What is the difference between First Ionisation Energy and Second Ionisation Energy?

First IE is the energy to remove the first electron from a neutral gaseous atom, while Second IE is the energy to remove a second electron from a 1+ gaseous ion. Second IE is always higher because the electron is removed from a positive ion, which exerts a stronger pull on the remaining electrons.

How does the trend in ionisation energy across a period differ from the trend down a group?

Across a period, IE generally increases due to increasing nuclear charge and decreasing atomic radius with constant shielding. Down a group, IE decreases because the increasing atomic radius and shielding from additional shells outweigh the increase in nuclear charge.

Why does Oxygen have a lower first ionisation energy than Nitrogen, despite being further right in the period?

Nitrogen has three unpaired electrons in its $2p$ subshell, while Oxygen has four, meaning one $2p$ orbital contains a pair of electrons. The repulsion between these two electrons in the same orbital (spin-pair repulsion) makes it easier to remove one, lowering the IE.

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

A common mistake is forgetting the gaseous state symbols $(g)$ or writing the product as $Mg^{2+}$. The correct equation must show the formation of a $1+$ ion: $Mg(g) \rightarrow Mg^+(g) + e^-$.

Why is it incorrect to say that ionisation energy decreases down a group because the nuclear charge decreases?

This is a misconception; the nuclear charge actually increases significantly down a group as more protons are added. The IE decreases because the effects of increased atomic radius and shielding are more dominant than the increase in nuclear charge.

What error is made when identifying an element's group from a list of IE values: 578, 1817, 2745, 11577, 14842 kJ/mol?

A common error is picking the highest value rather than the jump. The jump occurs between the 3rd and 4th IE (from 2745 to 11577), indicating there are 3 valence electrons, so the element is in Group 13, not Group 4.

Define 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 under standard conditions.

What is 'Shielding' in the context of atomic structure?

Shielding is the reduction in the effective nuclear charge felt by outer electrons due to the repulsive forces of inner-shell electrons that lie between the nucleus and the valence shell.

What is the general formula for the $n^{th}$ ionisation energy?

The formula is $X^{(n-1)+}(g) \rightarrow X^{n+}(g) + e^-$. This represents the removal of the $n^{th}$ electron from an ion that has already lost $n-1$ electrons.

Why does ionisation energy always increase for successive electrons removed from the same atom?

As electrons are removed, the ratio of protons to electrons increases, meaning the remaining electrons are held more tightly by the nucleus. Additionally, there is less electron-electron repulsion, allowing the remaining electrons to be pulled closer to the nucleus.

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Chemistry

1. Atomic Structure

Ionisation Energy Trends

Summary

Ionisation energy (IE) is a fundamental periodic property that measures the energy required to remove electrons from gaseous atoms or ions. These trends are governed by the electrostatic attraction between the nucleus and the outermost electrons, which is influenced by nuclear charge, atomic radius, and electron shielding. Understanding these trends allows for the prediction of chemical reactivity and the determination of an element's electronic configuration.

1. Definition & Core Concepts

First Ionisation Energy is defined as the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions. This process is always endothermic because energy must be supplied to overcome the electrostatic attraction between the positively charged nucleus and the negatively charged electron.
The general equation for the first ionisation energy is represented as: $X(g) \rightarrow X^+(g) + e^-$ where $X$ represents any element. It is crucial that the state symbols are always included as 'g' because the definition specifically refers to the gaseous state to ensure that no other energy changes (like lattice energy or atomisation) interfere with the measurement.
Successive Ionisation Energies refer to the energy required to remove the second, third, and subsequent electrons from the resulting ions. These values always increase because each subsequent electron is being removed from an increasingly positive ion, resulting in a stronger electrostatic pull on the remaining electrons.

2. Underlying Principles: The Three Factors

Diagram showing the relationship between nuclear attraction, atomic radius, and electron shielding.

3. Periodic Trends: Across a Period

4. Group Trends: Down a Group

5. Key Distinctions: Anomalies in the Trend

6. Exam Strategy & Tips