1.5 Evidence for the electronic structure of atoms

What is First ionisation energy?

The energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.

What is the formula for the first ionisation energy?

X (g) ——> X+ + e-

What is the unit of measurement for ionisation energy?

kJ mol⁻¹

is ionisation energy endothermic or exothermic and why?

Ionisation is always endothermic

because energy must be supplied to overcome the electrostatic attraction between the positive nucleus and the electron.

What is successive ionisation energy?

Successive ionisation energies are the energies required to remove electrons one after another from the same element.

Why do successive ionisation energies increase?

After removing an electron:

• the number of protons stays the same

• there are fewer electrons

• electron–electron repulsion decreases

• the remaining electrons experience a stronger net attraction to the nucleus

• therefore more energy is required to remove the next electron

The number of electrons decreases, reducing electron–electron repulsion, so the remaining electrons are more strongly attracted to the nucleus.

What are the three main factors affecting ionisation energy?

Nuclear charge

More protons:

nuclear charge ↑

→ stronger attraction between nucleus and electron

→ more energy required

→ IE increases

Distance from nucleus

Greater distance:

distance ↑

→ weaker electrostatic attraction

→ electron easier to remove

→ IE decreases

Shielding

Shielding is the effect of inner electrons reducing the attraction between the nucleus and an outer electron.

Inner electrons repel outer electrons and reduce the effective nuclear charge experienced by the outer electron.

More inner shells:

greater shielding

→ weaker attraction

→ electron easier to remove

→ IE decreases

What is the fourth factor affecting ionisation energy?

ELECTRON PAIRING / ELECTRON–ELECTRON REPULSION

Describe the trend of first ionisation energy across a period

First IE generally increases from left → right across a period.

Why?

Moving across a period:

Nuclear charge increases

More protons → greater nuclear charge → stronger attraction.

Same principal energy level

The electrons are being added to the same outer shell.

Shielding is approximately constant

No additional inner electron shell is being added.

Distance decreases slightly

The increasing nuclear charge pulls the outer electrons slightly closer to the nucleus.

Describe the trend of first ionisation energy down a group

First ionisation energy decreases down a group.

Why?

Moving down the group:

Nuclear charge increases

More protons → attraction tends to increase.

BUT:

A new principal energy level is added

The outer electron becomes further from the nucleus.

Shielding increases

There are more inner electron shells between the nucleus and outer electron.

How does the trend of first ionisation provide evidence for the existence of principle energy levels?

First IE generally increases across each period, then drops significantly at the start of a new period. The electron being removed is now in a new principal energy level further from the nucleus. This provides evidence for distinct quantum shells/principal energy levels.

Why are the small steps in ionisation graphs?

Each time an electron is removed:

• same nuclear charge

• fewer remaining electrons

• less electron–electron repulsion

• stronger net attraction on remaining electrons

What is atomic emission spectra?

When atoms are given energy, electrons can move from a lower energy level to a higher energy level.

When electrons fall back down:

energy is released as electromagnetic radiation.

A spectroscope shows this as bright lines.

Why are outer electrons easier to remove?

Outer electrons are generally:

• further from nucleus

• more shielded

• less strongly attracted to nucleus

Therefore:

higher-energy outer electrons are easier to remove.

Inner electrons are:

• closer to nucleus

• less shielded

• strongly attracted

Therefore:

inner electrons require much more energy to remove.

What does a large jump in successive ionisation energies indicate?

An electron is being removed from a new principal energy level closer to the nucleus and with less shielding, so much more energy is required.

What can the position of the first large jump tell you?

The number of electrons in the outer shell, and therefore the group for a main-group element.