Salt, Sugar and Copper: Three Kinds of Bond
- Chemistry
- Ages 14–18 · Grades 9–12
- Hands-on experiment
What you’ll find out
- Ionic bonds form when electrons transfer from metal to non-metal atoms, making oppositely charged ions.
- Covalent bonds share pairs of electrons; forces between small molecules are weak.
- Structure explains properties such as melting point and electrical conductivity.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Giving and sharing electrons
Atoms join together, or bond, because it makes them more stable, usually by giving them a full outer shell of electrons. There are two main ways. When a metal meets a non-metal, electrons move across. Sodium has one outer electron that it holds loosely. Chlorine needs one more electron to fill its shell. So the electron moves from sodium to chlorine. Sodium becomes a positive ion and chlorine becomes a negative ion, and opposite charges attract. In salt, huge numbers of these ions stack up in a neat pattern called a lattice. When two non-metals meet, they share electrons instead. In water, oxygen shares a pair of electrons with each hydrogen atom. Each shared pair is a covalent bond. In metals, the outer electrons move freely among the metal atoms, like a sea of electrons holding them together.
Structure explains properties
The type of bonding explains how a material behaves. To melt salt, you have to pull apart a whole lattice of ions that attract each other strongly. So salt melts only at about 800 °C. To melt sugar, you only need to separate its molecules, which are held to each other weakly. So sugar melts at around 186 °C. Bonding also decides whether something conducts electricity. Electricity is moving charge, so something charged must be free to move. Solid salt does not conduct, because its ions are stuck in place. Dissolve it in water and the ions can move, so it conducts. Sugar never conducts, because its molecules have no charge. Copper conducts even when solid, because its sea of electrons can flow. Quartz, the mineral in sand, is one giant network of bonded atoms, so it melts only above 1,700 °C.
Curriculum links: NGSS HS-PS1-3 · NGSS HS-PS1-1