# 4.15: Atomic Number

Athletes wearing the same-colored jerseys are all on the same team. In addition, each player's jersey has a unique number to distinguish him from his teammates. Imagine how confusing it would be if members of both teams wore the same-colored jerseys or all the members of a team had the same number on their jerseys. How could you tell the athletes apart?

## Telling Atoms Apart

It's often useful to have ways to signify different people or objects like athletes on teams. The same is true of atoms. It's important to be able to distinguish atoms of one element from atoms of other elements. Elements are pure substances that make up all other matter, so each one is given a unique name. The names of elements are also represented by unique one- or two-letter symbols, such as $$\ce{H}$$ for hydrogen, $$\ce{C}$$ for carbon, or $$\ce{He}$$ for helium. You can see other examples in the figure below.

The symbols in the figure above would be more useful if they revealed more information about atoms they represent. For example, it would be useful to know the numbers of protons and neutrons in the atoms. That's where atomic number and mass number come in.

## Atomic Number

The number of protons in an atom is called its atomic number. This number is very important because it is unique for atoms of a given element. All atoms of an element have the same number of protons, and every element has a different number of protons in its atoms. For example, all helium atoms have two protons, and no other elements have atoms with two protons. In the case of helium, the atomic number is 2. The atomic number of an element is usually written in front of and slightly below the element's symbol, like in the figure below for helium.

Atoms are neutral in electrical charge because they have the same number of negative electrons as positive protons. Therefore, the atomic number of an atom also tells you how many electrons the atom has. This, in turn, determines many of the atom's properties.

## Mass Number

There is another number in the figure above for helium. That number is the mass number, which is the mass of the atom is a unit called the atomic mass unit $$\left( \text{amu} \right)$$. One atomic mass unit is the mass of a proton, or about $$1.67 \times 10^{-27}$$ kilograms, which is an extremely small mass.

A neutron has just a tiny bit more mass than a proton, so its mass is often assumed to be one atomic mass unit as well. Because electrons have virtually no mass, just about all the mass of an atom is in its protons and neutrons. Therefore, the total number of protons and neutrons in an atom determines its mass in atomic mass units.

Consider helium again. Most helium atoms have two neutrons in addition to two protons. Therefore the mass of most helium atoms is 4 atomic mass units ($$2 \: \text{amu}$$ for the protons + $$2 \: \text{amu}$$ for the neutrons). However, some helium atoms have more or less than two neutrons. Atoms with the same number of protons but different numbers of neutrons are called isotopes. Because the number of neutrons can vary for a given element, the mass numbers of different atoms of an element may also vary. For example, some helium atoms have three neutrons instead of two. Therefore, they have a different mass number than the one given in the figure above.

## Summary

Elements are pure substances that make up all matter, so each one is given a unique name. The names of elements are also represented by unique one- or two-letter symbols. An atom's mass number is its mass in atomic mass units $$\left( \text{amu} \right)$$, which is about equal to the total number of protons and neutrons in the atom. Different isotopes of an element have different mass numbers because they have different numbers of neutrons.

## Explore More

Use the periodic table of the elements at the first URL below to fill in the blanks in the worksheet at the second URL.

## Contributors

• CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.