Build a Molecule

An atom is the smallest piece of an element you can have and still call it that element. In the middle sits a tiny nucleus packed with protons and neutrons, and around it buzzes a cloud of electrons.

• The number of protons decides which element it is: 1 proton is hydrogen, 6 is carbon, 8 is oxygen.
• Atoms are unbelievably small — about a million of them side by side would span the width of a human hair.
• The electrons on the outside are the part that does chemistry. The nucleus mostly stays out of it.

Atoms are happiest when their outer electron shell is full. Most atoms are a few electrons short, so they team up with neighbours to share. Those shared pairs are the bonds — the arms on the workbench.

• Hydrogen and chlorine each have 1 arm, oxygen has 2, nitrogen has 3, and carbon has 4.
• Sharing one pair of electrons is a single bond, two pairs is a double bond, three is a triple bond.
• A molecule works out when every arm has found a partner — no arms left dangling.

Not every bond is a friendly share. Sodium would rather hand its outer electron over completely, and chlorine is delighted to take it. Now sodium is positive, chlorine is negative, and opposite charges pull together.

• Sharing electrons is a covalent bond — water, methane and ammonia are all built this way.
• Giving an electron away makes an ionic bond — table salt is the classic example.
• Ionic compounds stack into crystals, which is why salt comes as little cubes you can see.

A formula is a recipe written in shorthand. The letters are the elements and the small number after each one counts how many of that atom are in the molecule.

• H₂O means 2 hydrogen atoms and 1 oxygen atom. No number means just one.
• CO₂ and CH₄ use the same carbon atom, but the partners change everything: one you breathe out, the other burns.
• In a reaction, atoms are never created or destroyed — they only get rearranged. 2H₂ + O₂ → 2H₂O uses up exactly the atoms it started with.