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Transcript of Trend Characteristics
Electron affinity is the change in energy of a neutral atom which is in the gaseous phase when an electron is added to a from a negative ion. The energy is in (kj/mole). Its the ability of an atom to accept an electron.
There are two electron affinities. The first electron affinity involves the energy released being released when 1 mole of of gaseous atoms gain an electron to form 1 mole of gaseous -1 ions.
The second electron affinity involves the energy that is required to produce 1 mole of gaseous -2 ions by adding an electron to each mole of of gaseous -1 ions.
First Electron Affinity. It is negative as energy is being released.
X(g)+e − →X − (g) (1)
Second Electron Affinity. It is positive because energy is being gained.
X − (g)+e − →X 2− (g) (2)
When an electron is added to a neutral atom, energy is released; therefore the first electron affinities are negative: negative energy because energy is released. However, when an electron is added to a negative ion, more energy is required, therefore, more energy is released to add the electron to the ion. This additional boost of energy is required because the negative ion must force the electron to go into its electron orbital. The resulting second electron affinity is positive because more energy is needed than is gained.
Electron affinity is increased moving from left to right across the periods and moving upwards in the groups of the periodic table. This is due to the energy levels getting closer to the nucleus therefore creating a stronger attraction between the nucleus and the electrons. Electron affinity decreases as you move from right to left across the periods and when moving down the groups of the periodic table. This is because the electrons are in a higher energy level far from the nucleus, creating a weak attraction between the electrons and the nucleus.
The electron affinity of metals is lower than of non-metals. This is because metals lose electrons and do so by absorbing energy. Whereas non-metals gain electrons by releasing energy.
When an electron is added to a metal element, energy is needed to gain that electron, which results in an endothermic reaction – a reaction that absorbs energy. It is much easier for metals to lose a valence electron to become a cation than it is to gain a new electron, and because of this, electron affinities of metals are low, and decrease down their column.
When an electron is added to a non-metal element, the resultant energy change is usually negative because energy is given off to form an anion; the reaction that takes place is an exothermic reaction. Non-metals have a much greater electron affinity than metals do; this is so for two main reasons: non-metals have more valence electrons than metals do, so it is far easier to gain electrons than it is to lose them, and, the valence shell of a non-metal is closer to the nucleus, making it harder to remove an electron but easier to attract electrons from other elements. This trend can be viewed as in increase as you move up in period, and as a decrease as you move down a group.