What is the formal definition of the first ionisation energy?

The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous $1+$ ions. It is measured in $kJ\text{ mol}^{-1}$ under standard conditions.

Why does ionisation energy decrease as you move down a group in the periodic table?

As you move down a group, the atomic radius increases and the number of inner shielding shells increases. These factors outweigh the increase in nuclear charge, resulting in a weaker attraction between the nucleus and the outer electron.

What is the difference between the first and second ionisation energy equations?

The first IE equation starts with a neutral gaseous atom ($X(g) \rightarrow X^+(g) + e^-$), while the second IE equation starts with a gaseous $1+$ ion ($X^+(g) \rightarrow X^{2+}(g) + e^-$).

What is a common error when writing the equation for the second ionisation energy of Magnesium?

A common error is writing $Mg(g) \rightarrow Mg^{2+}(g) + 2e^-$. This represents the sum of the first and second ionisation energies, not the second ionisation energy specifically, which must start from $Mg^+(g)$.

Why are ionisation energy values always positive?

Ionisation is an endothermic process. Energy must be absorbed to overcome the electrostatic attraction between the negatively charged electron and the positively charged nucleus.

How does shielding affect the ionisation energy of an atom?

Shielding occurs when inner-shell electrons repel outer electrons and block the nuclear attraction. Increased shielding reduces the effective nuclear charge, making it easier to remove outer electrons and thus lowering the ionisation energy.

When looking at successive ionisation energies, what does a sudden, large increase in energy signify?

It signifies that the next electron is being removed from a shell closer to the nucleus (an inner shell). This electron experiences much stronger nuclear attraction and significantly less shielding.

Compare the shielding effect in Sodium and Magnesium (both in Period 3).

In both Sodium and Magnesium, the shielding is approximately the same because they have the same number of inner-shell electrons ($1s^2 2s^2 2p^6$). The difference in their IE is primarily due to Magnesium's higher nuclear charge.

What error is made if the state symbols are omitted in an IE definition?

Omitting $(g)$ implies the atoms could be in a solid or liquid state, where energy would also be needed to overcome metallic or intermolecular bonds, leading to an inaccurate measurement of the atomic property.

Why does ionisation energy generally increase across a period?

Across a period, the nuclear charge increases (more protons) while shielding remains relatively constant. This increases the attraction for the outer electrons and pulls them closer, decreasing the atomic radius.

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Chemistry

1. Atomic Structure

Defining Ionisation Energy

Summary

Ionisation energy is a fundamental thermodynamic property that quantifies the energy required to remove electrons from atoms or ions in the gaseous state. It serves as a critical indicator of atomic structure, providing direct evidence for the existence of electron shells and subshells while explaining the chemical reactivity of elements across the periodic table.

1. Definition & Core Concepts

2. Successive Ionisation Energies

Sequential Removal: Successive ionisation energies refer to the energy required to remove the second, third, and subsequent electrons from an ion. For example, the second ionisation energy ( $IE_2$ ) involves removing an electron from a $1+$ ion: $X^+(g) \rightarrow X^{2+}(g) + e^-$
Increasing Energy Requirement: Successive ionisation energies always increase for a given element. As electrons are removed, the remaining electrons experience a greater effective nuclear charge (the ratio of protons to electrons increases), pulling them closer to the nucleus and making them harder to remove.
Shell Evidence: Large 'jumps' in successive ionisation energy data indicate that an electron is being removed from a new principal quantum shell that is closer to the nucleus and experiences significantly less shielding.

Graph showing successive ionisation energies with a significant jump indicating a change in electron shell.

3. Factors Influencing Ionisation Energy

4. Key Distinctions

5. Exam Strategy & Tips