Back to Basics: Atomic Structure

1

The identity of an element is determined by the number of protons in its nucleus. 

The nucleus contains positively charged atomic mass units called protons, and neutral atomic mass units called neutrons. 

So Hydrogen, for example, has one proton. All elements with one proton are Hydrogen atoms.

The mass number of an atom is the number of protons plus the number of neutrons.

Two atoms with the same number of protons, (thus the same identity) but have a different number of neutrons are called isotopes.

2

An example of an isotope would be Tritium, which is a Hydrogen atom (1 proton) with 2 neutrons.The superscript 3 in the image above is the sum of the protons and neutrons in Tritium. The subscript is the number of protons.

Protium is the most common isotope of Hydrogen, it has one proton and one electron only, no neutrons.

Another common example would be carbon- 12 and carbon-14. Carbon-12 has 6 protons and 6 neutrons, while carbon-14 contains 6 protons and 8 neutrons. There are many other isotopes of Carbon, and their molar masses average out to 12.01g per mole which is the value on the periodic table.

The molar mass we see on a periodic table is the average mass of all the isotopes of an element.

Sources:
1) "Blausen 0342 ElectronEnergyLevels" by BruceBlaus - Own work. Licensed under CC BY 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Blausen_0342_ElectronEnergyLevels.png#mediaviewer/File:Blausen_0342_ElectronEnergyLevels.png

2) "Protium deuterium tritium" by Lamiot for french version, from Dirk Hünniger - self, translation from Dirk Hünniger (german wikipedia). Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Protium_deuterium_tritium.jpg#mediaviewer/File:Protium_deuterium_tritium.jpg


3) Tro, Nivaldo J., Robert S. Boikess, Joseph H. Bularzik, and William M. Cleaver. "Liquids, Solids, and Intermolecular Forces." Chemistry A Molecular Approach. Upper Saddle River: Prentice Hall, 2008. 76. Print.