1Summary: Chapter 3© Cambridge University Press IGCSE Chemistry

Chapter 3SummaryYou should know that:

The Periodic Table lists the elements of the Universe in order of increasing atomic number.!The Periodic Table has a structure that provides useful information and the elements are grouped so that certain patterns of !properties repeat themselves.The elements are broadly divided into metals and non-metals.!The metals have certain characteristic properties, the most important being that they conduct electricity and are malleable and !ductile.The Periodic Table is divided into vertical Groups and horizontal Periods.!Certain groups of elements have specifi c names, for example the alkali metals, and show ‘family’ properties.!There is a trend from metallic to non-metallic properties as you go across the Periodic Table.!The halogens are a group of reactive non-metals – Group VII in the Periodic Table.!The halogens all exist as diatomic molecules (Cl! 2, for example).Their physical properties change as you go down the Group – their melting points and boiling points increase, and the intensity of !their colour deepens.Their chemical reactivity decreases down the Group – the more reactive halogens will displace the lower halogens from salts.!The structures of all substances are made up of either atoms, ions or molecules.!There are three main types of chemical bonding that hold these structures together – metallic, ionic and covalent bonding.!Covalent bonding occurs in some elements and non-metallic compounds, and involves the ‘sharing’ of electrons between atoms !to form stable molecules.Two types of covalent structure can be formed – simple molecules and giant molecular (macromolecular) structures.!Compounds of a metal plus a non-metal involve ionic bonding in which positive and negative ions are held together by strong !electrostatic forces.The physical properties of a substance are related to the type of bonding present.!Each chemical compound has a particular name and can also be represented by a chemical formula that summarises the atoms !or ions present.There is a link between the physical properties of a solid substance and the microscopic structure of its atoms, ions or molecules.!The characteristic properties of metals can be adapted to suit a specifi c purpose by alloying.!Alloys can be designed, for example, for strength (steel and duralumin), resistance to corrosion (stainless steel), or low melting !point (solder).Giant molecular structures linked by covalent bonds can be formed by some elements; carbon (diamond and graphite) and !silicon, for example.Diamond and graphite are two different forms of carbon, with different structures and distinctly different physical properties.!There is a close relationship between the electron arrangement of an element and its position in the Periodic Table.!Metallic bonding involves the closely packed atoms losing their outer electrons into a mobile ‘sea of free electrons’.!Understanding the closely packed structure of metals can explain the characteristic properties of metals and how one metal can !strengthen another when the two form an alloy.Ionic compounds are solid at room temperature and form crystals where the positive and negative ions are next to each other in !a framework (lattice).Ionic substances are often soluble in water because water molecules can break down the structure of the ionic crystal.!Giant molecular structures linked by covalent bonds can be formed by some elements and compounds.!Other covalent substances are simple molecular substances; these include elements such as oxygen and chlorine, and !compounds such as water.

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