Topic 3 – Covalent compounds and separation techniques • • • • • • •
COVALENT BONDS Most non-metal compounds are held together by pairs of electrons, which form covalent bonds The electrons used in covalent bonds come from the outermost shells of atoms When covalent bonds form, the sharing of electrons allows both atoms to have full outer shells (more stable)àformation of molecules Covalent bonding between atoms can be shown by dot-cross diagrams (often only outer electrons are shown because it is these that form covalent bonds)… Covalent bonds between atoms of the same (non-metal) element: E.g two hydrogen atoms (H) form one molecule of hydrogen gas (H2): (note: compounds are molecules containing at least two different elements) o Each hydrogen atom contributes one electron to the covalent bond (i.e electrons are shared)àboth hydrogen atoms have full outer shells o àstable H2 molecule is formed
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Note: in a dot-cross diagram there’s no difference between the electrons from different atoms…they’re shown as dots and crosses just to show which atom each electron is from Covalent bonds between atoms of different (non-metal) elements: E.g hydrogen chloride (HCl) – only outer electrons are shown:
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E.g2 water (H2O) – only outer electrons are shown:
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E.g3 methane (CH4) – only outer electrons are shown:
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Double bonds: Atoms can share more than one pair of electrons if this is needed for each atom to have a full outer shell… Two pairs of shared electrons form a double bond… o E.g O=O (O2) – only outer electrons are shown:
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