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 different atoms of the same element that have the same number of protons but different numbers of neutrons. Same protons = same element; different neutrons = different nucleon number and a different mass. Take chlorine-35, , and chlorine-37, : both have 17 protons and 17 electrons, but chlorine-35 has neutrons and chlorine-37 has . The number in the name ("chlorine-37") is the nucleon number.
Key idea🔑 Key idea: Isotopes = same protons, different neutrons. What's different between two isotopes? The number of neutrons. ⚖️
Worked example
Worked Example: Comparing Two Isotopes
Sulfur-33 vs Sulfur-36 ➗
Two isotopes of sulfur are and . Give the protons, neutrons and electrons in each, and say what makes them isotopes.
- 1Both have 16 protons and 16 electrons (neutral atoms).
- 2Neutrons: and .
Same protons (both sulfur), different neutrons (17 and 20): that's what makes them isotopes. ✅
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5. Reading Atom and Ion Symbols
In a symbol like , the bottom number is the protons, the top number is protons + neutrons, and the charge (top right) tells you electrons lost (+) or gained (−). So protons = , neutrons = , and electrons = minus the charge: a 2+ ion has 2 fewer electrons than protons, a 1− ion has 1 more. For that's 17 protons, 18 neutrons, 18 electrons. To write a symbol, work backwards: protons give and the element, protons + neutrons give , protons − electrons give the charge.
Key idea🔑 Key idea: protons = ; neutrons = ; electrons = minus the charge. A charge changes only the electrons, never the nucleus. 🔙
Worked example
Worked Example: Writing an Ion's Symbol
Sulfur Gains Two 🔬
A sulfur atom has proton number 16 and nucleon number 34. It gains two electrons to form an ion. Write the ion's symbol and give its numbers of neutrons and electrons.
- 1The atom has 16 protons, neutrons and 16 electrons. The nucleus doesn't change, so and stay.
- 2Gaining 2 electrons gives electrons and a 2− charge.
So the ion is , with 18 neutrons and 18 electrons. ✅
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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 electrons → positive ions (cations); non-metals gain → negative ions (anions). An ionic bond is a strong electrostatic attraction between oppositely charged ions. At Core you draw it for a Group I metal with a Group VII non-metal: the metal has one outer electron to give, the non-metal needs exactly one, so one electron moves across. In the dot-and-cross diagram, put each ion in square brackets with its charge: with an empty outer shell, with 7 crosses + 1 dot.
Key idea🔑 Key idea: Metals lose electrons (cations), non-metals gain (anions). Ionic bond = strong electrostatic attraction between oppositely charged ions. Group I + Group VII: one electron moves, ions 1 : 1. ➕➖
Worked example
Worked Example: Sodium Chloride
One Electron Moves 🔋
Sodium (Group I) and chlorine (Group VII) form sodium chloride. Describe what happens to the electrons, describe the dot-and-cross diagram, and give the formula.
- 1Sodium (2,8,1) loses its one outer electron → , 2,8. Chlorine (2,8,7) gains it → , 2,8,8. The oppositely charged ions attract: that's the ionic bond.
- 2Diagram: with an empty outer shell; with 7 crosses and 1 dot. One lost, one gained, so the ions are 1 : 1: NaCl.
Don't draw a shared pair between Na and Cl; that's covalent. ✅
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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 have very high melting points. Diamond (each C bonded to 4) is very hard and doesn't conduct, so it's used in cutting tools. Graphite (each C bonded to 3, in layers held by weak forces) is soft because the layers slide, so it's a lubricant, and it conducts (one spare electron per carbon), so it's used for electrodes.
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 (the attraction between the ions is strong, so it takes a lot of energy to overcome) 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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