# Why is the average atomic mass usually a decimal number?

## Why is the average atomic mass usually a decimal number?

The standard atomic weight is the average mass of an element in atomic mass units (“amu”). Though individual atoms always have an integer number of atomic mass units, the atomic mass on the periodic table is stated as a decimal number because it is an average of the various isotopes of an element.

## Why is the atomic mass of an element an average atomic mass?

The mass written on the periodic table is an average atomic mass taken from all known isotopes of an element. This average is a weighted average, meaning the isotope’s relative abundance changes its impact on the final average. The reason this is done is because there is no set mass for an element.

What is the average atomic mass of an element?

The atomic mass of an element is the average mass of the atoms of an element measured in atomic mass unit (amu, also known as daltons, D). The atomic mass is a weighted average of all of the isotopes of that element, in which the mass of each isotope is multiplied by the abundance of that particular isotope.

### How is average mass related to atomic mass?

The average atomic mass (sometimes called atomic weight) of an element is the weighted average mass of the atoms in a naturally occurring sample of the element. Average masses are generally expressed in unified atomic mass units (u), where 1 u is equal to exactly one-twelfth the mass of a neutral atom of carbon-12.

### Why isn’t the atomic mass a whole number?

The atomic masses of most elements are not whole numbers, because they are a weighted average of the mass numbers of the different isotopes of that element, with respect to their abundances in nature.

How do I find the average atomic mass?

The average atomic mass for an element is calculated by summing the masses of the element’s isotopes, each multiplied by its natural abundance on Earth.

#### What makes up the average mass of an element?

The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its natural abundance (the decimal associated with percent of atoms of that element that are of a given isotope).

#### Why is the atomic mass of an element not whole number?

Because the atomic mass is a calculated average of all the Isotopes of an Element with their % of which they occur also calculated in. The atomic mass of an element is the average mass of all its isotopes, weighted by their abundance.

Where is the atomic mass written on the periodic table?

This is a derived unit from the carbon-12 isotope, where 12 u is the atomic mass of carbon-12. On the periodic table, students can find the atomic mass written below the element’s name.

## How to calculate the mass of an isotope?

Average atomic mass = f 1 M 1 + f 2 M 2 + … + f n M n where f is the fraction representing the natural abundance of the isotope and M is the mass number (weight) of the isotope.