atomic mass

All about ATOMIC MASS:

Why is the mass number on the periodic table a decimal?

Most elements found in the universe exist in slightly different masses. Although proton numbers determine an element's identity (Hydrogen's atomic number is 1 and will always have 1 proton, Helium's is 2 and will always have 2 protons), the number of neutrons in an atom can vary. This means that although you have the same element (atom that behaves the same way and has a specific number of protons), your elements can weigh different amounts!

Atomic Mass is a WEIGHTED AVERAGE.

When you take the average of something in math class we assume that each amount counts the same -- you simply add all the numbers and divide by how many numbers you totaled. A weighted average factors in the "importance" of a number. Just like when grades are calculated, some categories are more important than others. That is to say, they "weigh" more than others

In the same way, some isotopes are more prevalent than others. So when you calculate a weighted mass, you are factoring in how abundant (how much there is) that specific isotope is into the total. The resulting number will closer reflect the most abundant isotope's mass.

In the same way, different amounts of elements exist in samples. That is to say their masses are WEIGHTED differently.

In the above picture there are 3 different isotopes

carbon 12 (12= mass number)

Carbon 13 (13= mass number)

Carbon 14 (14= mass number)

Carbon's WEIGHTED average (the one appearing on the periodic table) is

12.011 amu