Summary of electron counting

Most errors result either from placing too many electrons around an atom, e.g. CH5, or by placing the wrong charge on an atom, e.g. R3NH. 

The smallest unit of an element is the atom. Atoms are formed from protons, neutrons and electrons. The protons and neutrons together constitute the nucleus the electrons occupy orbitals, which are centred on the nucleus. The number of protons within the nucleus determines the element. This number is called the atomic number. The sum of the number of neutrons and protons is called the atomic mass number. 

Usually, there are approximately the same number of neutrons as there are protons. For a given element, atoms that contain different numbers of neutrons are called isotopes, and they have different physical properties. However, the chemical properties of different isotopes are the same. When referring to an element, it is usual to imply that the most stable isotope, or the naturally occurring mixture of isotopes, is meant unless otherwise stated. Usually, different isotopes are distinguished by 

The chemical properties of an element are determined by the element s electrical properties. In an atom, the number of electrons always equals the number of protons, and so every element in the atomic state is electrically neutral. If an element loses or gains one or more electrons then that element will 

Electrons in an atom are contained within atomic orbitals, each of which may only hold a maximum of two electrons. The principal quantum shell is indicated by a number, e g. 1, 2, 3, etc. Within each shell, there may exist a number of subshells s, p, d and f, depending upon which shell is being considered. There is only one s subshell, while there are three p subshells, which are degenerate pr, pv and pz Hund s rule states that if electrons are placed in degenerate orbitals, they 

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Table 1 Summary of electron-counting rules and structure relationship...

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