Structure 3.1.3 - periodicity refers to trends in properties of elements across a period
see 3.1.1 periods, groups and blocks
atomic radius, ionic radius, ionisation energy, electron affinity, electronegativity
atomic radius
- increases down a group due to increased number of occupied energy levels
- decreases across a period due to increased attraction between nucleus and outer electrons due to increase of nuclear charge
there is no sharp boundaries to an atom. the radius is considered to be the distance from the nucleus to the outermost electrons.
the atomic radius is measured as half the distance between neighbouring nuclei
ionic radius
- increases down a group due to increasing number of energy levels
positive ions are smaller than parent atoms
- loss of outer energy level
- decreases due to increase in nuclear charge
- decreases due to increased attraction between nucleus and electrons, pulling outer energy level closer to nucleus
negative ions are larger than parent atoms:
- increased repulsion between electrons in outer energy level causing electrons to move further apart
- decreases due to increase in nuclear charge
- decreases due to increased attraction between nucleus and electrons, pulling outer energy level closer to nucleus
ionisation energy
- decreases down a group due to increased distance and reduced attraction (effective nuclear charge is about the same due to shielding)
- increases across a period due to increase in effective nuclear charge increasing attraction between outer electrons and nucleus
electron affinity
the first electron affinity is the energy change
as the added electron is attracted to the positively charged nucleus, the process generally gives out energy and is exothermic
noble gases do not generally form negatively charged ions, so values are unavailable for them
some group 2 and 5 elements are not included.
- group 2 elements have an electron configuration
, so the added electron is placed into a orbital, which is further from the nucleus and experiences reduced electrostatic attraction. - group 5 elements have half-filled
sublevels and the addition of an electron must be into a singly-filled orbital - as all singly-occupied orbitals in a sublevel have parallel spin, the attraction is less as there is increased inter-electron repulsion
second and third electron affinities are defined similarly. this process is endothermic as the added electron is repelled by the negatively charged ion, and energy needs to be available for the electron to be added.
electronegativity
- decreases down a group as bonding electrons are further from the nucleus, reduced attraction
- increases across a period due to the increase in nuclear charge, resulting in increased attraction between bonding electrons and the nucleus
electronegativity is the measure of an element’s ability to attract electrons towards itself in a covalent bond
- it is a measure of the attraction between the nucleus and its outer electrons - in this case bonding electrons
- this does not apply to noble gases as they generally do not form covalent bonds