.redes {
    position: fixed;
    top: 400px;
    z-index: 2;
    right: 0px;
}

#Whatsapp a {
    background-color:#42e35f;
    padding: 10px 10px;
    color: #fff;
    text-decoration: none;
    width: 55px;
    transition: all .5s ease;
    font-size: 40px;
}

#Whatsapp a:hover {
    background-color: #fff;
    color: #42e35f;
}   

@media screen and (max-width: 400px) {
    #Whatsapp a {
        width: 40px;
    }
}