Topic 3.2: Periodic Trends Flashcards
Definition of atomic radius
a) Distance from the nucleus to the outermost electron
b) Half the internuclear distance in a molecule
Definition of ionic radius
Distance from the nucleus to the outermost electron in an ion
Definition of first ionization energy
Energy required to remove one mole of electrons from one mole of gaseous atoms
Definition of electron affinity
Energy change when one mole of electrons is added to one mole of gaseous atoms
Definition of electronegativityor
A measure of the attraction an atom has for a shared pair of electrons in a covalent bond
Definition of periodicity
Repeating trends of physical and chemical properties in elements
Why do elements in the same group react the same way?
Since reactions are determined by the valence electrons and elements in the same group posess the same number of valence electrons
Definition of nuclear charge
Attraction exerted by the nucleus on electrons
Trend of nuclear charge between successive elements
Nuclear charge is given by the atomic number and increases by one between successive elements
Why do outer electrons do not experience the full attraction of the nuclear charge?
Since they are shielded from the nucleus and repelled by inner electrons
What is the effective nuclear charge?
Nuclear charge experienced by an atom’s valence electrons
Trends in the effective nuclear charge across a period and down a group
It increases with the group number but remains approximately the same down a group
Explanation of the trends in atomic radius down a group
It increases
a) Number of electron shells increase and the shielding effect increases, counteracting any effects due to nuclear charge
Explanation of the trends in atomic radius across a period
It decreases
a) Electrons are added to the same main energy level and the effective nuclear charge increases due to no significant change in shielding.
Explanation of the difference between the atomic radius of cations and atoms
Cations are smaller than their parent atoms.
a) The positive nucleus remains the same with the same attractive force, but now pulling on fewer electrons
b) It involves the loss of outer shell
Explanation of the difference between the atomic radius of anions and atoms
Anions are larger that their parent atoms
a) The slightly increased electron repulsion between the electrons in the outer main energy level causes the electrons to move further apart and increases the radius of the outer shell
Explanation of the trends of cations’ ionic radius across a period
It decreases across a period
a) The number of protons in the nucleus increases but the number of electrons remain the same
b) The increased attraction between the nucleus and the electrons pulls the outer shell closer to the nucleus
Explanation of the trends of anions’ ionic radius across a period
It decreases across a period
a) The number of protons in the nucleus increases but the number of electrons remains the same
b) The increased attraction between the nucleus and the electrons pulls the outer shell closer to the nucleus
Explanation of the trends of ionization energy across a period
Ionization energy generally increase across a period
a) The increase in nuclear charge increases the attraction between outer electrons in the same energy level
b) Nucleus attracts the electrons more strongly and is more difficult to remove them
Explanation of the trends of ionization energy down a group
a) The electron being removed is further from the nucleus and less attracted by the nucleus
b) It gets easier to remove valence e- as atomic radius increases down a group
What are the regular discontinuities in the trends ionization energy across a period? (2)
a) Lower ionization energy in elements from Group 13 than 2
b) Lower ionization energy in elements from Group 16 than 15
Explanation of the lower ionization energy in elements from Group 13 than 2
a) The electron removed when the Group 13 elements are ionized is a p electron.
b) The electron removed when the Group 2 elements are ionized is an s electron.
c) Electrons in p orbitals are of higher energy and further away from the nucleus than s electrons, so they are easier to remove than electrons in an s orbital.
Explanation of the lower ionization energy in elements from Group 16 than 15
a) Group 15 elements have the configuration s2p1p1p1
b) Group 16 elements have the configuration s2p2p1p1
c) For Group 16 elements, the electron is removed from a doubly occupied 2p orbital. An electron in a doubly occupied orbital is repelled by its partner more strongly, so it is easier to remove than an electron in a half-filled orbital.
Explanation of trends across first, second, and third electron affinity
a) First electron affinity is exothermic since added electrons are attracted to the positively charged nucleus.
b) Second and third electron affinities are endothermic as the added electron is repelled by the negatively charged ion, so energy needs to be available for this to occur.