Atomic Weight — Definition & Overview

Atomic Weight: How to calculate atomic weight?

Calculate the atomic weight of boron from given isotopes.

You can calculate the atomic weight by summing each isotope's atomic mass times their abundance.

(Video) Isotopes and How to Calculate Atomic Weight

by The Science Classroom

Have you ever noticed on the periodic table that some elements have a weird number like $10.811$ listed below them? Does that mean they have $10.811$ protons per atom? Well, not quite. This number is known as the atomic weight of the element. It actually shows you the average mass of all naturally occurring isotopes of that element.

In this video, you will learn what isotopes are, how atoms are weighed, and how all of this gives us a number known as the atomic weight. At the end, you'll have a chance to try out a practice problem to see what you've learned.

Atomic Weight of Elements

The atomic weight or relative atomic mass describes the weight of specific element which takes into account the isotopes and their abundance.

For example, the element carbon (C) has two major, naturally occurring isotopes. The abundance of the isotopes, $^{13}C$ and $^{12}C$, are 1.1% and 98.9% respectively. The atomic weight of carbon can be calculated as: $$Atomic\ weight\ of\ C\ =\ (13 \times 0.011)\ +\ (12 \times 0.989)\ =\ 12.011$$

Sometimes atomic weight and atomic mass are used interchangeably, but this is not entirely correct. The atomic weight is the average weight of an element, whereas atomic mass is the weight of a single atom.