Periodic Trends in Ionisation Energies and Covalent Radii

Trends in the Periodic Table and Bonding

Covalent Radii of Elements

The size of an atom is measured by it’s covalent radius, the distance between the nucleus and it’s outer electrons.

Values for covalent radii can be found on page 5 of the data book

nucleus

energy levels

covalent radius

Looking across a periodAcross a period we can see the covalent radius decreasing.

So, from lithium to fluorine:

3+

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9+

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Lithium Atom Fluorine Atom

As we move left to right we are adding a proton to the nucleus and an electron to the outermost energy level.

Looking across a periodThe lithium atom

has a smaller nuclear charge

than neon and so a larger covalent

radius

Fluorine’s greater nuclear charge pulls

the outer energy level in closer.

3+

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radius = 134pm radius = 71pm

9+

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9+

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Looking down a group

The single electron in the outermost energy level is much further from the nucleus in caesium.

CsLi

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This causes the caesium atom to have a much larger covalent radius.

The caesium atom also has many more electrons between the single outer electron and the nucleus.

This screening effect counteracts the attraction from the greater nuclear charge.

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Atomic Size SummaryDecreasing Atomic Size

Across a period from left to right atomic size decreases

This is because of the atom having more electrons & protons and therefore a greater attraction which pulls the atom closer together hence the smaller size.

Atomic Size SummaryIn

crea

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Ato

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Down a group atomic size increases

This is because of the extra outer energy levels and the screening effect of the outer electrons.

Decreasing Atomic Size

Ionisation Energy

The ionisation energy is the energy required to remove one mole of electrons from one mole of atoms in the gaseous state.

The first ionisation energy of magnesium:

Mg (g) Mg+ (g) + e- 744 kJmol-1

Values for ionisation energies can be found on page 10 of the data book

Ionisation Energy

The third ionisation enthalpy shows a massive increase because it requires an electron to be removed from magnesium’s second energy level.

Mg2+ (g) Mg3+ (g) + e- 7750 kJmol-1

Mg+ (g) Mg2+ (g) + e- 1460 kJmol-1

The second ionisation energy of magnesium:

Looking across a period

From lithium to neon the first ionisation energy increases. Why?

Li (g) Li+ (g) + e- 526 kJmol-1

Ne (g) Ne+ (g) + e- 2090 kJmol-1

Li Be B C N O F Ne

An atom of Lithium

The lithium atom has 3 protons inside the nucleus

Li (g) Li+ (g) + e- 526 kJmol-1

3+

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The outer electron is attracted by a relatively small nuclear charge

An atom of Neon

The neon atom has 10 protons inside the nucleus

10+

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Ne (g) Ne+ (g) + e- 2090 kJmol-1

Each of neon’s eight outer electrons is attracted by a stronger nuclear charge

Looking down a groupThe first ionisation energy decreases down a group in the periodic table. Why?

Li (g) Li+ (g) + e- 526 kJmol-1

Cs (g) Cs+ (g) + e- 382 kJmol-1

1. More Energy LevelsAs we saw with atomic size, the single electron in the outermost energy level is much further from the nucleus in caesium than in lithium.

Li

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Caesium’s attraction for its outer electron is lowered by the screening effect caused by all its other electrons.

Cs

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2. Screening Effect

Ionisation Energy SummaryIncreasing Ionisation Energy

Across a period from left to right ionisation energy increases

This is due to the increase in atomic charge having a greater pull on the electrons and therefore more energy is required to remove electrons.

Ionisation Energy SummaryD

ecre

asin

g Io

nisa

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Ene

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Down a group ionisation energy decreases

This is due to the outer electrons being further away from the nucleus and so the attraction is weaker and they are more easily removed.

Increasing Ionisation Energy