I want to achieve this grid layout using floats (or another method):

But I cannot get to move grid number 8 (numbering in natural order left to right top to down) to occupy its position in left. HTML:
<div class="blockContainer">
<ul id="RnP" class=" rnp">
<li>1</li>
<li>2</li>
<li>3</li>
<li>4</li>
<li>5</li>
<li id="bigbox">6</li>
<li >7</li>
<li >8</li>
<li>9</li>
<li>10</li>
<li>11</li>
<li>12</li>
<li>13</li>
</ul>
</div>
CSS:
.blockContainer{
position: absolute;
top:20%;
left:20%;
border:1px solid red;
width: 37em;
}
.rnp li{
background: blue;
width: 8em;
height: 8em;
color: yellow;
margin-left: 1em;
margin-top: 1em;
float: left;
padding: 0;
list-style: none;
}
#bigbox{
width: 17em;
height: 17em;
}
.rnp{
margin: 0px;
padding: 0px;
}
JS FIDDLE:http://jsfiddle.net/ebbj2sch/
floatdue to the way it functions AFAIK.display: table-rowdisplay: table-celland so on?