Atoms, Elements and Bonding

2.1 Elements, compounds and mixtures

Iron and sulfur. A mixture of iron filings and sulfur powder can be separated with a magnet (the iron is attracted), and it can be any proportion of each. If the mixture is heated, the elements react to form the compound iron(II) sulfide, which is a different grey solid that the magnet does not attract. This cannot be separated by physical means.

$\ce{Fe(s) + S(s) -> FeS(s)}$

2.2 Atomic structure and the Periodic Table

An atom has a very small, dense nucleus in the centre, containing protons and neutrons. The electrons move around the nucleus in shells (energy levels).

An atom is neutral: it has the same number of electrons as protons.

Electronic configuration

Electrons fill the shells nearest the nucleus first. The first shell holds up to 2 electrons, and the second and third shells hold up to 8 each, for the elements with proton number 1 to 20. The arrangement is written as numbers, for example sodium is $2,8,1$.

The link with the Periodic Table.

• For Groups I to VII, the number of outer-shell electrons equals the group number. Sodium ($2,8,1$) is in Group I, and chlorine ($2,8,7$) is in Group VII.
• The number of occupied shells equals the period number. Sodium has three shells, so it is in Period 3.
• The Group VIII noble gases have a full outer shell (helium has $2$, the others have $8$), which is why they are so unreactive.

For an ion, the electronic configuration is that of the atom with the extra electrons added or removed: $\text{Na}^+$ is $2,8$, and $\text{Cl}^-$ is $2,8,8$.

$15$ electrons fill the shells as $2,8,5$. There are $5$ outer electrons, so it is in Group V, and three shells, so Period 3.

2.3 Isotopes

Isotopes of an element have the same chemical properties, because they have the same number of electrons and therefore the same electronic configuration. Chemical reactions depend on the electrons.

Symbols. An atom or ion is written with the nucleon number at the top and the proton number at the bottom, for example ${}^{12}_{6}\text{C}$ and ${}^{35}_{17}\text{Cl}^{-}$.

Protons $= 17$. Neutrons $= 35 - 17 = 18$. The ion has gained one electron, so electrons $= 17 + 1 = 18$.

Relative atomic mass

The relative atomic mass, $A_\text{r}$, of an element is the average mass of its isotopes, taking account of how much of each there is, compared with $\frac{1}{12}$ of the mass of an atom of carbon-12.

$A_\text{r} = \frac{\sum (\text{isotope mass} \times \text{abundance})}{100}$

$A_\text{r} = \frac{(35 \times 75) + (37 \times 25)}{100} = \frac{2625 + 925}{100} = 35.5$

2.4 Ions and ionic bonds

An atom becomes an ion when it gains or loses electrons so that it has a full outer shell, the electronic configuration of a noble gas.

• Metal atoms lose electrons to form positive ions (cations): $\ce{Na -> Na+ + e-}$.
• Non-metal atoms gain electrons to form negative ions (anions): $\ce{Cl + e- -> Cl-}$.

The charge on the ion depends on the group:

Ionic bonds form between a metal and a non-metal. The metal atoms transfer electrons to the non-metal atoms.

Sodium chloride. A sodium atom ($2,8,1$) gives its one outer electron to a chlorine atom ($2,8,7$). Both ions now have a full outer shell: $\text{Na}^+$ is $2,8$ and $\text{Cl}^-$ is $2,8,8$.

In a dot-and-cross diagram for an ionic compound, show only the outer shell of each ion, put the ions in square brackets, and write the charge at the top right. For magnesium oxide, magnesium transfers two electrons to oxygen, giving $\text{Mg}^{2+}$ and $\text{O}^{2-}$. For calcium chloride, one calcium atom gives one electron to each of two chlorine atoms, giving $\text{Ca}^{2+}$ and two $\text{Cl}^-$.

The giant ionic lattice

An ionic compound is a giant lattice: a regular, repeating arrangement of alternating positive and negative ions, held together by strong ionic bonds in every direction.

Properties of ionic compounds

2.5 Simple molecules and covalent bonds

Covalent bonds form between non-metal atoms. A molecule is a group of atoms held together by covalent bonds. A single bond is one shared pair, a double bond is two shared pairs, and a triple bond is three shared pairs.

In a dot-and-cross diagram, the outer electrons of one atom are drawn as dots and the other atom's as crosses, and the overlap shows the shared pairs. Outer electrons that are not shared are shown as lone pairs.

Hydrogen ($1$ electron) and chlorine ($2,8,7$) share one pair. The shared pair is drawn in the overlap of the two outer shells, and chlorine has three lone pairs (the other $6$ outer electrons). Both atoms now have a noble gas configuration: hydrogen has $2$ and chlorine has $8$.

Properties of simple molecular substances

2.6 Giant covalent structures

Some covalent substances form a giant covalent structure (macromolecule): a huge network of atoms held together by strong covalent bonds. The three to know are diamond, graphite and silicon(IV) oxide.

Uses related to structure.

• Diamond is used in cutting tools (drill bits and saws) because it is extremely hard.
• Graphite is used as a lubricant because the layers can slide over each other easily, as the forces between them are weak.
• Graphite is used as an electrode (in electrolysis) because it conducts electricity and is unreactive.

Diamond and silicon(IV) oxide have similar properties (hard, very high melting points, do not conduct) because both are giant covalent structures with each atom held by strong covalent bonds to the same number of neighbours.

2.7 Metallic bonding

The outer electrons of each metal atom leave the atom and move freely through the whole structure. The positive ions are left in a regular lattice.

Comparing the structures