1 - Period 3 elements

Question 1

What is periodicity?

Answer 1

The repeating pattern / trend of physical or chemical properties

Question 2

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

Answer 2

Atomic radii decreases

Question 3

Why does atomic radius decrease across period 3?

Answer 3

Atomic radius increases along period 3
- Nuclear charge increases as number of protons increase causing electrons to be pulled closer to the nucleus as there is a stronger force of attraction between electrons and nucleus
- Electrons are being added to the same shell so there is no shielding

Question 4

Define ‘1st ionisation energy’

Answer 4

The energy/enthalpy required to remove 1 mole of electrons from 1 mole of gaseous atoms

Question 5

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

Answer 5

1st ionisation energy increases

Question 6

Why do 1st ionisation energies increase across period 3?

Answer 6

1st ionisation energy increases across period 3
- Outer electron is closer to the nucleus as atomic radius decreases
- Nuclear charge increases so there is a greater attraction between the nucelus and outer electron

Question 7

Why is there a small drop in IE between Magnesium and Aluminium?

Answer 7

• For Al, the outer electron is removed from a p subshell whereas the outer electron in Mg is being removed from an s subshell
• P subshells are in a higher energy level and are slightly further away from the nucleus so electrons are more easily removed

- Evidence for subshells

Question 8

Why is there a small drop in IE between Phosphorus and Sulfur?

Answer 8

• For S, the outer electron being removed is paired up with another electron in the same 3p orbital
• There is slight repulsion between the two electrons which makes the second one easier to remove

- Evidence for electron repulsion

Question 9

Define ‘electronegativity’

Answer 9

The ability of an atom to attract electrons in a covalent bond

Question 10

What is the trend in electronegativity across period 3?

Answer 10

Electronegativity increases

Question 11

Why does electronegativity increase across period 3?

Answer 11

• Nuclear charge increases
• Atomic radius decreases so bonding pair of electrons is closer to the nucleus
• The electron pair in a covalent bond is more attracted to the atoms nucleus

Question 12

What is the trend in melting points across period 3?

Answer 12

Generally increases, however:
- Na, Mg, Al and Si mp increase
- Large increase in mp from Al to Si
- From Si, P, S, Cl and Ar mp decrease

Question 13

Why does the melting points increase for Na - Al?

Answer 13

• Metallic bonding occurs in Na, Mg and Al
• Increase in mp from Na - Al due to increasing electrostatic forces of attraction between positive ion and delocalised electrons
• This is due to increases nuclear charge and an increase in delocalised electrons
• More energy is needed to overcome the metallic bonds

Question 14

Why does the melting point for Si significantly increase after Na - Al?

Answer 14

• Silicon is giant covalent / macromolecular
• Has many 4 strong covalent bonds per silicon atom
• A lot of energy is required to overcome / break these bonds so Si has a very high mp

Question 15

Why does the melting point decrease for P - Cl?

Answer 15

• P, S and Cl are simple molecular
• There are weak VdW’s forces between molecules
• Little energy is needed to overcome the VdW’s forces

Question 16

Why does Sulfur have a higher melting point than Phosphorus?

Answer 16

Sulfur has a higher mp than phosphorus as it has more electrons, meaning it is a larger molecule than phosphorus so sulfur has stronger VdW forces that require more energy to overcome
- S₈ = 128 e^-
- P₄ = 60 e^-

Question 17

Why does Ar have the lowest melting point in period 3?

Answer 17

Argon is monatomic with only weak VdW forces between Ar atoms

Question 18

Are the trends of atomic radii, IE and melting points the same in period 2?

Answer 18

Yes
- Same explanations can be applied to other periods