Atoms, Elements and Compounds: What Stuff Is Made Of
Tiny Bits, Two 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 at Core, atoms join in two ways: they give/take electrons (ionic) or they share them (covalent). 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 at Core, atoms join in two ways: they give/take electrons (ionic) or they share them (covalent). 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? 🔍
Which is a mixture of exactly one element and one compound: (a) oxygen dissolved in water, or (b) glucose dissolved in water?
- 1Label each part. Oxygen () = element. Water () = compound. Glucose = compound.
- 2(a) = element + compound ✓. (b) = compound + compound ✗.
So it's (a). "Dissolved in water" always means a mixture, just name each thing. ✅
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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, basically no mass) 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
Krypton's Protons, Neutrons, Electrons 🧮
A krypton atom has nucleon number 84 and proton number 36. How many electrons and neutrons?
- 1Electrons = protons = 36 (neutral atom).
- 2Neutrons = .
So 36 electrons, 48 neutrons. Only the neutrons need a subtraction — don't just copy 84. ✅
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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 ➗
Chlorine is 75% mass 35 and 25% mass 37. Find to one decimal place.
- 1and . Add: .
- 2Divide by 100: .
So 35.5 — close to 35 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, solved the other way: let one isotope's percentage be , the other is , set the weighted average equal to the , and solve for . The answer always lands 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
Gallium-69 vs Gallium-71 🔬
Gallium has gallium-69 and gallium-71, and . Find the percentage of gallium-69 (call it ).
- 1, so , giving .
- 2.
So 60% gallium-69 (and 40% gallium-71). Check: gives 69.8 ✓. ✅
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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. To get a formula, just balance the charges so they cancel to zero.
Key idea🔑 Key idea: Metals lose electrons (+ ions), non-metals gain (− ions). Ionic = strong attraction in a giant lattice. Balance the charges to get the formula. ➕➖
Worked example
Worked Example: Make Magnesium Chloride
Balance the Charges 🔋
Magnesium forms and chlorine forms . What is the formula of magnesium chloride?
- 1is 2+, is 1−. To cancel a 2+, you need two 1− ions.
- 2One Mg to two Cl → .
Don't write "MgCl" — chlorine only takes one electron, so you need two of them. ✅
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7. Same Electron Arrangement
Here's a classic twist: different ions can have the exact same electron arrangement. A positive ion (a metal that lost electrons) and a negative ion (a non-metal that gained electrons) can both end up as, say, 2,8,8 (18 electrons) — they just have different numbers of protons, which keeps them different. To check, just count the electrons on each ion.
Key idea🔑 Key idea: Same electron arrangement ≠ same element. Count the electrons (after gaining/losing); the proton number is what really makes them different. 👯
Worked example
Worked Example: Who Else Is 2,8,8?
Match the Configuration 🎯
An ion has 2,8,8 (18 electrons). Could it be , , both, or neither?
- 1Sulfur atom = 16 electrons; gains 2 → 18 → 2,8,8 ✓.
- 2Calcium atom = 20 electrons; loses 2 → 18 → 2,8,8 ✓.
So both — one by gaining, one by losing, but both land on 18 electrons. ✅
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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). 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: Draw Water
Bonds and Lone Pairs 💧
In a dot-and-cross diagram of water, , how are the electrons arranged?
- 1Each hydrogen shares one pair with the oxygen → two O–H bonds.
- 2Oxygen has 6 outer electrons; 2 are in bonds, leaving 4 spare → two lone pairs.
So: two shared pairs (the bonds) and two lone pairs on the oxygen. ✅
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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 and graphite (both forms of carbon). To melt them you break loads of strong bonds, so they're super hard with very high melting points. Diamond (each C bonded to 4) is hard and doesn't conduct. Graphite (each C bonded to 3, in sliding layers) is soft, a lubricant, and conducts (one spare electron per carbon).
Key idea🔑 Key idea: Giant covalent = strong-bond network → hard, high melting point. Graphite conducts (3 bonds, spare electron, layers slide); diamond doesn't (4 bonds). 💎
Worked example
Worked Example: Statements About Graphite
Spot the Right One ⚡
Which is correct? 1: each carbon bonds to four others. 2: graphite conducts because of free ions. 3: graphite is a lubricant because its layers slide.
- 11 is wrong (each C bonds to three). 2 is wrong (free electrons, not ions).
- 23 is right — the layers slide.
So only statement 3. ✅
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10. Ionic Properties
Ionic compounds have high melting points (lots of energy needed to break the strong forces across the whole giant lattice) and conduct electricity only when molten or dissolved, never as a solid. Why? Conducting needs charged particles that can move. In a solid the ions are locked in place; melt it or dissolve it and the ions are free to move. And remember — the movers are ions, not electrons.
Key idea🔑 Key idea: Ionic = high melting point + conducts only when molten/dissolved (ions free to move). Never say "free electrons" for ionic. 🔌
Worked example
Worked Example: Does Potassium Iodide Conduct?
True or False ✔️
Potassium iodide is ionic. Which statements are correct? 1: it's made of potassium anions and iodide cations. 2: it conducts when molten or dissolved. 3: potassium shares electrons with iodine.
- 11 is wrong — the metal (potassium) is the positive ion; iodide is negative (the words are swapped). 3 is wrong — ionic bonding transfers electrons, doesn't share.
- 22 is right — molten or dissolved → ions free to move → conducts.
So only statement 2. ✅
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