The atomic number of an element is the number of protons present in the atom of the element. For a neutral atom, the proton number is equivalent to the number of electrons. The chemical nature of an element is defined by the atomic number

The mass number of an element is the sum of the protons and neutrons present in the atom of that element.

Consider an element $${_y^zX} $$ y= atomic number (number of protons)
z = mass number
= proton number + neutron number

This is the number of neutrons present in the atom of an element. It is the difference between the mass number and atomic number.
Neutron number = mass number - atomic number

It is the number of electrons present in the atom of an element. For a neutral atom,
Electron number = proton number = atomic number

For a charged atom,
Electron number = proton number ± charge

For positive ions i.e +, +2, +3
Electron number = proton number - respective value of charge
This is because the atom has lost or given out electrons

For negative ions i.e -, -2, -3
Electron number = proton number + respective charge
This is because the atom has gained electrons

Example 1: What is the mass number of an element 11 protons, 11 electrons and 12 neutrons.

$$\text{mass number} = \text{protons + neutrons} $$ $$\text{mass number} = \text{11 + 12} $$ $$\text{mass number} = {23} $$

Example 2: Consider an element $${_{17}^{35}Cl} $$ Find the

1. proton number
2. Neutron number
3. Electron number

1. proton number(atomic number) = 17
2. neutron number = mass no - atomic number
   neutron number = 35 - 17 = 18
3. electron number = proton number = 17 (atom is neutral)

Example 3: An element, X has 127 neutrons and 82 electrons. What is its atomic number (JAMB)

Atomic number = electron number = 82
This is because for a neutral atom, the number of protons is equivalent to the number of electrons.

Example 4: Consider the element $${_{8}^{16}O^{-2}}$$ Calculate the

1. atomic number
2. Neutron number
3. Electron number

1. Atomic number = 8
2. Neutron number = 16 - 8 = 8
3. Here, Oxygen has gained two electrons
   electron number = proton number + respective charge
   8 + 2 = 10 electrons

Example 5: Find the number of electrons in an Aluminum ion

Aluminum ion is $${Al^{3+}} $$ Atomic number = proton number = 13
Electron number = 13 - 3 = 10
Hence aluminum ion contains 10 electrons

Example 6: A metabolic atom is shown below with an inert has structure, contains 18 electrons. How many protons are in this ion $${X^{2+}} $$

The atom has donated two electrons.
Hence,
number of protons = number of electrons + value of charge
number of protons = 18 + 2
number of protons = 20

Example 7: An atom Z attains the electronic configuration of Argon when it forms an ion as shown below. Find its atomic number $${Z^{2-}} $$

Number of electrons in Argon = 18

Atom Z has gained two electrons
Hence,
number of protons = number of electrons - value of charge
Number of protons = 18 - 2
Number of protons = 16

Isoelectricity refers to a condition in which two or more chemical species or ions have the same electronic configuration, and therefore, they have the same number of electrons.

For example, chloride ions (Cl^-) and argon (Ar) are isoelectronic because they both have 18 electrons and share the same electronic configuration (1s² 2s² 2p⁶ 3s² 3p⁶).

Even though chlorine (Cl) and argon have different atomic numbers and charges, they possess the same number of electrons when they are in the Cl^- ion and neutral argon atom forms.

Example 1: Determine if the following is isoelectronic

1. Na⁺
2. Al³⁺
3. O^2-
4. N^3-
5. Ne

1. Na⁺ = 11 - 1 = 10 electrons
2. Al³⁺ = 13 - 3 = 10 electrons
3. O^2- = 8 + 2 = 10 electrons
4. N^3- = 7 + 3 = 10 electrons
5. Ne = 10 electrons