Как добавить классы дочерним элементам и удалить при повторном клике?

// external js: isotope.pkgd.js
$(document).ready(function() {
  // init Isotope
  var $container = $('.isotope').isotope({
    itemSelector: '.element-item',
   
  });
  
  //****************************
  // Isotope Load more button
  //****************************
  var initShow = 6; //number of items loaded on init & onclick load more button
  var counter = initShow; //counter for load more button
  var iso = $container.data('isotope'); // get Isotope instance
  loadMore(initShow); //execute function onload
  function loadMore(toShow) {
    $container.find(".hidden").removeClass("hidden");
    var hiddenElems = iso.filteredItems.slice(toShow, iso.filteredItems.length).map(function(item) {
      return item.element;
    });
    $(hiddenElems).addClass('hidden');
    $container.isotope('layout');
    //when no more to load, hide show more button
    if (hiddenElems.length == 0) {
      jQuery("#load-more").hide();
    } else {
      jQuery("#load-more").show();
    };
  }
  //append load more button
  $container.after('<button id="load-more"> Load More</button>');
  //when load more button clicked
  $("#load-more").click(function() {
    $('.element-item').toggleClass('active')
    $('.element-item.hidden').removeClass('active')
    
    $('#sorts').animate({scrollTop:0}, '300');
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      //when filter button clicked, set initial value for counter
      counter = initShow;
      $('#filters').data('clicked', false);
    } else {
      counter = counter;
    };
    counter = counter + initShow;
    loadMore(counter);
  });
  //when filter button clicked
  $("#filters").click(function() {
    $(this).data('clicked', true);
 
    loadMore(initShow);
  });
  
  
});

#load-more{
  float:right;
  margin:5px;
}
.hidden{visibility:hidden;
  width:0px!important;
  height:0px!important;
  margin:0px!important;
  padding:0px!important;
  }
* {
  -webkit-box-sizing: border-box;
     -moz-box-sizing: border-box;
          box-sizing: border-box;
}
body {
  font-family: sans-serif;
}
/* ---- button ---- */
.button {
  display: inline-block;
  padding: 0.5em 1.0em;
  background: #EEE;
  border: none;
  border-radius: 7px;
  background-image: linear-gradient( to bottom, hsla(0, 0%, 0%, 0), hsla(0, 0%, 0%, 0.2) );
  color: #222;
  font-family: sans-serif;
  font-size: 16px;
  text-shadow: 0 1px white;
  cursor: pointer;
}
.button:hover {
  background-color: #8CF;
  text-shadow: 0 1px hsla(0, 0%, 100%, 0.5);
  color: #222;
}
.button:active,
.button.is-checked {
  background-color: #28F;
}
.button.is-checked {
  color: white;
  text-shadow: 0 -1px hsla(0, 0%, 0%, 0.8);
}
.button:active {
  box-shadow: inset 0 1px 10px hsla(0, 0%, 0%, 0.8);
}
/* ---- button-group ---- */
.button-group:after {
  content: '';
  display: block;
  clear: both;
}
.button-group .button {
  float: left;
  border-radius: 0;
  margin-left: 0;
  margin-right: 1px;
}
.button-group .button:first-child { border-radius: 0.5em 0 0 0.5em; }
.button-group .button:last-child { border-radius: 0 0.5em 0.5em 0; }
/* ---- isotope ---- */
.isotope {
  border: 1px solid #333;
}
/* clear fix */
.isotope:after {
  content: '';
  display: block;
  clear: both;
}
/* ---- .element-item ---- */
.element-item {
  position: relative;
  float: left;
  width: 100px;
  height: 100px;
  margin: 5px;
  padding: 10px;
  background: #888;
  color: #262524;
}
.element-item > * {
  margin: 0;
  padding: 0;
}
.element-item .name {
  position: absolute;
  left: 10px;
  top: 60px;
  text-transform: none;
  letter-spacing: 0;
  font-size: 12px;
  font-weight: normal;
}
.element-item .symbol {
  position: absolute;
  left: 10px;
  top: 0px;
  font-size: 42px;
  font-weight: bold;
  color: white;
}
.element-item .number {
  position: absolute;
  right: 8px;
  top: 5px;
}
.element-item .weight {
  position: absolute;
  left: 10px;
  top: 76px;
  font-size: 12px;
}
.element-item.alkali          { background: #F00; background: hsl(   0, 100%, 50%); }
.element-item.alkaline-earth  { background: #F80; background: hsl(  36, 100%, 50%); }
.element-item.lanthanoid      { background: #FF0; background: hsl(  72, 100%, 50%); }
.element-item.actinoid        { background: #0F0; background: hsl( 108, 100%, 50%); }
.element-item.transition      { background: #0F8; background: hsl( 144, 100%, 50%); }
.element-item.post-transition { background: #0FF; background: hsl( 180, 100%, 50%); }
.element-item.metalloid       { background: #08F; background: hsl( 216, 100%, 50%); }
.element-item.diatomic        { background: #00F; background: hsl( 252, 100%, 50%); }
.element-item.halogen         { background: #F0F; background: hsl( 288, 100%, 50%); }
.element-item.noble-gas       { background: #F08; background: hsl( 324, 100%, 50%); }

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<h1>Isotope - Load more button</h1>
<h2>Filter</h2>
<div id="filters" class="button-group">  <button class="button is-checked" data-filter="*">show all</button>
  <button class="button" data-filter=".metal">metal</button>
  <button class="button" data-filter=".transition">transition</button>
  <button class="button" data-filter=".alkali, .alkaline-earth">alkali and alkaline-earth</button>
  <button class="button" data-filter=":not(.transition)">not transition</button>
  <button class="button" data-filter=".metal:not(.transition)">metal but not transition</button>
  <button class="button" data-filter="numberGreaterThan50">number > 50</button>
  <button class="button" data-filter="ium">name ends with &ndash;ium</button>
</div>
<h2>Sort</h2>
<div id="sorts" class="button-group">  <button class="button is-checked" data-sort-by="original-order">original order</button>
  <button class="button" data-sort-by="name">name</button>
  <button class="button" data-sort-by="symbol">symbol</button>
  <button class="button" data-sort-by="number">number</button>
  <button class="button" data-sort-by="weight">weight</button>
  <button class="button" data-sort-by="category">category</button>
</div>
<div class="isotope">
  <div class="element-item transition metal " data-category="transition">
    <h3 class="name">Mercury</h3>
    <p class="symbol">Hg</p>
    <p class="number">80</p>
    <p class="weight">200.59</p>
  </div>
  <div class="element-item metalloid " data-category="metalloid">
    <h3 class="name">Tellurium</h3>
    <p class="symbol">Te</p>
    <p class="number">52</p>
    <p class="weight">127.6</p>
  </div>
  <div class="element-item post-transition metal " data-category="post-transition">
    <h3 class="name">Bismuth</h3>
    <p class="symbol">Bi</p>
    <p class="number">83</p>
    <p class="weight">208.980</p>
  </div>
  <div class="element-item post-transition metal " data-category="post-transition">
    <h3 class="name">Lead</h3>
    <p class="symbol">Pb</p>
    <p class="number">82</p>
    <p class="weight">207.2</p>
  </div>
  <div class="element-item transition metal " data-category="transition">
    <h3 class="name">Gold</h3>
    <p class="symbol">Au</p>
    <p class="number">79</p>
    <p class="weight">196.967</p>
  </div>
  <div class="element-item alkali metal " data-category="alkali">
    <h3 class="name">Potassium</h3>
    <p class="symbol">K</p>
    <p class="number">19</p>
    <p class="weight">39.0983</p>
  </div>
  <div class="element-item alkali metal " data-category="alkali">
    <h3 class="name">Sodium</h3>
    <p class="symbol">Na</p>
    <p class="number">11</p>
    <p class="weight">22.99</p>
  </div>
  <div class="element-item transition metal " data-category="transition">
    <h3 class="name">Cadmium</h3>
    <p class="symbol">Cd</p>
    <p class="number">48</p>
    <p class="weight">112.411</p>
  </div>
  <div class="element-item alkaline-earth metal " data-category="alkaline-earth">
    <h3 class="name">Calcium</h3>
    <p class="symbol">Ca</p>
    <p class="number">20</p>
    <p class="weight">40.078</p>
  </div>
  <div class="element-item transition metal " data-category="transition">
    <h3 class="name">Rhenium</h3>
    <p class="symbol">Re</p>
    <p class="number">75</p>
    <p class="weight">186.207</p>
  </div>
  <div class="element-item post-transition metal " data-category="post-transition">
    <h3 class="name">Thallium</h3>
    <p class="symbol">Tl</p>
    <p class="number">81</p>
    <p class="weight">204.383</p>
  </div>
  <div class="element-item metalloid " data-category="metalloid">
    <h3 class="name">Antimony</h3>
    <p class="symbol">Sb</p>
    <p class="number">51</p>
    <p class="weight">121.76</p>
  </div>
  <div class="element-item transition metal " data-category="transition">
    <h3 class="name">Cobalt</h3>
    <p class="symbol">Co</p>
    <p class="number">27</p>
    <p class="weight">58.933</p>
  </div>
  <div class="element-item lanthanoid metal inner-transition " data-category="lanthanoid">
    <h3 class="name">Ytterbium</h3>
    <p class="symbol">Yb</p>
    <p class="number">70</p>
    <p class="weight">173.054</p>
  </div>
  <div class="element-item noble-gas nonmetal " data-category="noble-gas">
    <h3 class="name">Argon</h3>
    <p class="symbol">Ar</p>
    <p class="number">18</p>
    <p class="weight">39.948</p>
  </div>
  <div class="element-item diatomic nonmetal " data-category="diatomic">
    <h3 class="name">Nitrogen</h3>
    <p class="symbol">N</p>
    <p class="number">7</p>
    <p class="weight">14.007</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Uranium</h3>
    <p class="symbol">U</p>
    <p class="number">92</p>
    <p class="weight">238.029</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Plutonium</h3>
    <p class="symbol">Pu</p>
    <p class="number">94</p>
    <p class="weight">(244)</p>
  </div>
  <div class="element-item metalloid " data-category="metalloid">
    <h3 class="name">Antimony</h3>
    <p class="symbol">Sb</p>
    <p class="number">51</p>
    <p class="weight">121.76</p>
  </div>
  <div class="element-item transition metal " data-category="transition">
    <h3 class="name">Cobalt</h3>
    <p class="symbol">Co</p>
    <p class="number">27</p>
    <p class="weight">58.933</p>
  </div>
  <div class="element-item lanthanoid metal inner-transition " data-category="lanthanoid">
    <h3 class="name">Ytterbium</h3>
    <p class="symbol">Yb</p>
    <p class="number">70</p>
    <p class="weight">173.054</p>
  </div>
  <div class="element-item noble-gas nonmetal " data-category="noble-gas">
    <h3 class="name">Argon</h3>
    <p class="symbol">Ar</p>
    <p class="number">18</p>
    <p class="weight">39.948</p>
  </div>
  <div class="element-item diatomic nonmetal " data-category="diatomic">
    <h3 class="name">Nitrogen</h3>
    <p class="symbol">N</p>
    <p class="number">7</p>
    <p class="weight">14.007</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Uranium</h3>
    <p class="symbol">U</p>
    <p class="number">92</p>
    <p class="weight">238.029</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Plutonium</h3>
    <p class="symbol">Pu</p>
    <p class="number">94</p>
    <p class="weight">(244)</p>
  </div>
   <div class="element-item transition metal " data-category="transition">
    <h3 class="name">Cobalt</h3>
    <p class="symbol">Co</p>
    <p class="number">27</p>
    <p class="weight">58.933</p>
  </div>
  <div class="element-item lanthanoid metal inner-transition " data-category="lanthanoid">
    <h3 class="name">Ytterbium</h3>
    <p class="symbol">Yb</p>
    <p class="number">70</p>
    <p class="weight">173.054</p>
  </div>
  <div class="element-item noble-gas nonmetal " data-category="noble-gas">
    <h3 class="name">Argon</h3>
    <p class="symbol">Ar</p>
    <p class="number">18</p>
    <p class="weight">39.948</p>
  </div>
  <div class="element-item diatomic nonmetal " data-category="diatomic">
    <h3 class="name">Nitrogen</h3>
    <p class="symbol">N</p>
    <p class="number">7</p>
    <p class="weight">14.007</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Uranium</h3>
    <p class="symbol">U</p>
    <p class="number">92</p>
    <p class="weight">238.029</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Plutonium</h3>
    <p class="symbol">Pu</p>
    <p class="number">94</p>
    <p class="weight">(244)</p>
  </div>
   <div class="element-item transition metal " data-category="transition">
    <h3 class="name">Cobalt</h3>
    <p class="symbol">Co</p>
    <p class="number">27</p>
    <p class="weight">58.933</p>
  </div>
  <div class="element-item lanthanoid metal inner-transition " data-category="lanthanoid">
    <h3 class="name">Ytterbium</h3>
    <p class="symbol">Yb</p>
    <p class="number">70</p>
    <p class="weight">173.054</p>
  </div>
  <div class="element-item noble-gas nonmetal " data-category="noble-gas">
    <h3 class="name">Argon</h3>
    <p class="symbol">Ar</p>
    <p class="number">18</p>
    <p class="weight">39.948</p>
  </div>
  <div class="element-item diatomic nonmetal " data-category="diatomic">
    <h3 class="name">Nitrogen</h3>
    <p class="symbol">N</p>
    <p class="number">7</p>
    <p class="weight">14.007</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Uranium</h3>
    <p class="symbol">U</p>
    <p class="number">92</p>
    <p class="weight">238.029</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Plutonium</h3>
    <p class="symbol">Pu</p>
    <p class="number">94</p>
    <p class="weight">(244)</p>
  </div>
   <div class="element-item transition metal " data-category="transition">
    <h3 class="name">Cobalt</h3>
    <p class="symbol">Co</p>
    <p class="number">27</p>
    <p class="weight">58.933</p>
  </div>
  <div class="element-item lanthanoid metal inner-transition " data-category="lanthanoid">
    <h3 class="name">Ytterbium</h3>
    <p class="symbol">Yb</p>
    <p class="number">70</p>
    <p class="weight">173.054</p>
  </div>
  <div class="element-item noble-gas nonmetal " data-category="noble-gas">
    <h3 class="name">Argon</h3>
    <p class="symbol">Ar</p>
    <p class="number">18</p>
    <p class="weight">39.948</p>
  </div>
  <div class="element-item diatomic nonmetal " data-category="diatomic">
    <h3 class="name">Nitrogen</h3>
    <p class="symbol">N</p>
    <p class="number">7</p>
    <p class="weight">14.007</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Uranium</h3>
    <p class="symbol">U</p>
    <p class="number">92</p>
    <p class="weight">238.029</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Plutonium</h3>
    <p class="symbol">Pu</p>
    <p class="number">94</p>
    <p class="weight">(244)</p>
  </div>
   <div class="element-item transition metal " data-category="transition">
    <h3 class="name">Cobalt</h3>
    <p class="symbol">Co</p>
    <p class="number">27</p>
    <p class="weight">58.933</p>
  </div>
  <div class="element-item lanthanoid metal inner-transition " data-category="lanthanoid">
    <h3 class="name">Ytterbium</h3>
    <p class="symbol">Yb</p>
    <p class="number">70</p>
    <p class="weight">173.054</p>
  </div>
  <div class="element-item noble-gas nonmetal " data-category="noble-gas">
    <h3 class="name">Argon</h3>
    <p class="symbol">Ar</p>
    <p class="number">18</p>
    <p class="weight">39.948</p>
  </div>
  <div class="element-item diatomic nonmetal " data-category="diatomic">
    <h3 class="name">Nitrogen</h3>
    <p class="symbol">N</p>
    <p class="number">7</p>
    <p class="weight">14.007</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Uranium</h3>
    <p class="symbol">U</p>
    <p class="number">92</p>
    <p class="weight">238.029</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Plutonium</h3>
    <p class="symbol">Pu</p>
    <p class="number">94</p>
    <p class="weight">(244)</p>
  </div>
   <div class="element-item transition metal " data-category="transition">
    <h3 class="name">Cobalt</h3>
    <p class="symbol">Co</p>
    <p class="number">27</p>
    <p class="weight">58.933</p>
  </div>
  <div class="element-item lanthanoid metal inner-transition " data-category="lanthanoid">
    <h3 class="name">Ytterbium</h3>
    <p class="symbol">Yb</p>
    <p class="number">70</p>
    <p class="weight">173.054</p>
  </div>
  <div class="element-item noble-gas nonmetal " data-category="noble-gas">
    <h3 class="name">Argon</h3>
    <p class="symbol">Ar</p>
    <p class="number">18</p>
    <p class="weight">39.948</p>
  </div>
  <div class="element-item diatomic nonmetal " data-category="diatomic">
    <h3 class="name">Nitrogen</h3>
    <p class="symbol">N</p>
    <p class="number">7</p>
    <p class="weight">14.007</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Uranium</h3>
    <p class="symbol">U</p>
    <p class="number">92</p>
    <p class="weight">238.029</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Plutonium</h3>
    <p class="symbol">Pu</p>
    <p class="number">94</p>
    <p class="weight">(244)</p>
  </div>
   <div class="element-item transition metal " data-category="transition">
    <h3 class="name">Cobalt</h3>
    <p class="symbol">Co</p>
    <p class="number">27</p>
    <p class="weight">58.933</p>
  </div>
  <div class="element-item lanthanoid metal inner-transition " data-category="lanthanoid">
    <h3 class="name">Ytterbium</h3>
    <p class="symbol">Yb</p>
    <p class="number">70</p>
    <p class="weight">173.054</p>
  </div>
  <div class="element-item noble-gas nonmetal " data-category="noble-gas">
    <h3 class="name">Argon</h3>
    <p class="symbol">Ar</p>
    <p class="number">18</p>
    <p class="weight">39.948</p>
  </div>
  <div class="element-item diatomic nonmetal " data-category="diatomic">
    <h3 class="name">Nitrogen</h3>
    <p class="symbol">N</p>
    <p class="number">7</p>
    <p class="weight">14.007</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Uranium</h3>
    <p class="symbol">U</p>
    <p class="number">92</p>
    <p class="weight">238.029</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Plutonium</h3>
    <p class="symbol">Pu</p>
    <p class="number">94</p>
    <p class="weight">(244)</p>
  </div>
   <div class="element-item transition metal " data-category="transition">
    <h3 class="name">Cobalt</h3>
    <p class="symbol">Co</p>
    <p class="number">27</p>
    <p class="weight">58.933</p>
  </div>
  <div class="element-item lanthanoid metal inner-transition " data-category="lanthanoid">
    <h3 class="name">Ytterbium</h3>
    <p class="symbol">Yb</p>
    <p class="number">70</p>
    <p class="weight">173.054</p>
  </div>
  <div class="element-item noble-gas nonmetal " data-category="noble-gas">
    <h3 class="name">Argon</h3>
    <p class="symbol">Ar</p>
    <p class="number">18</p>
    <p class="weight">39.948</p>
  </div>
  <div class="element-item diatomic nonmetal " data-category="diatomic">
    <h3 class="name">Nitrogen</h3>
    <p class="symbol">N</p>
    <p class="number">7</p>
    <p class="weight">14.007</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Uranium</h3>
    <p class="symbol">U</p>
    <p class="number">92</p>
    <p class="weight">238.029</p>
  </div>
  <div class="element-item actinoid metal inner-transition " data-category="actinoid">
    <h3 class="name">Plutonium</h3>
    <p class="symbol">Pu</p>
    <p class="number">94</p>
    <p class="weight">(244)</p>
  </div>
</div>