Periodic Table – Terminologies

Periodic Table – Terminologies

Periodic Table Terminologies

In this session, we are going to discuss about various terms or terminologies that are used in relation to periodic table.

1. Atomic Particles

2. Atomic Number & Atomic Mass

3. Isotopes

4. Ionization Energy

5. Electron Affinity

6. Valency

Atomic Particles


Atom of an elements is represented with a combination of three small particles:

Atomic Structure
Atomic Structure
  • Electron (e)
    • Negative charge particles
  • Neutron (n)
    • No charge particles
  • Proton (p)
    • Positive charge particles

These three particles are structured in systemic manner in an atom:

  • Nucleus – Neutron & Proton combined are present at the center of atom and forms a Nucleus
  • Shell Orbital – Electron revolves around the nucleus in shell orbital

Number of electron in an atom are generally equal to number of proton in that atom. But, number of neutron may or may not be equal.

As shown in the below table, mass of electron in very small almost negligible as compared to mass of neutron and proton.

Atomic Particles Symbol & Mass
Atomic Particles Symbol & Mass

Atomic Number & Atomic Mass


Atomic Number (Z)

  • Number of Protons (p) present within atomic nucleus of chemical element
  • Atomic number is represented by Z
  • Number of proton (p) = Atomic Number (Z)

Atomic Mass (A)

  • It’s the total mass of the atom
  • Total mass of atom = Sum of mass of all the particles (including proton, neutron, electron) in atom, but mass of electron is negligible equivalent to zero.
  • Atomic mass is represented by A
  • Atomic mass (A) = p(mp) + n(mn) [Mass of electron is negligible)

Calculate the atomic number and atomic mass of Oxygen:

Oxygen Atomic Symbol
Oxygen Atomic Symbol
Oxygen Atomic Mass Calculation
Oxygen Atomic Mass Calculation
  • Number of Proton in Oxygen = 8
  • Number of Neutron in Oxygen = 8
  • Number of Electron in Oxygen = 8
  • Atomic Number of Oxygen = 8
  • Atomic Mass of Oxygen = 8 * mp + 8 * mn = 16



Isotopes are atoms of the same element which are different in their nucleus configuration.

Isotopes of an element have following two properties:

  • They have same number of proton in their nucleus. It means, they will have same atomic number
  • They have different number of neutron in their nucleus. It means, they will have different atomic mass
Isotopes of Carbon
Isotopes of Carbon

As shown in the diagram, Carbon has three isotopes.

  • Carbon-12 → Proton – 6, Neutron – 6
  • Carbon-13 → Proton – 6, Neutron – 7
  • Carbon-14 → Proton – 6, Neutron – 8
Isotopes of Hydrogen
Isotopes of Hydrogen

As shown in the diagram, Hydrogen has three isotopes.

  • Protium → Proton – 1, Neutron – 0
  • Deuterium → Proton – 1, Neutron – 1
  • Tritium → Proton – 1, Neutron – 2

Ionization Energy


An atom with completely filled outer orbital is more stable than partially filled outer orbital. An element always tries to move towards more stable electronic configuration either by losing electron or gaining electron.

Ionization energy is the energy required to remove an electron from an atom. Ionization Energy is expressed with +ve sign.

Ionization Energy of Sodium

  • Sodium atom converts into Sodium ion (Cation), by releasing an electron
  • Na + Energy(+495.8 kJ per mole) → Na+ + e
  • It represents, Energy require to covert Na into Na+

Elements in periodic table have:

  • Ionization Energy increases from left to right across a Period
  • Ionization Energy decreases from top to bottom within a Group

Electron Affinity


When an element gains an electron, energy is released.

Electron affinity is the change in energy that occurs when a neutral atom gains an electron. Electron affinity is expressed with -ve sign.

Energy change (DE) = Energy(final) – Energy (initial) = Electron Affinity (-EA)

Neutral atoms are higher in energy than their corresponding anion. Ex. Halogens

The type of reactions in which energy is released are known as exothermic reactions.

Electron Affinity Calculation for Chlorine

  • Formation of Chlorine Ion (Cl) (Anion)

Cl + e → Cl + Energy (-349 kJ per mole)

  • Energy (initial) = 1251 kJ per mole
  • Energy (final) = 902 kJ per mole

Energy change(DE) = Energy(final) – Energy (initial) = Electron Affinity (-EA)

  • Electron Affinity = Energy(final) – Energy(initial)

= 902 – 1251

= -349 kJ per mole

So, electron affinity of chlorine atom gaining one electron is -349 kJ per mole.



Valency of an Element – Number of valence electrons present in the outermost shell of an atom

Valence Electrons – Number of electrons that an atom can use for bonding

  • Valency of Sodium
    • Atomic Number (Z) of Sodium = 11
    • Electronic Configuration of Sodium can be written as = 2, 8, 1
Valency of Sodium
Valency of Sodium

Valence electron in Sodium = 1

It needs to lose 1 electron from its outermost orbit to attain full octet

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