CHEM SAC CHAPTER 2

Question 1

Effective nuclear charge

Answer 1

• the measure of an attractive force felt by the valence shell electrons towards the nucleus.
• the valence shell is shielded from the attraction of the nucleus because of inner shell electrons.
• remains constant down a group (held less strongly due to being far away from the nucleus, with more shells in an atom)
• increases from left to right across a period (more attracted to the nucleus as ENC increases)

Question 2

Electronegativity

Answer 2

the ability of an atom to attract electrons towards itself when forming a chemical bond.
- Electronegativity increases when ENC is greater.
- Decreases down a group (less strongly attracted to nucleus due to being further away)
- Increases across a period from left to right (valence become strongly attracted to the nucleus while the number of occupied shells in atoms remains constant.)

Question 3

Atomic radius

Answer 3

• Measurement used for the size of atoms, and distance from the nucleus to valence shell electrons (halving distance between nuclei of two atoms of the same element that are bonded together)
• Increases down a group, ENC stays constant and the number of shells increases as you move down a group.
• Decreases left to right across a period, the number of occupied shells in atoms remains constant but ENC increases. Valence become more strongly attracted to the nucleus (pulled tightly towards the nucleus)

Question 4

First ionisation energy

Answer 4

The energy required to remove one electron from an atom of an element in the gas phase.
- reflects how strongly valence electrons are attracted to the nucleus of the atoms.
- Down a group, it decreases as effective nuclear charge stays constant and the number of shells down a group increases. This makes valence electrons less attracted to the nucleus as they are further from the nucleus. Energy required to overcome the attraction between the nucleus and valence electron is less (first ionisation energy decreases down a group)
- Left to right across a period increases as ENC increases and the number of occupied shells remains constant across a period. Valence electrons become more strongly attracted to the nucleus, and more energy is required to remove an electron.