nth-child code will not work

@media screen and (min-width: 480px) { 

  #gallery li{ 
    width: 28.333%;
  } 

  #gallery li:nth-child(4n) {
    clear: left;
  }
}

/*****************
GENERAL
*****************/

body {
  font-family: 'Open Sans', sans-serif;
}

a {
  text-decoration:none;
}


#wrapper {
  max-width: 940px;
  margin: 0 auto;
  padding: 0 5%;
}

img {
  max-width: 100%;
}

h3{
 margin: 0 0 1em 0; 
}
/*****************
HEADING
*****************/

header {
  float: left;
  margin: 0 0 30px 0;
  padding: 5px 0 0 0;
  width: 100%;
}

#logo{
  text-align: center;
  margin: 0;
}

h1 {
  font-family: 'Changa One', sans-serif;
  margin: 15px 0;
  font-size: 1.75em;
  font-weight: bold;
  line-height: 0.8em;
}

h2 {
  font-size: 0.75em;
  margin: -5px 0 0;
  font-weight: normal;
}

/*****************
NAVIGATION
*****************/

nav {
  text-align: center;
  padding: 10px 0;
  margin: 20px 0 0;
}

nav ul {
  list-style:none;
  margin: 0 10px;
  padding: 0;
}

nav li{
  display: inline-block;
}

nav a{
  font-weight: 800;
  padding: 15px 10px;
}

/*****************
FOOTER
*****************/

footer{
  font-size: 0.75em;
  text-align: center;
  clear: both;
  padding-top: 50px;
  color: #ccc;
}

.social-icon {
  width: 20px;
  height: 20px;
  margin: 0 5px;
}

/*****************
PAGE: PORTFOLIO
*****************/

#gallery {
  margin: 0;
  padding: 0;
  list-style: none;
}

#gallery li{
  float: left;
  width: 45%;
  margin: 2.5%;
  background-color: #f5f5f5;
  color: #bdc3c7;
}

#gallery a li p {
  margin: 0;
  padding: 5%;
  font-size: 0.75em;
  color: #bdc3c7;
}

/*****************
PAGE: ABOUT
*****************/

.profile-photo {
  display: block;
  max-width: 150px;
  margin: 0 auto 30px;
  border-radius: 100%;
}

/*****************
PAGE: CONTACT
*****************/

.contact-info{
  list-style: none;
  padding:0;
  margin:0;
  font-size:0.9em;
}

.contact-info a {
  display: block;
  min-height:20px;
  background-repeat:no-repeat;
  background-size: 20px 20px;
  padding: 0 0 0 30px;
  margin: 0 0 10px;
}

.contact-info li.phone a{
  background-image: url('../img/phone.png');
}

.contact-info li.mail a{
  background-image: url('../img/mail.png');
}

.contact-info li.twitter a{
  background-image: url('../img/twitter.png');
}
/*****************
COLORS
*****************/

/* site body*/

body {
 background-color: #fff;
  color: #999;
}

/* green header*/
header {
  background: #6ab47b;
  border-color: #599a68;
}

/*logo text*/
h1, h2 {
  color: #fff;  
}

/* nav background on mobile devices*/
nav {
  background: #599a68;
}

/*Anchor links*/
a {
  color: #6ab47b;
}

/*nav link*/
nav a, nav a:visited {
  color: #fff;
}

/*selected nav link*/
nav a.selected, nav a:hover{
 color:#32673f; 
}