C2: Elements, compounds, mixtures

  1. Elements, compounds and mixtures

  • Element: substance made up of just one type of atom

  • Compounds: different types of atoms are chemically bonded

  • Mixtures: compounds, elements, molecules can mix without chemically bonding

  1. Atomic structure and the periodic table

  • The atom has a central nucleus containing neutrons and protons, surrounded by electrons in shells

  • Atomic number = number of protons in the nucleus

  • Nucleon number = protons + neutrons in the nucleus

  • Electronic configuration

    • group number = outer shell electrons

    • period number = number of shells

  • Noble gas (VIII) electronic configuration

    • Unreactive gases

    • Full outer shell (8 electrons)

    • Do not make chemical bonds with atoms of other elements

  1. Isotopes

  • Pure elements with atoms of different masses

    • Same number of protons & electrons, different number of neutrons.

    • Defined by their difference in mass number. Eg. carbon-12, carbon-13

  • Characteristics

    • Same elements → same properties (same number of electrons and electronic configuration)

  1. Chemical bonding

  • positive ions: cations

  • negative ions: anions

  • Ionic bonds: strong electrostatic attraction between oppositely charged ions

  • involves the outer electron shells

    • Molecular compounds: atoms bonded by shared electrons. Eg. water, methane,…

    • Ionic compounds: ions (charged atoms) are held together in a regular structure. Eg. Sodium chloride

  1. Ions and ionic bonds (metal)

  • Ionic compounds

    • Have a formation of positive and negative ions

    • Metal atoms lose their electrons to become positive ions

    • Non-metals gains electrons to become negative ions

    • Ions formed achieve stable electron arrangement of the nearest noble gas to them

    • Alkali metals + halogens → greater number of electrons that get transferred

  • Physical properties

    • Solid at room temperature

    • High m.p and b.p

    • Often soluble in water

    • Conduct electricity in water when liquid/dissolved

  • Reason:

    • Regular lattice structure (each ion is surrounded by opposite charge ions)

    • Ions attracted by strong electrostatic forces → large amounts of energy is needed to break them

    • Water is attracted to charged ions

    • Liquid/solution electrons are attracted to free ions moving about

  1. Simple molecules and covalent bonds (non-metal)

  • A pair/multiple pairs of electrons are shared between atoms

  • Simple molecular elements (diatomic molecules)

  • Held together by the forces of attraction between shared negative electrons and the nuclei involved

  • Main features:

    • formed by sharing a pair of electrons → noble gas configuration

    • each atom contributes an equal number of electrons to each bond

  • Group 7 halogens exist as diatomic particles (covalent molecules)

  • Hydrogen and halogens are held together by a single covalent bond.

    • Uses 2 electrons, one from each atom

  • Physical properties

    • Liquids/gas at room temp → made of simple molecules

    • low m.p and b.p → intermolecular forces are very weak

    • Do not conduct electricity→ no ions to carry a charge

  1. Giant covalent structures

  • Macromolecules: giant molecular crystals held together by strong covalent bonds

  • Structures

    • Diamond: 1 carbon atom is attached to 4 others

  • Graphite:

    • Does not conduct electricity

    • Arranged in flat layers of linked hexagons

  • Uses of diamond and graphite

  1. Metallic bonding

  • positive ions in a giant metallic lattice surrounded by a sea of delocalized electrons

  • Physical properties of metals:

    • Good conductors of electricity → delocalized electrons can carry a charge

    • Malleable → positive ions are arranged in layers that slide over each other