working, I don't know what I am doing wrong.

There is the button

```
 <button onclick="document.getElementsByClassName('.nemetale').style.backgroundColor = 'red';">Nemetale</button>

```

There is one of the `<td>s`. 

```
<table class="tabel_periodic">
  <!--Randul 1-->
  <tr>

   <td class="nemetale">
    <strong>1</strong><br>
    <acronym>H</acronym><br>
    <em>Hidrogen</em><br>
    <i>1,008</i>
   </td>
 ...

```