Periodicity (F6examonly)

Question 1

How does the periodic table arrange elements?

Answer 1

• by increasing atomic (proton) number
• in periods
• in groups

Question 2

What do the periods on the periodic table show?

Answer 2

• all elements in a period have the same number of electron shells
• therefore there are repeating trends in the physical and chemical properties of the elements across each period
• these trends are known as periodicity

Question 3

What do the groups on the periodic table show?

Answer 3

• all elements in a group have the same number of electrons in their outer shell
• therefore elements in a group have similar chemical properties

Question 4

What is first ionisation energy?

Answer 4

the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions

Question 5

What is meant by successive ionisation energies?

Answer 5

• after the first ionisation energy, there is the second ionisation energy, the energy required to remove 1 mole of electrons from 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions
• an element has as many ionisation energies as there are electrons
• as ionisation energy succession increases, the energy to remove the electrons increases as well

Question 6

Explain the general trend in first ionisation energies across periods 2 and 3:

Answer 6

• the nuclear charge increases across periods 2 and 3
• the shielding stays the same, and the atomic radius decreases slightly across periods 2 and 3
• so the attraction between the nucleus and the electron being removed increases across periods 2 and 3
• therefore it takes more energy to remove an electron across periods 2 and 3

Question 7

Why is the first ionisation energy of aluminium lower than magnesium in period 3?

Answer 7

• in Mg, an electron is being removed from a 3s orbital
• in Al, an electron is being removed from a 3p orbital
• the 3p subshell is further away from the nucleus the 3s subshell, so the attraction between the nucleus and the electron being removed in Al is weaker than the attraction between the nucleus and the electron being removed in Mg
• therefore less energy is required to remove an electron from Al
• (this is the same for boron and beryllium in period 2)

Question 8

Why is the first ionisation energy of sulfur lower than phosphorus in period 3?

Answer 8

• in P, an electron is being removed from an orbital with only 1 electron in it
• in S, an electron is being removed from an orbital with 2 electrons in it
• because electrons repel each other, there is more electron to electron repulsion in sulfur, so less energy is required to remove an electron in sulfur compared to in phosphorus
• (this is the same for oxygen and nitrogen in period 2)

Question 9

Explain the general trend in first ionisation energies down a group:

Answer 9

• although the nuclear charge increases down a group
• both atomic radius and shielding increases down a group as well, which outweighs the effect of increasing nuclear charge
• therefore there is a weaker attraction between the nucleus and the electron being removed down a group
• so less energy is required to remove an electron from elements down a group

Question 10

Explain why successive ionisation energies increase:

Answer 10

• as electrons are being removed, the ion becomes increasingly positive
• there is also less repulsion amongst the remaining electrons
• therefore the attraction between the nucleus and the remaining electrons becomes stronger as more electrons are removed

Question 11

How do successive ionisation energies tell us the group number of an element?

Answer 11

• the number of electrons removed before the first big jump (the most left one) in ionisation energy tells us the group number
• e.g. there is 1 electron being removed from Na before the first big jump in ionisation energy, so Na is in group 1

Question 12

How do successive ionisation energies tell us the number of electrons in each shell of an element?

Answer 12

• the number of electrons before each big jump in ionisation energy tells us the electronic structure (reading from right to left)
• e.g. in Na there are 2 electrons before a jump, then 8 electrons before another jump, then 1 final electron. this shows that Na has 2 electrons in the first shell, then 8 in the second, and 1 in the third