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All substances are made from atoms. Each atom is made of a nucleus - containing protons and neutrons - surrounded by electrons.The atomic number is the number of protons in an atom. The elements are arranged in the periodic table in ascending order of atomic number.

The mass number of an atom is the total of protons plus neutrons. Atoms of the same element with different numbers of neutrons (and hence different mass numbers) are called isotopes of that element. Every atom is made of a nucleus consisting of protons and neutrons. The nucleus is surrounded by electrons. Protons and electrons are oppositely charged. Neutrons have no charge. This means the nucleus of an atom is always positively charged.An atom has a neutral overall charge because it has the same number of electrons as protons.Protons and neutrons have the same mass. Electrons have such a small mass that this can usually be taken as zero.

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