First ionization potential | ionization Energy

First Ionization Energy and Its Variation in the Periodic Table

The first ionization energy (also called ionization potential) is the amount of energy required to remove one mole of electrons from one mole of atoms in their gaseous state. In simple terms, it is the amount of energy required to detach the outermost electron from a single atom in the gaseous state.

What is Ionization Energy?

When an atom loses an electron, it becomes a positively charged ion (cation). When an atom sheds one of its electrons, it forms a positively charged ion known as a cation. The first ionization energy refers to the energy needed to remove that initial electron. For example, in the case of sodium (Na), removing one electron forms a Na⁺ ion:

Na (g) → Na⁺ (g) + e⁻

Factors Affecting Ionization Energy

• Atomic size: Larger atoms have their outer electrons farther from the nucleus, so less energy is needed to remove an electron.
• Nuclear charge: More protons in the nucleus attract electrons more strongly, increasing ionization energy.
• Shielding effect: Inner electrons block the attraction between the nucleus and outer electrons, reducing ionization energy.

How Ionization Energy Changes Across the Periodic Table

1. Across a Period (Left to Right)

As we move from left to right across a period, ionization energy generally increases. This happens because:

• The number of protons (nuclear charge) increases.
• Since electrons are being added to the same energy level, the shielding effect stays nearly unchanged.
• The increased nuclear charge pulls electrons closer, making them harder to remove.

For example, ionization energy increases from sodium (Na) to chlorine (Cl) across the third period.

2. Down a Group (Top to Bottom)

As we move down a group, ionization energy generally decreases. This is because:

• Atoms become larger as more electron shells are added.
• Outer electrons are farther from the nucleus.
• Inner electrons cause greater shielding, which lowers the pull of the nucleus on the outer electrons.

For instance, ionization energy decreases from lithium (Li) to cesium (Cs) in Group 1.

To summarize, the first ionization energy is the energy needed to remove the outermost electron from a gaseous atom. Ionization energy rises when moving across a period because the nuclear charge becomes stronger, while it falls down a group as atoms get larger and the shielding effect grows. Understanding these trends helps explain the reactivity and chemical behavior of elements.