Study notes

Chemical Bonding & Structure

Ionic, covalent and metallic bonding, how each type of structure explains melting point and electrical conductivity, and the distinction between elements, compounds, mixtures and alloys.

Learn it step by step

Ionic bonding is electron transfer between a metal and a non-metal

In ionic bonding, one or more electrons transfer from a metal atom to a non-metal atom. The metal atom loses electrons to become a positively charged ion, and the non-metal atom gains those electrons to become a negatively charged ion. Both ions end up with a full outer shell matching the electronic configuration of the nearest noble gas.

Sodium chloride shows electron transfer in ionic bonding

A sodium atom (electronic configuration 2,8,1) transfers its single outer electron to a chlorine atom (electronic configuration 2,8,7). This leaves $N a^{+}$ with configuration 2,8, the same as neon, and $C l^{-}$ with configuration 2,8,8, the same as argon. The oppositely charged ions then attract each other strongly.

Ionic compounds form a giant lattice held by electrostatic attraction

The positive and negative ions in an ionic compound pack together in a regular, repeating three-dimensional pattern called a giant ionic lattice. Strong electrostatic forces of attraction act between every positive ion and its neighbouring negative ions throughout the whole lattice, not just between one pair of ions.

Covalent bonding is electron sharing between non-metals

In covalent bonding, two non-metal atoms each contribute one or more electrons to a shared pair, and that shared pair of electrons is what holds the two atoms together. Both atoms count the shared electrons toward their own outer shell, so each atom can reach a full, stable outer shell without fully gaining or losing electrons.

Simple molecules show single, double and multiple shared pairs

In hydrogen, $H_{2}$ , one shared pair joins the two atoms. In oxygen, $O_{2}$ , two shared pairs form a double bond. In water, $H_{2} O$ , the oxygen atom shares one pair with each of the two hydrogen atoms. In methane, $C H_{4}$ , the carbon atom shares one pair with each of the four hydrogen atoms. In carbon dioxide, $C O_{2}$ , the carbon atom forms a double bond with each oxygen atom.

Metallic bonding is a lattice of positive ions in a sea of electrons

In a metal, the outer shell electrons of every metal atom become delocalised, meaning they are free to move throughout the whole structure rather than staying attached to one atom. This leaves a regular lattice of positive metal ions held together by their attraction to this shared "sea" of delocalised electrons.

Structure explains why metals conduct, are malleable, and have high melting points

Metals conduct electricity because the delocalised electrons are free to move and carry charge through the structure. Metals are malleable because the layers of positive ions can slide over one another without breaking the metallic bond, since the sea of electrons simply flows around them. Metals generally have high melting points because strong electrostatic attraction between the positive ions and the delocalised electrons must be overcome throughout the whole lattice.

Structure explains differences in melting point and conductivity

Ionic compounds have high melting points because the electrostatic forces throughout the giant lattice are strong; molten or dissolved (aqueous) ionic compounds conduct electricity because their ions are then free to move and carry charge, but a solid ionic compound does not conduct, since its ions are fixed in the lattice and cannot move. Simple covalent molecules, such as water or carbon dioxide, have low melting points because only weak forces act between separate molecules, and they do not conduct electricity as they have no free ions or delocalised electrons. Giant covalent structures such as diamond, graphite, and silicon dioxide have very high melting points because a vast network of strong covalent bonds must be broken throughout the whole structure, not just weak forces between molecules.

Elements, compounds and mixtures are distinguished by composition and bonding

An element contains only one type of atom. A compound contains two or more different elements chemically bonded together in a fixed ratio, and it has different properties from the elements it is made of. A mixture contains two or more substances that are not chemically bonded and can, in principle, be separated by physical means, retaining their own individual properties.

An alloy is a mixture of a metal with another element

An alloy, such as brass (copper and zinc) or steel (iron and carbon), is a mixture of a metal with one or more other elements, usually other metals or carbon. Because it is a mixture rather than a compound, an alloy does not have a fixed, exact ratio of its components in the way a compound does.

Worked examples

Magnesium (electronic configuration 2,8,2) reacts with oxygen (electronic configuration 2,6) to form magnesium oxide,

M g O

. Deduce, step by step, whether magnesium oxide is ionic or covalent, and describe the electron transfer.

Magnesium is a metal and oxygen is a non-metal, and bonding between a metal and a non-metal is ionic, formed by electron transfer, not electron sharing.
Magnesium has 2 electrons in its outermost shell (2,8,2) and needs to lose both to reach the stable configuration 2,8, matching neon.
Oxygen has 6 electrons in its outermost shell (2,6) and needs to gain 2 electrons to reach the stable configuration 2,8, matching neon.
Each magnesium atom transfers its 2 outer electrons to one oxygen atom, forming $M g^{2 +}$ and $O^{2 -}$ ; the oppositely charged ions then attract electrostatically, confirming magnesium oxide is ionic, held together in a giant ionic lattice.

Two students argue about carbon dioxide,

C O_{2}

. Student A says it must be ionic because it contains oxygen, the same element found in ionic magnesium oxide. Student B says it is covalent. Determine who is correct and explain the actual bonding in

C O_{2}

Student A's reasoning is flawed: whether a compound is ionic or covalent depends on the TYPES of elements bonded together, not on which specific element is present.
Carbon and oxygen are both non-metals, and bonding between two non-metals is covalent, formed by sharing electrons, never by full electron transfer.
In $C O_{2}$ , the carbon atom shares two pairs of electrons with each oxygen atom, forming a double covalent bond on each side, so every atom ends up with a full outer shell through sharing.
Student B is correct: $C O_{2}$ is a simple covalent molecule, not ionic, so it exists as separate molecules with only weak forces between them, giving it a low melting point and no ability to conduct electricity.

Solid sodium chloride does not conduct electricity, but molten sodium chloride and aqueous sodium chloride solution both conduct electricity well. Explain this difference in terms of structure and bonding.

Sodium chloride is ionic, made of $N a^{+}$ and $C l^{-}$ ions held in a fixed, regular giant lattice by strong electrostatic attraction.
In the solid state, the ions are locked in fixed positions in the lattice and can only vibrate, they cannot move from place to place, so there are no mobile charge carriers and the solid does not conduct.
When sodium chloride is melted or dissolved in water, the rigid lattice breaks down and the $N a^{+}$ and $C l^{-}$ ions become free to move throughout the liquid.
These now-mobile ions can carry electric charge from place to place, which is exactly what allows both molten sodium chloride and aqueous sodium chloride solution to conduct electricity.

Diamond and iodine are both solids made of covalently bonded atoms/molecules, yet diamond has an extremely high melting point (over 3500 degrees Celsius) while iodine melts at a much lower temperature (around 114 degrees Celsius). Explain this difference.

Both diamond and iodine involve covalent bonds, so it is a common mistake to assume they should behave similarly, but the SCALE of the covalent structure is very different.
Diamond is a giant covalent structure: every carbon atom is covalently bonded to four neighbouring carbon atoms in a continuous network extending through the whole crystal, so melting diamond requires breaking a vast number of strong covalent bonds throughout the structure.
Iodine is a simple molecular substance made of separate, individual $I_{2}$ molecules; the atoms within each $I_{2}$ molecule are strongly covalently bonded to each other, but only weak forces act BETWEEN separate $I_{2}$ molecules.
Melting iodine only requires overcoming those weak forces between molecules, not breaking the strong covalent bond inside each molecule, so iodine melts at a far lower temperature than diamond, whose strong covalent bonds themselves must be broken to melt it.

Mind map

Chemical Bonding and Structure

Ionic bonding

metal + non-metal, electron transfer · forms ions with noble-gas configuration · giant lattice, electrostatic attraction

Covalent bonding

non-metal + non-metal, electron sharing · H2, O2, H2O, CH4, CO2

Metallic bonding

lattice of positive ions · sea of delocalised electrons · explains conductivity, malleability, high melting point

Structure and properties

ionic: high melting point, conducts molten/aqueous only · simple covalent: low melting point, no conduction · giant covalent (diamond, graphite, silica): very high melting point

Elements, compounds, mixtures

element: one type of atom · compound: fixed ratio, chemically bonded · mixture: not bonded, separable · alloy: metal mixed with another element

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