IGCSE Chemistry Atoms, elements and compounds notes

All matter is built from atoms, but not every material is the same kind of substance. An element is a substance made of only one type of atom — nothing simpler can be obtained from it by chemistry. A compound is formed when two or more elements react and their atoms become chemically joined in a fixed ratio. A mixture contains two or more substances that are simply mixed together and are not chemically bonded.

• One kind of atom

  Element

  Iron contains only iron atoms; it cannot be broken down into anything simpler by a chemical reaction.

• Chemically joined

  Compound

  In water, hydrogen and oxygen atoms are bonded in a fixed 2 : 1 ratio that is the same in every sample.

• Just mixed

  Mixture

  Air is a mixture of gases, and the proportions of nitrogen and oxygen can vary from place to place.

An atom has a very small, dense nucleus at its centre containing protons and neutrons. Around the nucleus, electrons are arranged in shells. Almost all of the mass of an atom is in the nucleus, but almost all of its volume is the space taken up by the electrons.

The proton number (or atomic number) is the number of protons in the nucleus, and it is what decides which element an atom is. The mass number (or nucleon number) is the total number of protons and neutrons in the nucleus. In a neutral atom the number of electrons equals the number of protons, so electron arrangements can be worked out directly from the proton number.

1. 1Find the number of electrons

   For a neutral atom this equals the proton number. For an ion, add or subtract the charge.

2. 2Fill the shells

   Put 2 electrons in the first shell, 8 in the second, 8 in the third — for the first twenty elements this always works.

3. 3Write it in order

   Write the shells from the nucleus outwards, for example sodium is 2,8,1 and magnesium is 2,8,2.

• The number of occupied shells equals the period number of the element.
• For Groups I–VII the number of outer-shell electrons equals the group number.
• Group VIII noble gases have full outer shells, which is why they are so unreactive.

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Because the proton number is unchanged, isotopes of an element react in exactly the same way (Supplement): they have the same number of electrons and the same electronic configuration, so they bond identically. Isotopes are written with the mass number as a superscript and the proton number as a subscript, such as ¹²₆C for carbon-12 or ³⁵₁₇Cl⁻ for a chloride ion.

Supplement: the relative atomic mass, Aᵣ, is a weighted average that takes account of how much of each isotope a sample contains. Multiply the mass of each isotope by its fractional abundance, then add the results. For chlorine, 75% is chlorine-35 and 25% is chlorine-37, so Aᵣ = (35 × 0.75) + (37 × 0.25) = 35.5. Because the isotopes are not equally abundant, the answer is usually not a whole number.

When a metal atom reacts with a non-metal atom, electrons are transferred. A metal atom loses its outer-shell electrons to form a positive ion, or cation. A non-metal atom gains electrons to complete its outer shell and forms a negative ion, or anion. Both ions now have the electronic configuration of a noble gas.

An ionic bond is the strong electrostatic attraction between oppositely charged ions. Dot-and-cross diagrams show it clearly: one dot or cross represents each electron, and the transfer from the metal to the non-metal is drawn so the outer shells become full, for example Na⁺ and Cl⁻ in sodium chloride.

Supplement: ionic compounds form a giant ionic lattice — a regular, repeating arrangement in which positive and negative ions alternate in every direction. Every ion is held by attractions to many oppositely charged neighbours, so melting or boiling means breaking a great many strong electrostatic attractions, not just one bond. Sodium chloride melts at about 800 °C for this reason.

A covalent bond is a shared pair of electrons between two atoms. By sharing, each atom gains the electrons it needs to reach the configuration of a noble gas. Covalent bonds form between non-metal atoms, and a group of atoms joined this way is a molecule.

Substances made of simple molecules have low melting and boiling points, because only weak forces between the separate molecules need to be overcome. The covalent bonds inside each molecule stay unbroken. They also do not conduct electricity, because their molecules carry no charge and there are no free ions or electrons.

In a giant covalent structure the covalent bonds do not stop at a small group of atoms — they continue through the whole solid. In diamond every carbon atom is joined to four others by strong covalent bonds in a rigid three-dimensional network, which makes diamond extremely hard, so it is used in cutting tools and drill tips. In graphite each carbon atom is joined to only three others, forming layers of hexagonal rings; the layers are held together weakly and can slide over one another, which is why graphite works as a lubricant. Each carbon atom also has one delocalised electron that is free to move along the layers, so graphite conducts electricity and is used as an electrode.

Supplement: silicon(IV) oxide, SiO₂, also has a giant covalent structure, with silicon and oxygen atoms joined by strong covalent bonds in a rigid network. This is very like diamond, and the two substances share similar properties: both are very hard, have very high melting points, and do not conduct electricity.

Supplement: a metal is a giant metallic lattice of positive ions packed closely together. The outer-shell electrons of each atom are not held by any one ion; they form a sea of delocalised electrons that is shared throughout the structure. Metallic bonding is the electrostatic attraction between the positive ions and this sea of delocalised electrons.

• Electrical conductivity: the delocalised electrons are free to move through the lattice and carry charge, so all metals conduct electricity.
• Malleability and ductility: the layers of positive ions can slide over one another without breaking the attraction to the electron sea, so metals can be hammered into sheets or drawn into wires.
• Thermal conductivity: the closely packed ions and mobile electrons pass on energy quickly, so metals conduct heat well.
• High melting points: many strong electrostatic attractions must be overcome, so most metals melt at high temperatures.