Atomic Number (Z)

Each atom has its atomic number. It indicates the number of electrons and protons in the atom. If it has zero electrical charge it is neutral, ie it is a neutral atom.

The atomic number is indicated by the letter (Z).

Atomic Number is the number of protons and electrons (neutral atom) that exist in the atom. Examples:

Na (sodium) Z = 11
He (helium) Z = 2
V (vanadium) Z = 23
Br (bromine) Z = 84
Po (polonium) Z = 84

It can be said that the atomic number is equal to the number of nucleus protons. If the atom is neutral, it is equal to the number of electrons as well.

Z = p = is

Mass Number (A)

Mass number is the weight of the atom. It is the sum of the number of protons (Z) and neutrons (n) that exist in an atom.
A = p + nor A = Z + n

It is this number that tells you whether the atom is lighter or heavier. It is the protons and neutrons that give the mass of the atom, since the electrons are very small, with negligible mass in relation to these particles. Examples:

Na (sodium) A = 23
If Na has A = 23 and Z = 11, what is the number of n (neutrons)?

A = 23
Z = p = is

A = p + n
23 = 11 + n
n = 12

From Z we have the number of protons and electrons in the atom. From the formula A = p + n, we isolate n to find it, replacing A and p in the formula. Then we can also use the formula:

n = A-p

Notice the model:

a) K (potassium)
A = 39
Z = 19
p = 19
é = 19
n = 20

We find these values ​​in the Periodic Table of the Elements. Every table has its legend stating the atomic number and mass number. By applying the correct formula, we can find the neutron value.


The atom having p = is, that is, the number of protons equal to the number of electrons is electrically neutral.

Neutral atom = p = is

If the atom has too many or too few electrons, then it will no longer be a neutral atom. This atom will be called ION.

Ion = p ≠ is

Ion is an atom that loses or gains electrons. It can be negative or positive. So:

Positive ion (+) donates electrons - cation ion. Ex. In+
Negative ion (-) receives electrons - anion ion. Ex. Cl-
When a cation donates electrons, it becomes positive.
When an anion gains electrons, it becomes negative.