Atoms, Elements and Compounds: What Stuff Is Made Of
Tiny Bits, Three Ways to Stick ⚛️
Introduction
1. Introduction
Okay, this is the foundation chapter. Get this one solid and half of chemistry stops being scary. It's basically two questions: what is an atom made of, and how do atoms stick together?
An atom = a tiny nucleus (protons + neutrons) with electrons buzzing around in shells. And atoms join in exactly three ways: they give/take electrons (ionic), they share them (covalent), or they pool them (metallic). The exam's favourite move: use the structure to explain a property (melting point, conducting, hardness). This is your fast refresh, not the full textbook: one key idea per section, one worked example, and the exact thing the marker wants. Let's go. ⚛️
An atom = a tiny nucleus (protons + neutrons) with electrons buzzing around in shells. And atoms join in exactly three ways: they give/take electrons (ionic), they share them (covalent), or they pool them (metallic). The exam's favourite move: use the structure to explain a property (melting point, conducting, hardness). This is your fast refresh, not the full textbook: one key idea per section, one worked example, and the exact thing the marker wants. Let's go. ⚛️
2. Elements, Compounds and Mixtures
An element is one type of atom (iron, oxygen). A compound is different elements chemically joined in a fixed ratio, with brand-new properties (water, CuO). A mixture is stuff that's not joined, in any ratio, that you can split with physical tricks (filtering, a magnet) — like air. The magic question is always: are the bits chemically joined? Joined = compound. Not joined = mixture.
Key idea🔑 Key idea: The deciding word is "chemically". Chemically joined → compound. Separated physically → mixture. One type of atom → element. 🧩
Worked example
Worked Example: Spot the Type
Element, Compound or Mixture? 🔍
True or false: (a) in an element all atoms are the same type; (b) a compound has different atoms chemically joined; (c) a mixture can be separated physically.
- 1Element = one type of atom ✓. Compound = joined chemically ✓. Mixture = not joined, split physically ✓.
- 2All three are correct. The trap options swap clauses (like saying a compound splits physically — nope).
Underline "chemically" and the answer pops out. ✅
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3. Atomic Structure
An atom = protons (charge +1, mass 1) and neutrons (0, mass 1) in the nucleus, plus electrons (−1, relative mass , almost nothing) in shells. Two numbers run everything: proton number (= protons = electrons if neutral) and nucleon number (= protons + neutrons). So neutrons = . Electrons fill shells 2, 8, 8, and that tells you the Periodic Table spot: period = number of shells, group = number of outer electrons.
Key idea🔑 Key idea: protons = ; electrons = (if neutral); neutrons = . The only sum you do is the neutrons. 🔢
Worked example
Worked Example: Count the Particles
Protons, Neutrons, Electrons 🧮
A zinc atom has proton number 30 and nucleon number 65. How many protons, electrons and neutrons?
- 1Protons = 30. Electrons = 30 (neutral).
- 2Neutrons = .
So 30 protons, 30 electrons, 35 neutrons. Only the neutrons need a subtraction. ✅
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4. Isotopes
Isotopes are atoms of the same element (same protons) with different numbers of neutrons — like and . Because they have the same electrons, they have identical chemical properties (reactions are all about electrons). And because an element is a mix of isotopes, its relative atomic mass () is a weighted average, which is why values aren't whole numbers.
Key idea🔑 Key idea: Isotopes = same protons, different neutrons, same chemistry. . ⚖️
Worked example
Worked Example: Relative Atomic Mass
Weighted Average ➗
Potassium is 90% mass 39 and 10% mass 41. Find to one decimal place.
- 1and . Add: .
- 2Divide by 100: .
So 39.2 — close to 39 because most atoms are the lighter isotope. ✅
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5. Working Backwards
Sometimes they give you the and ask for the percentages. Same formula, just solved the other way: let one isotope's fraction be , the other is , set the weighted average equal to the , and solve. The answer always sits closer to whichever isotope is more common, so use that as a sanity check.
Key idea🔑 Key idea: Set and solve for . The bigger percentage is the isotope nearer the . 🔙
Worked example
Worked Example: Find the Percentages
Copper-63 vs Copper-65 🪙
Copper has copper-63 and copper-65, and . Find the abundances.
- 1Let copper-63 be : , so .
- 2, so .
So 75% copper-63 and 25% copper-65. (Check: gives 63.5 ✓.) ✅
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6. Ions and Ionic Bonds
An ion is a charged atom that lost or gained electrons to get a full outer shell. Metals lose → positive ions; non-metals gain → negative ions. Ionic bonding is the strong attraction between opposite charges, and the ions pack into a giant lattice. In a dot-and-cross diagram, use dots for one atom's electrons and crosses for the other's, then put each ion in square brackets with its charge. For a Group I metal with a Group VII non-metal, like sodium chloride, sodium gives its one outer electron to chlorine: with an empty outer shell and with 8 (7 crosses + 1 dot). That's why ionic compounds have high melting points (strong forces everywhere) and conduct only when molten or dissolved (the ions are then free to move).
![A simple dot-and-cross diagram titled "Ionic bonding in magnesium chloride". A magnesium atom (two dots in its outer shell) with an arrow to each of two chlorine atoms (seven crosses each), giving "[Mg]²⁺" and two "[Cl]⁻". Caption: "Mg gives one electron to each Cl → MgCl₂". Clean black-on-white line diagram, outer electrons only.](https://firebasestorage.googleapis.com/v0/b/practice-book.appspot.com/o/chapter-notes-images%2FlPoK0EPRymmvqQ1hUos8%2F1782447644466.png?alt=media&token=9aae764e-7006-4be8-8cd6-5f5e74fd2bba)
Key idea🔑 Key idea: Metals lose electrons (+ ions), non-metals gain (− ions). Ionic = strong attraction in a giant lattice. Conducts only when molten (ions free to move) — never say "free electrons" for ionic. ➕➖
Worked example
Worked Example: Make Magnesium Chloride
Transfer and Balance 🔋
Magnesium (2,8,2) reacts with chlorine. Describe the electron transfer and give the formula.
- 1Magnesium gives away its 2 outer electrons → . Each chlorine needs 1 → .
- 2So one Mg gives one electron to two chlorine atoms. Charges balance → .
Don't write "MgCl" — chlorine only needs one electron, so you need two of them. ✅
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7. Same Electron Arrangement
Here's a classic Extended twist: different particles can have the exact same electron arrangement. An atom, a positive ion and a negative ion can all end up as, say, 2,8,8 (18 electrons) — they just have different numbers of protons, which keeps them different species. To find matches, just count the electrons and look for anything with that total.
Key idea🔑 Key idea: Same electron configuration ≠ same element. Count the electrons; the proton number is what actually makes them different. 👯
Worked example
Worked Example: Who Else Is 2,8,8?
Match the Configuration 🎯
is 2,8,8 (18 electrons). Which atom and which negative ion are also 2,8,8?
- 118 electrons as an atom → proton number 18 → argon (Ar).
- 218 electrons as a negative ion → chlorine (2,8,7) gains 1 → .
So argon and . Same 2,8,8, different protons. ✅
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8. Covalent Bonds and Simple Molecules
A covalent bond is a shared pair of electrons between two non-metals (they both want electrons, so they share). One pair = single bond, two = double, three = triple. In a dot-and-cross diagram each bond is one dot and one cross in the overlap, and every atom ends with 8 outer electrons (hydrogen 2). Learn , , HCl, , and , plus the Extended ones with multiple bonds or lone pairs: (O=O), (N≡N), (O=C=O), (one C=C) and (2 lone pairs on O). Simple molecular substances are small separate molecules: strong bonds inside each molecule, but weak forces between molecules. So they have low melting/boiling points (you only break the weak forces) and don't conduct (neutral molecules, no free charges).
Key idea🔑 Key idea: Covalent = shared pair. Simple molecular = low melting point (weak forces between molecules, not the strong bonds inside) and no conducting. 🤝
Worked example
Worked Example: Count the Bonds
Ethanol's 8 🔗
Ethanol is . How many covalent bonds in one molecule?
- 1C–H bonds: 3 + 2 = 5.
- 2Plus C–C (1), C–O (1), O–H (1): .
So 8 bonds. Count the lines you'd draw — and don't forget the O–H. ✅
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9. Giant Covalent Structures
Some covalent substances aren't little molecules. They're giant networks of atoms all joined by strong covalent bonds: diamond, graphite and silicon(IV) oxide. To melt them you break loads of strong bonds, so they all have very high melting points. Diamond (each C bonded to 4) is very hard, so it's used in cutting tools, and doesn't conduct. Graphite (each C bonded to 3, in layers held by weak forces) is soft and slippery, so it's a lubricant, and it conducts (one delocalised electron per carbon), so it's used for electrodes. is diamond-like: very hard, very high melting point, no conducting.
Key idea🔑 Key idea: Giant covalent = strong-bond network → very high melting point. Diamond and are very hard and don't conduct (4 bonds each). Graphite is soft (layers slide) and conducts (3 bonds, spare electron). Uses: diamond cutting tools; graphite lubricant and electrodes. 💎
Worked example
Worked Example: Diamond vs Graphite
Why Only One Conducts ⚡
How many carbons does each carbon bond to in graphite and diamond? Why does graphite conduct but diamond not?
- 1Graphite: each C bonds to 3 → one spare electron is free → conducts.
- 2Diamond: each C bonds to 4 → all electrons used → none free → doesn't conduct.
Graphite 3 (conducts), diamond 4 (doesn't). ✅
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10. Metallic Bonding
A metal = positive metal ions sitting in a "sea" of delocalised (free) electrons. Metallic bonding is the strong attraction between the ions and that electron sea. This explains what metals do: they conduct (free electrons move, even when solid), they're malleable (hammered into sheets) and ductile (drawn into wires) because layers of ions slide while the electron sea still holds them together, and most have high melting points because the attraction is strong. (How alloys change this sliding is covered in the Metals chapter.)
Key idea🔑 Key idea: Metal = positive ions + sea of free electrons. Conducts by free electrons (even solid); malleable and ductile because layers of ions slide and the electron sea holds them. 🌊
Worked example
Worked Example: Why Copper Conducts
Free Electrons 🔌
Describe the bonding in copper and explain why solid copper conducts.
- 1Copper = positive metal ions in a sea of delocalised electrons, held by strong attraction between them.
- 2The free electrons move, carrying charge — even when solid.
Metal = free electrons carry charge (not ions). ✅
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