Periodicity

Question 1

What is the element structure of Na, Mg and Al?

Answer 1

giant metallic

Question 2

What is the element structure of Si?

Answer 2

macro-molecular (giant covalent structure)

Question 3

What is the element structure of P, S and Cl?

Answer 3

(simple) molecular

Question 4

What is the element structure of Ar?

Answer 4

atomic

Question 5

What is the general trend in atomic radius across period 3?

Answer 5

decreases as you move across from left to right

Question 6

Why does atomic radius get smaller going across period 3?

Answer 6

• the nuclear charge (proton number) increases which means there is a greater attraction of the electrons towards the nucleus
• the amount of shielding across any period stays the same

Question 7

What is the general trend in 1st IE across period 3?

Answer 7

general increase

Question 8

Why does 1st IE across period 3 (left to right) generally increase?

Answer 8

• the nuclear charge (proton number) increases
• the amount of shielding stays the same

Question 9

Explain why Aluminium has a lower 1st IE than Magnesium (3 marks)

Answer 9

• the first e- removed from Mg is from a 3s sub level
• the first e- removed from Al is from a 3p sub level
• The 3s sub level is lower in energy than 3p

Question 10

Explain why Sulfur has a lower 1st IE than Phosphorus (3 marks)

Answer 10

• the first e- removed from P is from a 3p sub level and is unpaired
• the first e- removed from S is also from a 3p sub level, but is from a paired orbital
• this means sulfur has a lower first ionisation energy due to electron pair repulsion

Question 11

What is the general trend in melting and boiling points across period 3 from left to right?

Answer 11

they both increase and then decrease as you move across period 3

Question 12

Why do the melting points increase from Na to Mg to Al?

Answer 12

because the strength of the metallic bonding increases as the charges on the ions they form increase by 1 and the size of the ions they form decrease - this means small charge spread over a large ion leads to weak metallic bonding for Na and a large charge spread over a smaller ion leads to stronger metallic bonding for Al.

Question 13

Which of the group 3 elements has the highest melting point?

Answer 13

silicon

Question 14

Why does silicon have the highest melting point?

Answer 14

it has a giant macromolecular structure held together by strong covalent bonds which require a lot of energy to break

Question 15

What do P, S and Cl all exist as?

Answer 15

simple covalent molecules: P - P4, S - S8, Cl - Cl2

Question 16

What do the melting and boiling points of P, S and Cl depend on?

Answer 16

as they are simple molecular substances, their melting and boiling points don’t depend on the strength of the covalent bonds they form, but the strength of the Van der Waals forces which occur between molecules e.g. S8 is the biggest of the three molecules so they have the strongest VdW and so the higher melting and boiling point etc.

Question 17

Why does Argon have the lowest overall melting and boiling points of all the period 3 elements?

Answer 17

as argon only exists as single atoms, it has the weakest VdW forces of all the non-metals

Question 18

Are the general trends for melting and boiling points exactly the same across period 3?

Answer 18

• Yes with one exception: silicon has the highest melting point, but aluminium has the highest boiling point
• This is because in order to melt Si a lot of energy is needed to break strong covalent bonds but once molten relatively little more energy is then needed to vaporise it and so the boiling point isn’t that much higher than the melting point

Question 19

Which period 3 element has the highest boiling point and why?

Answer 19

aluminium because once molten a lot of energy is still needed to overcome strong electrostatic metallic bonds and so Al has a very high boiling point

Question 20

What is the general trend in successive IEs of period 3 elements?

Answer 20

increases each time because the number of protons in the nucleus remains the same but each time there are fewer electrons and so these are held more tightly so they therefore require more energy to be removed each time

Question 21

Why might there appear to be large ‘jumps’ in successive IEs of an element?

Answer 21

the ‘jumps’ indicate moving from one shell/energy level to another shell/energy level