Having trouble understanding XML DTD. Getting getting root element not well formed error when I parse

I'm new to XML. I'm working on an assignment and I'm stuck on external DTD validation. I get an error when I parse the file that says root element is not well formed. Both files are in the same directory.

Here is the external DTD:

   <!ELEMENT atoms (atom*)>
   <!ELEMENT atom (hydrogen, carbon, oxygen, titanium, uranium)>
   <!ELEMENT hydrogen (#PCDATA)>
   <!ELEMENT carbon (#PCDATA)>
   <!ELEMENT oxygen (#PCDATA)>
   <!ELEMENT titanium (#PCDATA)>
   <!ELEMENT uranium (#PCDATA)> 

Here is my XML file:

<?xml version="1.0" encoding="UTF-8"?>
<!--DTD is in a local directory file named atoms.dtd -->
<!-- Week 3 Homework -->
<!DOCTYPE atoms SYSTEM "atoms.dtd">    
  <atoms>

    <atom>
      <hydrogen>   
        <properties symbol="H" name="Hydrogen" atomicNumber="1" atomicWeight="1.00794"/>
      </hydrogen>

      <carbon>   
        <properties symbol="C" name="Carbon" atomicNumber="6" atomicWeight="12.011"/>
      </carbon>

      <oxygen>   
        <properties symbol="O" name="Oxygen" atomicNumber="8" atomicWeight="15.999"/>
      </oxygen>

      <titanium>   
        <properties symbol="Ti" name="Titanium" atomicNumber="22" atomicWeight="47.867"/>
      </titanium>

      <uranium>   
        <properties symbol="U" name="Uranium" atomicNumber="92" atomicWeight="238.03"/>
      </uranium>
    </atom>

  </atoms>