#periodic_table {
    width: 100%;
    min-width: 600px;
}

#periodic_table > div {
    margin-bottom: 2.5%;
}

#periodic_table > div > div {
	width: 5.5%;
	display: inline-block;
	margin-right: -7px;
}

#periodic_table > div > div > div {
    border: 3px solid black;
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
    text-align: center;
    background: #00FF00;
    margin-top: -3px;
    padding: 20% 0;
}