What is a molecule?
Atoms joined into a neutral unit.
The smallest piece of water
Keep dividing a drop of water into smaller and smaller pieces.
Which is the smallest piece that still behaves like water?
What this lesson covers
The idea
A molecule is an electrically neutral entity of more than one atom, capable of independent existence and showing all the properties of the substance; it may contain atoms of one element or of different elements.
The smallest piece of water
Keep dividing a drop of water into smaller and smaller pieces.
Which is the smallest piece that still behaves like water?
- A group of atoms joined together, a water molecule
- A single hydrogen atom
- A single oxygen atom
Sort the particles
Sort each particle into the right group.
Neutral and independent
A molecule has more than one atom and can exist on its own. It can be made of the same element or of different elements. Helium atoms are stable, so helium stays as single atoms.
A molecule is an electrically neutral entity of more than one atom, capable of independent existence and showing all the properties of the substance. It may contain atoms of one element or of different elements.
Notes
A molecule is an electrically neutral entity of more than one atom, able to exist independently. Its atoms may be of one element or of different elements.
Check yourself
Which of these is a molecule of an element?
How many atoms are there in one molecule of water, H₂O?
Answer: 3
Two hydrogen atoms and one oxygen atom make 3 atoms.
Why does helium exist as single atoms and not as molecules?
Which description fits a molecule?
- HCl. HCl has atoms of two different elements, hydrogen and chlorine, so it is a molecule of a compound.
- O₂ — correct. Yes!
- H₂O. H₂O has atoms of two different elements, hydrogen and oxygen, so it is a molecule of a compound.
- Its atoms are too heavy to join. The book gives the reason that helium atoms are stable, not that they are heavy.
- Helium atoms are electrically charged. A stable helium atom is not charged. Molecules are electrically neutral, and so are helium atoms.
- Its atoms are stable — correct. Yes!
- An electrically neutral entity of more than one atom, able to exist on its own — correct. Yes!
- A single charged atom. A molecule has more than one atom and is electrically neutral.
- Any atom that has lost an electron. A molecule is electrically neutral and has more than one atom.