Botão estilo google

float.css

/* Botão float */
.floating-btn{
    position: fixed;
    background: #9b59b6;
    border: none;
    font-size: 2.5em;
    border-radius: 50%;
    position: fixed;
    display: inline-block;
    padding: 9px;
    line-height: 1;
    right: 0;
    bottom: 0;
    margin: 2.5%;
    color: white;
    outline: none;
    -webkit-transition: 0.3s;
    -o-transition: 0.3s;
    transition: 0.3s;
}
.floating-btn:hover{
    -webkit-transition: 0.3s;
    -o-transition: 0.3s;
    transition: 0.3s;
    background: #8e44ad;
    box-shadow: 0px 5px 15px #c9c9c9;
}
.floating-btn:active{
    -webkit-transition: 0.3s;
    -o-transition: 0.3s;
    transition: 0.3s;
    box-shadow: 0px 5px 15px black inset;
}