Periodic Trends

Question 1

What is electronegativity?

Answer 1

The attraction of an atom for a bonded pair of electrons.

Question 2

What is the trend in electronegativity?

Answer 2

As a period is crossed, electronegativity increases. As a group descends, electronegativity decreases.

Question 3

Why does electronegativity increase as a period is crossed?

Answer 3

More protons are added for the number of electrons increasing electrostatic attraction.

Question 4

Why does electronegativity decrease as a group descends?

Answer 4

More electron shells are added meaning valence electrons are further away from the nucleus, decreasing electrostatic attraction.

Question 5

What is ionisation energy?

Answer 5

The energy required to remove one electron from an atom of an element in the gas phase.

Question 6

What is first ionisation energy?

Answer 6

The energy required to remove the first valence electron from an atom in the gas phase.

Question 7

What is the trend in first ionisation energy?

Answer 7

As a group descends, first ionisation energy decreases. As a period is crossed, first ionisation energy increases.

Question 8

Why does first ionisation energy increase as a period is crossed?

Answer 8

Higher electrostatic attraction occurs between electrons and protons making it harder to remove electrons, therefore requiring more energy.

Question 9

Why does first ionisation energy decrease as a group descends?

Answer 9

Valence electrons are further from the nucleus making it easier for electrons to be removed, therefore requiring less energy.

Question 10

What is atomic radius?

Answer 10

The measurement of size of atoms.

Question 11

What is the trend for atomic radius?

Answer 11

As a group descends, atomic radius increases. As a period is crossed, atomic radius decreases.

Question 12

Why does atomic radius decrease as a period is crossed?

Answer 12

Electrostatic attraction increases as atomic number (electrons and protons) increase.

Question 13

Why does atomic radius increase as a group descends?

Answer 13

A new electron shell (energy level) is added.

Question 14

What is the decrease in atoms radii due to?

Answer 14

Protons (electrostatic attraction increases)

Question 15

Why does group 1 become more reactive as you move down it?

Answer 15

The electrons get further away from the nucleus because of added electron shells.

Question 16

Why is helium in group 18, not group 2?

Answer 16

Helium only has 2 electrons however it has a full valence shell. The chemical properties of helium are more close to those of group 18 than they are of group 2.

Question 17

Explain the relationship between ionisation energy, atomic radius and core charge? (4 marks)

Answer 17

• Ionisation energy definition
• Atomic radius definition
• Core charge definition
• Higher ionisation energy from a higher core charge which causes a smaller atomic radius due to an increase in number of protons and neutrons.

Question 18

Trend in metallic character?

Answer 18

The elements with the most weakly held valence electrons, having the smallest first ionisation energy and the greatest metallic character, are towards the bottom left-hand side of the periodic table.

Question 19

Explain why does chlorine have a higher 1st ionisation energy than magnesium?
(3 marks)

Answer 19

• moving to the right of the periodic table, the first ionisation energy increases as more protons are added for electrostatic attraction.
• chlorine has more electrons in its valence shell making attraction higher.
• more energy is required to remove an electron, therefore higher first ionisation energy.

Question 20

What element has a higher 1st ionisation energy, iodine or chlorine?

Answer 20

chlorine

Question 21

The first four ionisation energies of an element in a previous periodic table in group 2, period 3 are 736kJ mol-1, 1450kJ mol-1, 7740kJ mol-1 and 10500kj mol-1 respectively. Explain why the ionisation energies for successive electrons being removed from the atom have this pattern?
(3 marks)

Answer 21

• the element has 2 electrons in its valence shell.
• the ionisation energies from the 1st to the 2nd show a rising trend as the electron has to be removed from an increasing core charge attracting the electron.
• the 3rd ionisation energy is so high because both valence electrons would have been removed from a shell much closer to the nucleus and the 7740kJ value represents the energy needed to pull an electron from a full shell below, which is extremely stable (noble gas structure).