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  • Chemistry
    • Structure of the atom
      • The nuclear model
        • 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.
        • Atoms contain three sub-atomic particles called protons, neutrons and electrons.
      • Properties of sub-atomic particles
        • The number of electrons in an atom is always the same as the number of protons, so atoms are electrically neutral overall.
    • Isotopes
      • Atomic number and mass number
        • The proton number is shown below the chemical symbol, and the mass number is shown above. In chlorine the atomic number is 17 and the mass number is 35.
      • Isotopes are atoms of an element with the normal number of protons and electrons, but different numbers of neutrons. Isotopes have the same atomic number, but different mass numbers.
    • Relative atomic mass
      • Different atoms have different masses. Atoms have such a small mass it is more convenient to know their masses compared to each other. Carbon is taken as the standard atom and has a relative atomic mass (Ar) of 12.
      • Ar values of elements
        • These values tell you that a magnesium atom has twice the mass of a carbon atom, and 24 times more mass than a hydrogen atom. They also tell you that hydrogen atoms have 12 times less mass than a carbon atom. The Ar values also allow you to work out that three oxygen atoms have the same mass as two magnesium atoms.
      • The relative atomic mass of an element is a weighted average of the masses of the atoms of the isotopes - because if there is much more of one isotope then that will influence the average mass much more than the less abundant isotope will.
    • Formation of ionic compounds
      • When metals react with non-metals, electrons are transferred from the metal atoms to the non-metal atoms, forming ions. The resulting compound is called an ionic compound.
      • In all of these reactions, the metal atoms give electrons to the non-metal atoms. The metal atoms become positive ions and the non-metal atoms become negative ions.

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