Chemistry Atom

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Structure of the atom

The nuclear model

Atoms contain three sub-atomic particles called protonsneutronsand electrons.

The protons and neutrons are found in the nucleus at the centre of the atom. The nucleus is very much smaller than the atom as a whole. The electrons are arranged in shells around the nucleus.

Electrons orbiting a central nucleus. (

Properties of sub-atomic particles

ParticleRelative massRelative charge Proton 1 +1 Neutron 1 0 Electron Equation: frac{1}{1836} ( -1

The number of electrons in an atom is always the same as the number of protons, so atoms are electrically neutral overall.

Atoms can lose or gain electrons. When they do, they form charged particles called ions:

  • if an atom loses one or more electrons, it becomes a positively charged ion
  • if an atom gains one or more electrons, it becomes a negatively charged ion


    Atomic number and mass number

    The number of protons in the nucleus of an atom is called its atomic number:

    • the atoms of a particular element all have the same number…


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