Electron Configurations and the Periodic Table
elements are organized by atomic number, the number of protons in an atom. The vertical columns in a periodic table are called
while the horizontal rows are called
. Atomic number increases from left to right across a period and from top to bottom down a group.
A neutral atom has the same number of electrons as protons. You can use the atomic number to find the number of electrons in the neutral atom, and start
building an electron configuration. The
outer electron configuration
of an element includes everything except the noble-gas core. For example, the element