Atomic Number and Atomic Mass

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-letter or two-letter symbols, such as H for hydrogen, C for carbon, and He for helium. You can see other examples by looking at the periodic table below.

Q: The table shown above is called the periodic table of the elements. Each symbol stands for a different element. What do you think the symbol K stands for?

A: The symbol K stands for the element potassium. The symbol comes from the Latin name for potassium, which is kalium.

The symbols in the table above would be more useful if they revealed more information about the 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.

[Figure 3]

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 box above for helium. That number is the mass number, which is the mass of the atom in a unit called the atomic mass unit (amu). One atomic mass unit is the mass of a proton, or about 1.67 × 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 amu for the protons + 2 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 box above.

Q: What is the mass number of a helium atom that has three neutrons?

A: The mass number is the number of protons plus the number of neutrons. For helium atoms with three neutrons, the mass number is 2 (protons) + 3 (neutrons) = 5.

Q: How would you represent this isotope of helium to show its atomic number and mass number?

A: You would represent it by the element’s symbol and both numbers, with the mass number on top and the atomic number on the bottom:

Subatomic Particles

Atoms, as we have learned, are composed of three subatomic particles — protons, neutrons, and electrons. Only the protons and neutrons contribute to the atomic mass and it is only protons that determine the atomic number. Regarding ionic or atomic charge, this is determined by comparing the numbers of protons and electrons.

Protons, Neutrons, and Atomic Mass

As mentioned before, protons and neutrons are used to determine the atomic mass. This is done by adding protons and neutrons together, as the below formula displays:

Atomic Mass = p⁺ + n (where “p⁺” means protons and “n” mean neutrons)

However, understanding this formula can also be used to determine the number of protons and the number of neutrons, as well. To use this formula to determine the number of neutrons, one must know the atomic mass and the number of protons; then, if the formula is rearranged, we can successfully determine the number of neutrons. For example, if the atomic mass of an element is 23 amu and it has 11 protons:

n = atomic mass - p⁺ → 23 - 11 = 12 n 

If the atomic mass and the number of neutrons are known, we can also calculate the number of protons. For example, if the atomic mass is 23 and if there are 12 neutrons in the nucleus:

p⁺ = atomic mass - n → 23 - 12 = 11 p⁺ 

Rounding to Nearest Whole Number (Atomic Mass)

Warning, on the periodic table, the atomic mass is usually written as a decimal number; therefore, it is highly recommended they be rounded to the nearest whole number before it is used to determine the number of protons or neutrons. To do this, we look at the number just to the right of the decimal to see if the number to the left of the decimal will round up to the next number or stay the same. If the number to the right of the decimal is 5 or greater, the number to the left of the decimal will round up by one; if the number to the right of the decimal is 4 or less, then the number to the left of the decimal will not round up. In both cases, all numbers to the right of the decimal are dropped from the number and it is written as a whole number.

For example, if the atomic mass is 22.99, the first 9 to the right of the decimal is in the 5 or greater category, this will cause the 2 to the left of the decimal to go up by one. Therefore, 22.99 rounded to the nearest whole number is 23. If, on the other hand, if the atomic mass is 22.49, the 4 to the immediate right of the decimal is in the 4 or less category, thus the 2 to the immediate left of the decimal will not round up. Therefore, 22.49 rounded to the nearest whole number becomes 22.

Protons, Electrons, and Atomic Number

This section is easier to understand and do than the prior section, because there is no formula to use or to rearrange. When we consider what atomic number is, we are really talking about the number of protons. In other words, the atomic number is the same as the number of protons. Therefore:

Atomic Number = p⁺

From this understanding, if we know the number of protons, then we also know the atomic number, and vice versa, if we know the atomic number then we know the number of protons, because they are the same thing.

How does this relate to the number of electrons? When we look at the elements on the periodic table, we must assume their charge is 0, neutral. Since it is the case that the ion charge (atomic charge) is calculated by subtracting the number of protons from the number of electrons, if the charge is 0, then the number of protons must be the same as the number of electrons. Therefore, from the atomic number, we can determine the number of protons and the number of electrons.

For example, if an atom has an atomic number of 11 = 11 p⁺ and 11 e^- (where “e^-” is electrons).

Use the skills required to complete #9 and the below formula to complete #10.

Atomic Mass = p⁺ + n

n = atomic mass - p⁺

p⁺ = atomic mass - n

Atomic Number = p⁺

Assume all atoms on the periodic table are neutral.

Ion Charge = p⁺ - e^-