Classifying substances by structure and bonding

Chemistry · Types of substance

Which of the five types is it?

Take each substance and ask: what are its particles? Then place it in one of the five types.

8 of 8 still to sort.

Pick a substance, drop it in a type, and the board tells you the particle-level reason.

Predict, then check

Sodium chloride is made of ions. So what happens when you try to pass electricity through it?

Which of these describes how sodium chloride behaves?

What you need to know

• Every substance is one of five types: monatomic, simple covalent, giant covalent, ionic or metallic.
• Helium and the other group 0 elements are monatomic: single atoms, not bonded to anything, with a full outer shell.
• Simple covalent substances are molecules: a fixed number of two or more non-metal atoms covalently bonded, like H₂ and H₂O.
• Have a goA classmate announces, very confidently, that helium gas is made of He₂ molecules, just like hydrogen is H₂. What do you tell them?

  Helium is monatomic: single atoms that are not bonded to anything, so there are no molecules.

  A molecule needs two or more atoms covalently bonded together, and helium's atoms are not bonded to anything.

• Melting or boiling a simple covalent substance overcomes the weak forces between molecules, while the covalent bonds inside stay put.
• Simple covalent substances cannot conduct, because there are no free moving charge carriers: no ions and no delocalised electrons.
• Giant covalent structures, like diamond and silicon dioxide, have an undefined number of atoms all covalently bonded, so their melting points are very high.
• Have a goA solid's atoms are all covalently bonded together in one huge arrangement, with no fixed number of atoms. Simple covalent or giant covalent, and what is the clue?

  Giant covalent. The clue is that there is no fixed number of atoms.

  A molecule has a fixed number of atoms, so an undefined number of atoms all joined together means one giant structure, not separate molecules.

• Ionic substances are positive and negative ions held by strong electrostatic attraction in a giant lattice, like sodium chloride. They do not form molecules.
• Have a goSomeone writes 'a molecule of sodium chloride' in their notes. Is that a good description?

  No. Sodium chloride is a giant lattice of positive and negative ions, not molecules.

  Ionic substances do not form molecules. Their ions are held in all directions by electrostatic attraction in one giant lattice.

• Ionic compounds have high melting points and conduct only when molten or dissolved, because their ions are then free to move.
• A metal is a lattice of positive ions in a sea of delocalised electrons: high melting point, good conductor, and its layers can slide.

The big picture

Every substance falls into one of five structure types: monatomic, simple covalent, giant covalent, ionic or metallic. Work out the particles, then work out what has to be overcome or freed to move, and the melting point and conductivity follow.

⚠ Watch out

Thinking that melting or boiling a simple covalent substance breaks its covalent bonds. It does not. Only the weak forces between the molecules are overcome, and the molecules stay intact.

Memory hook

Particles first, properties second. Name the particles (atoms, molecules, a giant network, ions, or ions in a sea of electrons), then ask: what has to be overcome, and is anything free to move?

Check yourself

Sodium chloride and a metal both have high melting points. Which test on the solid separates them, and what does each result tell you about the particles?