Periodic Trends

Question 1

Atomic Radius

Answer 1

The radius of an atom (the mean distance from the nucleus of an atom to the border of the outmost energy level)

Question 2

Ionisation energy

Answer 2

The energy needed per mole to remove electrons from atoms in the gaseous phase

Question 3

First Ionisation Energy

Answer 3

The energy needed per mole to remove the first electron from an atom in the gaseous phase

Question 4

Electron Affinity

Answer 4

The energy released when an electron is attached to an atom or molecule to form a negative ion

Question 5

Electronegativity

Answer 5

A measure of the tendency of an atom in a molecule to attract bonding electrons

Question 6

Periodicity

Answer 6

The repetition of similar properties in chemical elements, as indicated by their positioning on the Periodic Table

Question 7

Valence electrons

Answer 7

The electrons which occupy the outermost energy level in an atom

Question 8

Atomic Radius Trend

Answer 8

Decreases across (left to right) a period
Increases down a group

Question 9

Ionisation Energy Trend

Answer 9

Increases from left to right across a period

Decreases from top to bottom down a group

Question 10

Why does atomic radius increase down a group?

Answer 10

• As you go down a group the valence electrons are placed in a higher energy level.
• Proton number increases too, but the charge of the nucleus is shielded by the core electrons.
• This reduces the effective nuclear charge.
• This effect is known as screening.
• The valence electrons feel the reduced effective nuclear charge and hence are not as strongly attracted to the nucleus.
• This causes the atomic radius to increase as you go down a group.

Question 11

Why does atomic radius decrease across a period?

Answer 11

Going from one element to the next you have to add one proton to the nucleus and add one electron into the same energy level. The core electrons do not change!
This means that the effect of screening is constant.
All electrons (including the new one) feel a greater attractive pull towards the more positive nucleus.	
This causes the atomic radius to decreases as you go across a period from left to right.

Question 12

Positive ions and atomic radius

Answer 12

Positive ions are smaller than their parent atoms:

• A cation comes from an atom that has lost an electron.
• If an atom loses an electron it becomes overall positive. -The remaining electrons are attracted more strongly towards the nucleus.
• This causes positive ions to be smaller than their parent atoms.

Question 13

Negative ions and atomic radius

Answer 13

Negative ions are bigger than their parent atoms:

• An anion comes from an atom that has gained an electron.
• If an atom gains an electron it becomes overall negative. -The electrons repel each other more strongly than before.
• This causes negative ions to be larger than their parent atoms.

Question 14

Electron Affinity Trend

Answer 14

Increases across a period

Decreases down a group

Question 15

Why does ionisation energy increase across a period?

Answer 15

As you go from left to right across a period, atomic radius decreases. Electrons are therefore closer to the nucleus and thus they are harder to remove resulting in ionisation energy increasing

Question 16

Why does ionisation energy decrease down a group?

Answer 16

As you go down a group, atomic radius increases. This means that the valence electrons are further away from the nucleus and are less strongly attracted to the nucleus and are therefore easier to remove resulting in ionisation energy decreasing

Question 17

Crystal lattice formation

Answer 17

Formed in ionic bonding. e.g. NaCl

Na and Cl alternate and form a strong crystal

Question 18

Properties of Metallic compounds

Answer 18

• Shiny
• Conduct electricity
• Conduct heat
• High density
• Malleable and ductile
• High melting point

Question 19

Properties of Ionic compounds

Answer 19

• Arranged in crystal lattice structure
• Solids are crystalline at room temperature
• High melting and boiling points
• Brittle

Question 20

Properties of Covalent compounds

Answer 20

• Lower melting and boiling points than ionic bonds
• More flexible than ionic compounds
• Not soluble in water
• Doesn’t conduct electricity

Question 21

What are the exceptions for Electron Affinity?

Answer 21

• Group 1A has a higher electron affinity Group 2A.

- Group 4A has a higher electron affinity Group 5A.

Question 22

What are the exceptions for Ionisation energy?

Answer 22

• Elements that are equally distant cannot be identified which one has a higher IE.
• Group 2A has higher IE than Group 3A. Group 5A has a higher IE than Group 6A.