Atoms and Molecules

Why it matters

Zoom into a grain of salt until it vanishes, then rebuild it from invisible building blocks. Matter is not continuous; it is a collection of tiny particles.

Predict first

If you keep breaking a piece of chalk into smaller and smaller pieces, what happens eventually?

There is a limit to how small a piece of a substance can be while keeping its identity: a single atom (for elements), a molecule (for molecular substances), or a formula unit / ions in a lattice (for substances like salt and chalk).

• It becomes a smooth, continuous powder with no gaps
• You reach a point where the pieces are individual atoms or molecules — correct
• It disappears completely into nothingness

What you do

Oxygen atoms never wander alone. Each O atom needs 2 more electrons — share TWO pairs so they snap together into an O₂ molecule with a double bond.

Check yourself

Which statement best describes the relationship between atoms and molecules?

Atoms are the smallest units of an element. Molecules are formed when two or more atoms bond together.

Why does a drop of ink spread in water?

Diffusion occurs because particles are in constant motion, moving from areas of higher concentration to lower concentration.

• Atoms are made of molecules
• Molecules are groups of atoms bonded together — correct
• Atoms and molecules are the same thing
• The water pushes the ink down
• The particles of ink and water move and mix — correct
• The ink dissolves because it is heavier