<a href="https://wa.me/56968128889?text=¡Hola Radio Atacama!" class="whatsapp" target="_blank"> <i class="fa fa-whatsapp whatsapp-icon"></i></a>

<style>
  .whatsapp {
  position:fixed;
  width:60px;
  height:60px;
  bottom:100px;
  right:40px;
  background-color:#25d366;
  color:#FFF;
  border-radius:50px;
  text-align:center;
  font-size:30px;
  z-index:100;
}

.whatsapp-icon {
  margin-top:13px;
}
</style>

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">

Whatsapp Bottom Fixed