Atoms, elements, and compounds

Atomic structure

• Proton: Relative mass = 1; Relative charge = +1
• Neutron: Relative mass = 1; Relative charge = none
• Electron: Relative mass = negligible (0.00054); Relative charge = -1
• Proton number: the number of protons in the nucleus of an atom.
• Nucleon number: the total number of protons and neutrons in the nucleus of an atom.
• Elements in the Periodic Table are arranged in order of their proton number.
• Electrons in atoms are arranged in shells or energy levels. The first shell is nearest to the nucleus and can hold a maximum of 2 electrons. The shells after that can hold a maximum of 8 electrons, and fill up after the previous shell is filled.
• The number of outer shell (valence) electrons in an atom determines the chemical properties of that element.
• Noble gases have a full outer shell of electrons, meaning that they have a very stable structure and are very unreactive.

Isotopes

• Isotopes are atoms of the same element which have the same proton number but a different nucleon number.
• Isotopes have the same properties because they have the same number of electrons in their outer shell.
• There are two types of isotopes:
• Radioactive
• Non-radioactive
• A medical use for radioactive isotopes is in cancer treatment to kill cancer cells.
• An industrial use for radioactive isotopes is to check for leaks in oil and gas pipelines.
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Bonding: the structure of matter

• Elements contain only one type of atom and cannot be broken down to anything simpler.
• Compounds are substances containing two or more different types of atoms chemically bonded together. It is possible to separate them by chemical means.
• Mixtures are substances containing two or more elements or compounds that are not chemically bonded together. The substances in a mixture can easily be separated by physical means.
• Alloys are a mixture of a metal with other elements, e.g. brass.
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Ions and ionic bonds

• Ions are formed when atoms gain or lose electrons. Electrons are always transferred from a metal atom to a non-metal atom.
• The metal atom has a positive (+) charge as it has given up an electron, and the non-metal atom has a negative (-) charge as it has taken an electron away from the metal atom.
• When elements in Group 1 and 7 react, the atom of the Group 1 element loses the electron from its outer shell, This can be transferred to the outer shell of the atom of a Group 7 element.
• The lattice structure of ionic compounds is a regular arrangement of alternating positive and negative ions.
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• Bildunterschrift: Sodium and chlorine ionically bonding

Molecules and covalent bonds

• The formation of single covalent bonds in H2, Cl2, H20, CH4, NH3, and HCl is the sharing of pairs of electrons leading to a noble gas configuration.
• In complex covalent molecules, such as N2, C2H4, CH3OH and CO2, two or three pairs of electrons are shared between two atoms as double or triple bonds must be made to form a stable octet of electrons.
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• Bildunterschrift: Carbon dioxide - covalently bonded

Ionic v.s. covalent compound properties

• IONIC
• High melting and boiling points - strong attractive forces between the ions
• Soluble in water but insoluble in organic solvents
• Conduct electricity only when molten or in aqueous solution.
• COVALENT
• Low melting and boiling points - weak attractive forces between the ions
• Insoluble in water
• Do not conduct electricity

Macromolecules (1)

• DIAMOND
• A carbon atom forms 4 covalent bonds with other carbon atoms, forming a giant lattice.
• It is used for cutting tools, because as well as having a high melting and boiling point it is very hard.
• GRAPHITE
• Each carbon atom is joined to 3 other carbon atoms in hexagonal layers.
• It is used as a lubricant and a conductor as the bonds between the layers are weak, so they can slide over each other if a force is applied.
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Macromolecules (2)

• SILICON DIOXIDE
• Each silicon atom is bonded to 4 oxygen atoms, but each oxygen atom is only bonded to 2 silicon atoms.
• It is similar to diamond - tetrahedral shape, high melting and boiling point, and doesn't conduct electricity as it has no delocalized electrons - unlike graphite.
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Metallic bonding

• A lattice of positive ions in a 'sea of electrons'.
• This means that metals can conduct electricity because the delocalized electrons can move freely throughout the structure.
• It also means that they are malleable as if a force is applied, the layers can slide over each other.
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