Atomic Structure and Bonding I

Everything you can touch is built from atoms — fantastically tiny building blocks (about 10⁻¹⁰ m across). An element is a substance made of just one kind of atom. Hydrogen contains only hydrogen atoms; gold contains only gold atoms.

There are ~118 known elements, arranged in the Periodic Table. Each has its own one or two-letter symbol: H for hydrogen, O for oxygen, Na for sodium (from Latin natrium), Au for gold (aurum).

Atoms of the same element have the same number of protons in their nucleus. That's what makes them THE element.

A compound is formed when atoms of two or more different elements join together by chemical bonds. The properties of a compound are usually completely different from the elements it's made from:

• Water (H₂O) = hydrogen (a flammable gas) + oxygen (gas that supports burning). The compound: liquid that puts fires out.
• Sodium chloride (NaCl) = sodium (soft, reactive metal) + chlorine (poisonous green gas). The compound: table salt.

A chemical formula tells you the ratio of atoms. H₂O has 2 hydrogens and 1 oxygen per molecule. CO₂ has 1 carbon and 2 oxygens. CaCO₃ (limestone) has 1 calcium, 1 carbon and 3 oxygens.

A mixture is two or more substances together but NOT chemically bonded. Each substance keeps its own properties, and you can usually separate them by physical means (filtering, evaporation, distillation, chromatography).

• Sea water = water + dissolved salts → evaporation separates them.
• Air = N₂ + O₂ + CO₂ + Ar + ... → fractional distillation of liquid air.
• Iron filings + sulfur (before any heating) → a magnet picks out the iron.

Once you HEAT iron filings with sulfur, however, they react chemically to form iron sulfide (FeS) — a COMPOUND, no longer a mixture. The properties change: the magnet no longer picks out the iron.