Ionisation Energy Review

Question-and-answer flashcards covering definitions, trends, anomalies, and explanations related to first and successive ionisation energies.

What is ionisation energy?

The energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous +1 ions.

Write the equation for the first ionisation energy of element X.

X(g) → X⁺(g) + e⁻

What are successive ionisation energies?

The energies needed to remove additional electrons from an atom/ion one at a time.

Name the three main factors that affect ionisation energy.

1) Nuclear charge 2) Atomic radius 3) Electron shielding

How does increasing atomic radius affect ionisation energy?

A larger atomic radius places the outer electron farther from the nucleus, so less energy is required to remove it.

Define electron shielding.

Repulsion by inner-shell electrons that reduces the effective nuclear attraction felt by outer electrons.

State the general trend in first ionisation energy across a period.

Ionisation energy increases across a period due to greater nuclear charge, similar shielding, and a smaller atomic radius.

Why is aluminium’s first ionisation energy lower than expected in Period 3?

Its outer electron is in a 3p orbital, which is higher in energy than the 3s orbital, making it easier to remove.

Why is sulfur’s first ionisation energy lower than phosphorus’s?

Pairing of electrons in a 3p orbital causes repulsion, so an electron is removed more easily.

Describe the trend in first ionisation energy down a group.

Ionisation energy decreases down a group because atomic radius and shielding increase, weakening nuclear attraction on outer electrons.

How do successive ionisation energies change for an element?

Each successive ionisation energy is larger than the previous one; a big jump signals removal from a new, inner shell.

What information can a successive ionisation energy graph provide?

The number of electrons before the first large jump indicates how many electrons were in the outer shell.

Why is sodium’s second ionisation energy much higher than its first?

The second electron is removed from a lower, full shell with less shielding and stronger nuclear attraction.

Why is oxygen’s first ionisation energy lower than nitrogen’s?

Oxygen has a paired electron in a 2p orbital causing repulsion, so that electron is easier to remove.

Why does helium have the highest first ionisation energy of all elements?

Helium has no electron shielding, a very small atomic radius, and a high nuclear charge relative to its size, leading to very strong attraction for its electrons.