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Electron affinity is the measure of the energy released when a neutral atom absorbs an electron. Absorbing an additional electron requires energy because a negatively charged particle is not naturally attracting to a neutral atom. Elements in groups 6 and 7 of the periodic table are the most likely to attract one or two additional electrons.
In order to attract an additional electron into orbit around its nucleus, an atom must release energy. The measure of this energy is written as a negative number because the atom gives off energy and thus has a net of energy loss. Atoms that give off less energy when attracting an additional electron are said to have a lower electron affinity and are more likely to lose extra electrons.
The measure of electron affinity is lower in atoms with a greater molecular weight. Part of the reason for this is that heavier atoms naturally contain more electrons to balance with the number of protons in the nucleus. With many electrons orbiting an atom’s nucleus, a free electron has a greater chance of being repelled from the atom.
Extra electrons are pulled into the outermost electron orbital in an atom. The greater distance of the outer orbital in atoms with a greater molecular weight does not have an effect on an atom’s ability to attract these electrons, though atoms of greater molecular weight have a lower measure of electron affinity. All atoms in group 7 give off an attraction of +7 regardless of the number of electrons already in orbit. Likewise, all atoms in group 6 give off an attraction of +6. This is because an electron is attracted by the number of protons in the nucleus minus the number of electrons in all lower orbitals.
Oxygen and sulfur are capable of attracting two additional electrons into their orbits. This is unusual because a negatively charged atom requires a great deal of energy to attract an additional negative particle. These elements have a high electron affinity and are the only two elements known to commonly exist with a charge of -2.
An atom is comprised of positively and negatively charged particles as well as particles that have no charge. The nucleus of an atom contains protons, which have a positive charge, and neutrons, which have no charge. The number of protons in an atom is equal to that atom’s atomic number, which means that every atom of the same type contains the same number of protons. Individual atoms may have more or less neutrons or electrons than one another, though most atoms maintain a balance of positive and negative particles.