The periodic table shows all the elements


The atom



Electronic shells

Atomic number

Electronic subshells

Atomic orbitals

Atomic orbitals d

Rutherford's experiment




Atomic and mass numbers

Everything we know is formed using the 92 elements in the periodic table, which represent atoms.

What is the difference between one element (atom) in the table and another?

Atomic number

The difference is in the number of protons on the nucleus (atomic number - Z).

By looking at the fraction of the periodic table showed on the left, we see that the element with atomic number Z=5 is B. Nitrogen has atomic number Z=7, oxygen has Z=8, and so on.

All the elements in the periodic table are ordered according to their atomic numbers. The atomic number is the identity of an element.

Mass number

The mass number (A) corresponds to the mass of the atom, in atomic units (amu: atomic mass units). That is obtained by adding the number of protons and neutrons present (electrons are not included because they are relatively light weight).

The mass numbers appears under the element symbol. For instance, the atomic mass of oxygen is A = 16. For nitrogen A = 14.

Finding out the number of neutrons

We know that Z is the number of protons and that A is the total of protons plus neutrons. Hence, the number of neutrons is A - Z.



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