Periodic trend in electron configuration and ionisation energy

Question 1

the periodic trend in electron configurations across Periods 2

Answer 1

The 2s sub-shell fills with two electrons, followed by the 2p sub shell with six electrons

Question 2

the periodic trend in electron configurations across Periods 3

Answer 2

The 3s sub shell fills with two electrons, followed by the 3p sub shell with six electrons

Question 3

classification of elements into s-, p- and d-blocks

Answer 3

Question 4

first ionisation energy

Answer 4

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

X(g) –> X+(g) + e-

Question 5

Atomic radius

Answer 5

The larger the atomic radius, the greater the distance between the nucleus and the outer electrons, the less the electrostatic attraction.

• across a period the radius decreases (Electrons in smaller atoms are held closer to the nucleus so the attraction is greater)
• down a group the radius increases

Question 6

Nuclear charge

Answer 6

The more protons there are in the nucleus of an atom, the greater the electrostatic attraction between the nucleus and the outer electrons

Question 7

Electron shielding

Answer 7

Electrons are negatively charged and so inner shell electrons repel outer shell electrons. This repulsion (shielding effect) reduces the attraction between the nucleus and the outer electrons. More electron shells provide more shielding.

Question 8

Trends in first ionisation energies

Answer 8

• The electrons with the largest ionisation energy are from the shell closest to the nucleus
• a general increase in first ionisation energy across each period (H-He, Li-Ne, Na-Ar)
• a sharp decrease in first ionisation energy between the end of one period and the start of the next period (He-Li, Ne-Na, Ar-K)

Question 9

Helium and hydrogen

Answer 9

Its first electron is in the first shell closest to the nucleus and has no
shielding effects from inner shells. He has a bigger first ionisation
energy than H as it has one more proton

Question 10

trend in first ionisation
energies across Periods 2

Answer 10

• Nuclear charge increases
• the number of electrons in the outer shell increases
• same shell; similar shielding
• nuclear attraction increases
• atomic radius decreases because the shell is held by a greater nuclear charge and is held inwards
• first ionisation energy increases

Question 11

Boron and Beryllium

Answer 11

The fall in first ionisation energy from beryllium to boron marks the start of filling the 2p sub-shell. The first ionisation energy of boron is less than the first ionisation energy of beryllium,

The 2p sub shell of boron has a higher energy than the 2s sub shell in beryllium, therefore it is easier to remove the 2p electron in boron than one of the 2s electrons in beryllium

Question 12

nitrogen and oxygen

Answer 12

The fall in first ionisation energy from nitrogen to oxygen mark the start of electron pairing in the p-orbitals of the 2p sub shell. The first ionisation energy of oxygen is less than the first ionisation of nitrogen.

• In nitrogen and oxygen the highest energy electrons are in a s- sub shell
• In oxygen the paired electrons in one of the 2p orbitals repel one another, making it easier to remove an electron

Question 13

trend in first ionisation
energies across Periods 3

Answer 13

• Nuclear charge increases
• the number of electrons in the outer shell increases
• same shell; similar shielding
• nuclear attraction increases
• atomic radius decreases
• first ionisation energy increases

Question 14

sodium and neon

Answer 14

This is because Na will have its outer electron in a 3s shell further from the nucleus and is more shielded. So Na’s outer electron is easier to remove and has a lower ionisation energy.

Question 15

magnesium and aluminium

Answer 15

Al is starting to fill a 3p sub shell, whereas Mg has its outer electrons in the 3s sub shell. The electrons in the 3p subshell are slightly easier to remove because the 3p electrons are higher in energy so the outermost electron is on average further from the nucleus
The p orbital is also slightly shielded by the 3s electrons

As a result, less energy is required to remove the outermost p electron from the group 3 element compared to removing the outermost s electron from the group 2 element.

Question 16

phosphorus and sulphur

Answer 16

With sulphur there are 4 electrons in the 3p sub shell and the 4th is starting to fill the first 3p orbital. When the second electron is added to a 3p orbital there is a slight repulsion between the two negatively charged electrons which makes the second electron easier to remove.

As a result, less energy is needed to remove one of these paired electrons in the group 6 element compared to the unpaired electron (singly occupied orbital) in the group 5 element.

Question 17

trend in first ionisation
energies down a group

Answer 17

First ionisation energies decrease down every group in the periodic table
- nuclear charge increases
- atomic radius increases
- more inner shells so shielding increases
- nuclear attraction on outer electrons decreases (less energy is required to remove the outermost electron)
- first ionisation energy decreases

Question 18

Successive ionisation energies

Answer 18

An element has as many ionisation energies as there are electrons

Question 19

Making predictions from successive ionisation energies

Answer 19

Successive ionisation energies allow predictions to be made about
- the number of electrons in the outer shell
- the group of the element in the periodic table
- the identity of an element