• Tells you how many protons are in the nucleus.

• Unique to each element.

• Will always be the smaller of the two, may not always be on the bottom.

• In a NEUTRAL atom, the atomic number also tells you the # of electrons.

• Mass Number, or the atomic mass unit, tells you the mass of the atom.

• Includes all the protons and all the neutrons added up.

• Always the larger number.

• Because there are different types (or isotopes) of the same element, the mass is an average.

• Average atomic mass is usually written as a decimal.

• Round this number to the nearest whole number to find the Mass Number.

• Often simply referred to as atomic mass.

Three ways we can represent an atom.

​Bohr model of Carbon

​Nuclear symbol of Carbon

​carbon-12

Hyphen Notation for Carbon