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An atom is the smallest component of an element. An element is a substance that is made entirely of one type of atom (e.g. lithium, Li). Elements have chemical symbols which is a one or two letter code for an element.
A compound is a substance made up of two or more atoms which are chemically bonded together (e.g. water, H₂O). Compounds also have chemical formulas made up from the chemical symbols of the elements involved.
The atom consists of a central nucleus, which is tiny compared to the atom as a whole. The nucleus contains protons and neutrons. Electrons orbit around the nucleus in energy levels, also known as shells. The nuclear radius is around 1 × 10⁻¹⁴ m whilst the radius of the atom is about 1 × 10⁻¹⁰ m so the nucleus is less than 1/10,000 of the size of the atom.
The relative electronic charges of a proton, neutron and electron are +1, 0 and −1 respectively. In atoms the number of electrons is the same as the number of protons. Since they have opposite charges, this ensures that every atom is neutral.
The relative mass of a proton or neutron is 1 atomic units, while electron has a mass of approximately 1/1840 a. u. Therefore, the mass of an electron is often approximated to be 0, especially in comparison to the other two subatomic particles.
The number of electrons and protons in an atom is the same as the atomic number. The number of neutrons in an atom can be found by subtracting the atomic number from the mass number.
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