HTML Tables with CSS Styles

Hi
i tried using the css style sheet (child theme) to make a custom table:

table.chem {
    font-family: "Trebuchet MS", Arial, Helvetica, sans-serif;
    width: 100%;
    border-collapse: collapse;
}

table.chem td, table.chem th {
    font-size: 19px;
    border: 1px solid #062073;
    padding: 3px 7px 2px 7px;
}

table.chem th {
    font-size: 19px;
    text-align: left;
    padding-top: 5px;
    padding-bottom: 4px;
    background-color: #3a9ed1;
    color: #062073;
}

table.chem tr.alt td {
    color: #062073;
    background-color: #fafafa;
}

then use html tag like:
<table class="chem">
but it doesn’t work’ can’t understand why