To download and install Atom in Ubuntu 14.04 ( the version i am using ) you have to type following command one by one in your terminal.
sudo add-apt-repository ppa:webupd8team/atom
Press Enter if it ask your permission.
sudo apt-get update
It will take some time. Be patient.
sudo apt-get install atom
It will download and install the Atom editor on your Ubuntu 14.04. Here is screen shot of Atom running in Ubuntu.