Chemistry - Chapter 7: Trends in the Periodic Table

atomic radius (covalent radius)

half the distance between the nuclei of two atoms of the same element joined by a single covalent bond

decreasing atomic radius across periods

1. Increase in number of nuclear charge (no. of protons) → greater attractive force on the outer electrons

2. no screening effect → due to no energy levels/shells added

increasing atomic radius down groups

1. an increase in the number of energy levels → extra electrons are occupying a new shell added to the atom, which is further that the nucleus, increasing the atomic radius

2. increasing screening effect → inner electrons shield, i.e. blocks out the outer electrons from the nuclear charge

screening effect

the innermost electrons shielding the nuclear charge from the outermost electrons, meaning the outermost electrons are relatively unaffected by the nuclear charge

1st ionisation energy

the minimum amount of energy required to completely remove the first most loosely bound electron from a mole of isolated atoms of an element in its neutral gaseous ground state

this is due to how the outermost the electron is from the nucleus, the easier it is to remove & also due to its weaker attractive force

decreasing 1st ie down groups

1. screening effect of some of the nuclear attraction due to more shells

2. increasing atomic radius due to extra shells being added

increasing ie across periods

1. increase in effective nuclear charge : holds the outer electrons and more energy required to remove them & no change in the screening effect

2. decreasing atomic radius : causes outer electrons to be closer to the nucleus

exceptions to 1st ie trends → stability

• group 2 elements → filled outer sublevel

• group 5 elements → half-filled outer sublevel

this gives stability, hence higher ionisation needed

2nd ionisation energy

refers to ions:

the energy required to remove an electron from an ion with one positive charge in its gaseous state

so… X+ (g) → X²+ (g) + e-

• it is always greater than the first as there now being more protons than electrons

• electrons are now held more firmly by the positive ion, hence the atomic radius of an ion is smaller than the neutral atom

• therefore, there is an increase in ionisation energy for the removal of the 2nd e-

electronegativity

the relative power attraction an atom in a molecule has for the shared pair of electrons in a single covalent bond

increasing e/n across periods

1. increasing nuclear charge (no. of protons increase)

2. decreasing atomic radius

so w/ a small atomic radius and relatively large nuclear charge, the atom has great attraction for electrons

decreasing e/n down a group

1. increasing atomic radius due to addition of extra shells

2. increasing screening effect of inner completed shells on outer electrons (determines the attraction)

so w/ a large radius, the nucleus finds it more difficult to attract electrons

trends within alkali metals

reactivity of the alkali elements increasing going down because…

• decreasing ionisation energies (low)

  • outershell e-s are further away from the nucleus and so give up their electrons easily

• decreasing electronegativities

  • e-s with potential to form bonds are further away from the nucleus, hence there is very little pull on them

so they will have the lowest 1st ie as they only have 1 e- in their outermost shell and will lose it easily to attain a stable configuration (u can compare using electron configuration

trends within halogens

• high e/n values

• quite reactive due to great attraction for electrons

• oxidising agents - substances capable of removing electrons from other atoms