Atomic number

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The atomic number is used to identify the amount of positive charge in the nucleus of atoms of chemical elements. With it, it was possible to organize the periodic table as it exists today. Learn more about this atomic feature.

What is atomic number?

The atomic number, represented by the capital letter Z, corresponds to the number of protons, that is, positive charges, present in the nucleus of atoms of chemical elements. Hydrogen, for example, is made up of a proton and an electron, so its Z is equal to 1.

This characteristic is unique for each chemical element. There is not more than one element with the same Z. Therefore, this number can be used to identify and distinguish the elements. 

The accepted representation for the atomic number is subscript on the left side of the chemical element symbol ( Z X). Thus, hydrogen is represented as 1 H.

What is the atomic number for?

With atomic number (Z), it is possible to determine the position of the elements in the periodic table and establish the elements’ main characteristics. See more about the uses of the atomic number.

  • Organization in the periodic table: as each atom has a unique Z, they are arranged in ascending order in the periodic table, and this makes the characteristics of the elements, such as whether it is a metal or halogen, for example, defined;
  • Determination of the electronic distribution: as Z = p = e, knowing the atomic number of an element, it is possible to know the electronic distribution of the electrons in its electrosphere, in addition to the energy sublevels that are filled;

You can calculate the number of neutrons or the mass of an atom by the relationship of A = Z + N. The atomic number is the identity of each atom, and two elements never share the same Z.

How to calculate Atomic number

As the atomic number of an element corresponds to its number of protons in the nucleus, there is no equation to determine Z. It is always in the periodic table above the chemical element’s symbol. Still, there is a relationship of equality between atomic numbers, protons, and electrons. You can use the formula below to find atomic numbers.

Z = p = e

  • Z: atomic number
  • p: number of protons in the atom
  • e: number of electrons

This means that sulfur ( 16 S) in its electronically neutral state, for example, has Z = 16, with 16 protons in the nucleus and 16 electrons in the electrosphere, since that is the amount of negative charge needed to neutralize the positive charges of the protons. On the other hand, it is possible to calculate the mass number (A) of a chemical element by adding Z to the number of neutrons in the element (N).

The atomic number works as an identifying code for chemical elements, as each atom of an element has a specific number of protons. Therefore, the 118 elements in the periodic table are arranged in order of increasing atomic number.

For example, in nature, there are 3 isotopes of the element hydrogen (H): protium ( 1 H 1 ), deuterium ( 1 H 2 ) and tritium ( 1 H 3 ). Note that the masses are different, but the atomic number is always the same.

Here are some examples of chemical elements and their respective atomic numbers.

  • 2 He: The helium atom has atomic number 2 because it contains 2 protons in its nucleus.
  • 11 Na: The sodium atom has atomic number 11 because it contains 11 protons in its nucleus.
  • 20 Ca: The calcium atom has an atomic number of 20 because it contains 20 protons in its nucleus.

An atom in its ground state is electrically neutral, that is, the positive and negative charges must be in the same amount. Therefore, if an atom has 3 protons in its nucleus, then there are 3 electrons in its electrons.

Atomic Number and Mass Number

It is worth noting that the atomic number (Z) and the mass number (A) are information that make up the structure of chemical elements.

However, attention should be paid to the concepts so that there is no confusion, since the atomic number represents the number of protons in an atom, and the mass number corresponds to the sum of the number of protons and the number of neutrons that exist in the nucleus.

The mass number is expressed by the following formula:

A = p + n

Note that from this expression, one can also calculate:

  • Number of protons: Z = A – n or P = A – n
  • Number of neutrons: n = A – Z

Worked example: If the atom of the chemical element sodium has mass number 23 and atomic number 11, how many neutrons are there in its nucleus?

n = A – Z
n = 23 – 11
n = 12

Therefore, a sodium atom contains 11 protons and 12 neutrons in its nucleus.


The atomic number is a fundamental concept that requires special care when solving the exercises so that there is no confusion with similar terms.
In short, the atomic number of an atom corresponds to its number of protons. It’s as if it were the atomic identity of the elements since each element has its own Z. Don’t stop studying here, see also about the periodic table.


Bachelor in Industrial Chemistry from Covenant University with experience in academic research in the area of ​​environmental ecology. M.Sc. (Petrochemical and Hydrocarbon Chemistry),(Manchester), Ph.D. (Chemistry) - Industrial/Applied( Manchester).