Electron affinity is the measure of energy released when an electron is added to an atom to create a negative ion in a gaseous. This property is measured in a gaseous state only due to the separation of the atoms. Electron affinity is measured in kj/mol much like ionisation energy and is the opposite of ionisation energy. This is an exothermic reaction that gives out energy in the form of heat. Electron affinities are measured in levels as it requires different amounts of energy to add electrons each time and depending on the position of the elements in the periodic table some elements prefer to gain electrons and some prefer to lose electrons.
For example when a metal element has electrons added to it, it requires a large energy amount due to its preference to lose electrons than to gain electrons. This is also due to the smaller Strong nuclear force as metals tend to be on the left hand side of the periodic table with fewer protons and a lower nuclear charge. Non metals in general have lower electron affinities as they are more likely to gain electrons due to their larger nuclear charge and preference to fill their electron configuration.
Most first electron affinities are negative as energy is released and when an electron is added to a neutral atom to create a negative ion this is an exothermic process. We can define the first electron affinity as the energy released when 1 mole of gaseous atoms are given 1 mole of electrons to form 1 mole of gaseous negative ions. Metals have lower electron affinities than non metals as it is easier for non metals to gain electrons to form a noble gas configuration. Often for non metals the second electron affinity value is positive and an endothermic process as energy is needed to gain a second electron as there is already a negative ion which will repel any electrons that are being aded to it.
Electron affinity decreases down a group because the electrons are placed in higher energy levels and have a lower attraction to the nucleus as it is further away from the nuclear charge in a further valence shell. It will also decrease as going down a group there are more shells and shielding reducing the nuclear attraction and also increasing repulsion of the electrons as there are more electrons. Fo example looking at group 1 the electron affinity of lithium is 60Kjmol but for caesium it is 46Kjmol this is due to the larger size of caesium, more shielding, increase in repulsion of electrons and also lower nuclear charge.
Across a period from left to right in the periodic table the electron affinity increases this is because from left to right in a period the same shielding is experienced but the number of protons in the nucleus increases increasing the nuclear charge of the nucleus on the outer electron.
One thing to remember is that electron affinity and electronegativity sound similar but they are different. Electronegativity is the measure of how much an atom will attract a pair of electrons in a bond. Where as, electron affinity is the energy required to add electrons to an atom to create a negative ion. Thy both rely on the same principle of having more protons and a higher nuclear charge to attract electrons more readily.
Electron affinity is a measure of the amount of energy in kj/mol required to add one mole of electrons to an atom to create one mole of negatively charged ions
An ion is the term used to describe an atom that has either lost or gained an electron so that it is charged
Gaseous is the term used to describe a substance in the gas phase
Ionisation energy is the amount of energy required to remove an electron from an atom creating a positvely charged ion
Exothermic is a term used to describe a chemical process where heat energy is given out from the making of chemical bonds. When excess chemical energy is released as heat energy making the surroundings warmer increasing temperature.
The electron is the smallest sub atomic particle that make up the atom. Has a negative charge and is located in shells that orbit the nucleus
The strong nuclear force is the attractive force between protons and neutrons that keeps them together in the nucleus
A proton is a positive particle that makes up the atom in the nucleus with a positive charge
A mole is the term used to describe the number of atoms in a substance. One mole is equal to 6.02x10^23 atoms
Endothermic is a term used to describe a chemical process where heat energy is taken in by chemicals and turned into chemical potential energy. Energy has to be put in to get the process/reaction to work an the temperature of the surroundings decreases
Repulsion is when two charges that are the same come into contact they prefer to be further away from each other so push each other away. For example when two electrons come close to each other the negative charges cause repulsion
Shielding is the effect of inner shell electrons close to the nucleus reducing the nuclear charge on the valence electron.
Nuclear charge is the attraction exerted by the nucleus on electrons due to the positive charge of the protons and negative charge of the electron