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Atoms first: matter, atomic theory, particles, isotopes, light, quantum numbers and electron configurations.
Read the idea in Learn, then practise. Each tab has Hint, Reset and Skip, 3 levels and a crown to win.
Track 0 · Introductory Chemistry · part of ChemTracks
Atoms first. Every substance you will ever meet is built from atoms, so we start there: what matter is, how we found out about atoms, what is inside one, and how its electrons are arranged. That last idea (the electron configuration) explains the periodic table and, later, all of bonding and reactions.
Read the idea in Learn, then practise it. Every tab has Hint, Reset and Skip, 3 levels, and a crown to win. You never need to memorize: you will see each idea again and again until it is yours.
Keep ElementWorks open in another tab. The atomic number, symbol and atomic mass of every element are one click away, and so is the toolbox here.
Matter is anything that has mass and takes up space. We sort it by asking one question at a time: can it be separated by physical means?
| Kind | What it is | Examples |
|---|---|---|
| Element | One kind of atom | gold, O2, He |
| Compound | Two or more elements bonded in a fixed ratio | H2O, NaCl, CO2 |
| Homogeneous mixture | A mixture that is uniform all the way through (a solution) | salt water, air, brass |
| Heterogeneous mixture | A mixture with visibly different parts | sand and water, granite, pizza |
Solid: particles packed in fixed positions, fixed shape and volume. Liquid: particles close but free to slide, fixed volume, no fixed shape. Gas: particles far apart and fast, fills its container. Changes of state: melting, freezing, vaporization, condensation, sublimation (solid to gas) and deposition (gas to solid). Melting, vaporization and sublimation absorb heat; the reverse changes release it.
A mixture keeps the properties of its parts, so you separate it with a physical property: size (filtration), boiling point (distillation), magnetism, solubility or volatility (evaporation), or how far a substance travels on paper (chromatography).
The atom was not seen; it was figured out, one experiment at a time. Each new result showed that the previous picture was incomplete.
| Who | Evidence | Idea |
|---|---|---|
| Dalton | Fixed mass ratios in compounds | Matter is made of atoms; atoms of an element are alike; reactions rearrange atoms |
| Thomson | Cathode rays bend toward the + plate | Atoms contain negative electrons (plum pudding) |
| Millikan | Oil drop charges are multiples of one value | The electron has a fixed charge, 1.60 × 10−19 C |
| Rutherford | Gold foil: most alpha particles go straight through, a few bounce back | A tiny, dense, positive nucleus with mostly empty space around it |
| Chadwick | Neutral radiation from beryllium | The nucleus also contains neutrons |
| Bohr | Hydrogen gives only a few colors of light | Electrons have fixed, quantized energy levels |
| Particle | Charge | Mass (u) | Where |
|---|---|---|---|
| Proton | +1 | 1.0073 | nucleus |
| Neutron | 0 | 1.0087 | nucleus |
| Electron | −1 | 0.00055 | outside the nucleus |
In AXq: Z (the atomic number) = protons, A = protons + neutrons, so neutrons = A − Z. The charge q tells you the electrons: electrons = Z − q.
An atom that loses electrons becomes a cation (positive). One that gains electrons becomes an anion (negative). The protons never change in chemistry.
Isotopes are atoms of the same element (same Z) with different numbers of neutrons. Chlorine-35 and chlorine-37 both have 17 protons, but 18 and 20 neutrons. The mass on the periodic table is a weighted average of the isotopes found in nature.
With two isotopes, let x be the fraction of the first, so the second is 1 − x. Then x(m1) + (1 − x)(m2) = average. Solve for x and multiply by 100 for a percent.
Electrons give atoms their chemistry, and we learn about them through light. Light is a wave: wavelength λ (distance between peaks) and frequency ν (peaks per second) are linked by c = λν, with c = 3.00 × 108 m/s. A packet of light, a photon, has energy E = hν = hc/λ (h = 6.626 × 10−34 J·s).
Long wavelength, low frequency, low energy: radio, microwave, infrared. Then visible light from red to violet. Short wavelength, high frequency, high energy: ultraviolet, X-ray, gamma.
Bohr explained hydrogen’s colors: its electron can only be in certain energy levels (n = 1, 2, 3 …), with En = −2.178 × 10−18 J ÷ n2. When the electron falls to a lower level it emits a photon; when it jumps up it absorbs one. Today we describe electrons with four quantum numbers:
| Number | Meaning | Allowed values |
|---|---|---|
| n | Shell (size, energy) | 1, 2, 3 … |
| l | Subshell (shape): s, p, d, f | 0 to n − 1 |
| ml | Which orbital | −l to +l |
| ms | Spin | +½ or −½ |
The electron configuration lists which subshells hold the electrons. Fill the lowest-energy subshell first (Aufbau): 1s 2s 2p 3s 3p 4s 3d 4p. Capacities: s 2, p 6, d 10. Oxygen (8 electrons) is 1s2 2s2 2p4; with a noble-gas core, sulfur is [Ne] 3s2 3p4.
An orbital diagram draws every orbital as a box. Pauli: a box holds at most two electrons, with opposite spins (↑↓). Hund’s rule: in a subshell, put one electron in each box (all spinning the same way) before you pair any.
Cations lose electrons from the highest shell first, so Fe2+ loses its 4s electrons before any 3d: [Ar] 3d6. Anions gain electrons until they reach a noble-gas arrangement: Cl− is [Ne] 3s2 3p6, the same as argon.
Exceptions: chromium is [Ar] 3d5 4s1 (not 3d4 4s2) and copper is [Ar] 3d10 4s1. A half-filled or filled d subshell is especially stable.
AtomWorks is Track 0 of ChemTracks. Each topic has a short lesson, a practice tab with three levels of randomly generated problems, hints and worked solutions, and a crown for mastering it. Earn all eight crowns for a certificate.
Matter and its changes, the history of atomic theory, protons, neutrons and electrons, isotopes and average atomic mass, light and photon energy, quantum numbers and orbitals, and electron configurations with orbital diagrams.
Click each box to add an up arrow, a down arrow, both or none. The check follows the Pauli exclusion principle and Hund's rule, so any equivalent placement of paired electrons is accepted.
Track 0 (Introductory Chemistry) of ChemTracks. Deeper configurations and periodic trends come later in Track 1.
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