Ionisation Energy

What is 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.

Why is ionisation energy always endothermic?

Energy must be supplied to overcome the electrostatic attraction between the positively charged nucleus and the negatively charged electron.

What happens to an atom when an electron is removed?

It becomes a positively charged ion (cation).

What is the second ionisation energy?

The energy required to remove one mole of electrons from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions.

Why is the second ionisation energy always greater than the first?

The remaining electrons experience a greater effective nuclear attraction because there are fewer electrons, less electron-electron repulsion, and a smaller ionic radius.

Why does ionisation energy increase as more electrons are removed?

• ion becomes more positively charged

• less electron–electron repulsion.

• remaining electrons experience a stronger electrostatic attraction to the nucleus

• more energy is required to remove each successive electron.

What is nuclear charge?

The positive charge of the nucleus, determined by the number of protons.

How does increasing nuclear charge affect ionisation energy?

It increases ionisation energy because the stronger positive charge produces a stronger electrostatic attraction between the nucleus and outer electrons.

What is shielding?

Shielding is the reduction in attraction between the nucleus and an outer electron caused by inner-shell electrons.

How does shielding affect ionisation energy?

Increased shielding decreases ionisation energy

reduces the electrostatic attraction between the nucleus and the outer electron.

less energy is required to remove the electron

How does shielding change down a group?

Shielding increases

new principal energy levels (electron shells) are added with each subsequent element.

This increases the number of inner-shell electrons between the nucleus and the outermost valence electrons.

How does shielding change across a period?

It remains approximately constant because electrons are added to the same principal energy level while the inner shells remain unchanged.

How does atomic radius affect ionisation energy?

A larger atomic radius means the outer electron is further from the nucleus, so the attraction is weaker and ionisation energy is lower.

Why does atomic radius increase down a group?

Additional electron shells are added, increasing the distance of the outer electrons from the nucleus.

Why does atomic radius generally decrease across a period?

Nuclear charge increases while electrons are added to the same shell, increasing attraction and pulling the electrons closer to the nucleus.

What is the general trend in first ionisation energy across a period?

First ionisation energy generally increases from left to right.

Why does first ionisation energy generally increase across a period?

Nuclear charge increases while shielding remains approximately constant and the outer electrons are added to the same shell, so attraction between the nucleus and outer electrons increases.

What is the general trend in first ionisation energy down a group?

First ionisation energy generally decreases down a group.

Why does first ionisation energy decrease down a group?

There is increased shielding and greater distance between the nucleus and outer electron, which outweighs the increase in nuclear charge, reducing the attraction between them.

Why is the first ionisation energy of B lower than Be?

B's outermost electron is in a higher-energy 2p subshell, whereas Be's is in the lower-energy 2s subshell. The 2p electron is therefore easier to remove.

Why is the first ionisation energy of Al lower than Mg?

Al's outermost electron is in a higher-energy 3p subshell, whereas Mg's is in the lower-energy 3s subshell. The 3p electron is therefore easier to remove.

Why is the first ionisation energy of O lower than N?

O has four electrons in its 2p subshell, so one 2p orbital contains a pair of electrons. Repulsion between the paired electrons makes one electron easier to remove.

Why is the first ionisation energy of S lower than P?

S has four electrons in its 3p subshell, so one 3p orbital contains a pair of electrons. Repulsion between the paired electrons makes one electron easier to remove.

Why does N have a higher first ionisation energy than O?

N has a half-filled 2p subshell with one electron in each 2p orbital, which is relatively stable. O has a paired 2p orbital, so electron-electron repulsion makes an electron easier to remove.

Why does P have a higher first ionisation energy than S?

P has a half-filled 3p subshell with one electron in each 3p orbital, while S has a paired 3p orbital. Repulsion between paired electrons makes an electron easier to remove from S.

What is special about a half-filled p subshell?

Each p orbital contains one electron, giving a relatively stable arrangement with reduced electron-electron repulsion.

What causes the dip in first ionisation energy from N to O?

The fourth 2p electron in O must pair with another electron in a 2p orbital, creating electron-electron repulsion and making that electron easier to remove.

What causes the dip in first ionisation energy from P to S?

The fourth 3p electron in S must pair with another electron in a 3p orbital, creating electron-electron repulsion and making that electron easier to remove.

What are the three main factors affecting ionisation energy?

Nuclear charge, shielding, and distance of the outer electron from the nucleus.

How does increasing distance from the nucleus affect ionisation energy?

It decreases ionisation energy because electrostatic attraction decreases as the distance increases.

How does electron-electron repulsion affect ionisation energy?

Repulsion makes electrons easier to remove, decreasing ionisation energy.

What happens to ionisation energy when an electron is removed from a new inner shell?

There is a large jump in ionisation energy because the electron is closer to the nucleus and experiences less shielding.

What does a large jump in successive ionisation energies indicate?

The electron has begun to be removed from an inner shell rather than the outer shell.

How can successive ionisation energies be used to determine the number of outer-shell electrons?

Identify the large jump in ionisation energy; the number of electrons removed before the jump corresponds to the number of outer-shell electrons.

Why is there a large jump when removing an electron from an inner shell?

Inner-shell electrons are closer to the nucleus and experience less shielding, so they are much more strongly attracted to the nucleus.

How does ionisation energy relate to electron configuration?

The pattern of successive ionisation energies can reveal how electrons are arranged in shells and subshells.

What is the trend in first ionisation energy across the periodic table?

It generally increases from left to right across a period and decreases down a group.

What is the key explanation for the periodic trend in first ionisation energy?

Across a period, increasing nuclear charge strengthens attraction while shielding is similar; down a group, increased shielding and distance weaken attraction.