1 - Period 3 elements

Question 1

What is periodicity?

Answer 1

The repeating pattern of physical or chemical properties going across the periods

Question 2

In order, what are the period 3 elements?

Answer 2

Na, Mg, Al, Si, P, S, Cl, Ar

Question 3

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

Answer 3

Atomic radii decreases

Question 4

Why does atomic radius decrease across period 3?

Answer 4

• Number of protons increase so positive charge of nucelus increases
• Electrons are pulled closer to nucleus, making atomic radii smaller
• Electrons are also being added to same shell so no further shielding

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

Number of protons increase so nuclear charge increases
- Greater attraction between outer electrons and nucleus, increasing IE

Question 7

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

Answer 7

• Mg has its outer electrons in the 3s subshell, whereas Al is starting to fill the 3p subshell
• Al’s electron is slightly easier to remove as 3p electrons are higher in energy

Question 8

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

Answer 8

• Sulfur’s outer electron is paired up with another electron in the same 3p orbital
• There is repulsion between electrons making them easier to remove

Question 9

What is the trend in melting points across period 3?

Answer 9

• Na, Mg, Al and Si mp increase
• P, S, Cl and Ar mp decrease
• Large increase in mp in Si

Question 10

Why does the melting points increase for Na - Al?

Answer 10

• They are metals so metallic bonding occurs
• Mp increases as metallic bonds get stronger due to increasing amount of delocalised electrons and decreased atomic radii
• Higher energy needed to break bonds

Question 11

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

Answer 11

• Silicon is macromolecular
• Has many strong covalent bonds between atoms, high energy needed to break bonds
• Therefore very high mp

Question 12

Why does the melting point decrease for P - Cl?

Answer 12

• P, S and Cl are simple molecular
• Weak VdW forces between molecules so little energy needed to break them
• Therefore low mp

Question 13

Why does Sulfur have a higher melting point than Phosphorus?

Answer 13

Sulfur (S₈) has a higher mp as it has more electrons than phosphorus (P₄) so has stronger VdW forces between molecules
- Sulfur = 128 e^-
- Phosphorus = 60 e^-

Question 14

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

Answer 14

Argon is monatomic with weak VdW forces between atoms

Question 15

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

Answer 15

Yes
- Same explanations can be used for trends in period 2