Zoom from a sugar cube down to a single gold atom and its nucleus. Each step shows the real size, the power of ten and a scale bar. Then predict the answers to 3 questions and check them with the numbers. Mission: all 3 answered, at least 2 right on the first try.
Stadium analogy. If an atom were as wide as a 100 m stadium, the nucleus would be a bead only a few millimeters across on the center spot (gold: about 5 mm; hydrogen: about 2 mm). The electrons are found far out in the stands. Everything in between is empty space, yet the bead holds more than 99.9 % of the mass.
Sizes: grain of table salt about 0.5 mm; human hair about 80 µm wide; a typical animal cell 10–20 µm; many viruses about 100 nm; a gold atom 0.29 nm across (radius 0.144 nm); a gold nucleus about 14 fm (1.4 × 10⁻¹⁴ m). 1 nm = 10⁻⁹ m. An atom has no hard edge, so “size” means the usual distance between atom centers.
In 1909 Hans Geiger and Ernest Marsden, working with Ernest Rutherford, fired fast alpha particles (helium nuclei, charge +2) at thin metal foils. About 1 in 8,000 bounced back from platinum; later runs with gold foil about 1,400 atoms thick also gave rare bounce-backs. In 1904 J.J. Thomson had pictured the atom as a ball of spread-out positive charge with electrons in it (the “plum pudding”). Predict, then fire. Compare Thomson’s prediction with what really happened. Mission: fire at least 1,000 particles with each model, then build a claim–evidence–reasoning answer.
| Model | Fired | Nearly straight (< 10°) | Deflected (10°–90°) | Bounced back (> 90°) | Bounced back: 1 in … |
|---|
Real result (Geiger and Marsden, 1909, platinum): about 1 in 8,000 bounced back by more than 90°; with thin gold foil it was about 1 in 20,000 (Rutherford, 1911). This model uses 1 in 8,000. Rutherford explained the bounce-backs in 1911 with a tiny, dense, positive nucleus; he said it was “as if you fired a 15-inch shell at a piece of tissue paper and it came back.” In the model here, each particle’s deflection comes from how close it passes to a nucleus (tan(θ/2) = b₀/b, with b₀ set so that 1 in 8,000 bounces back). The counts restart from the same sequence after a clear, so a class gets the same numbers.
Scientific models change when new evidence appears. Put the six models of the atom in the order they were proposed, using the ▲ and ▼ buttons, then match each model to the evidence that led to it. Mission: the order right and at least 5 of 6 evidence matches right on one check.
The electron-cloud model (1926) is the modern model; Chadwick’s neutron (1932) came later and completed the picture of the nucleus. Each model kept what still worked from the one before. More on energy levels and spectra: Bohr atom; on orbitals: electron configuration (Aufbau).
Each card shows an atom or ion as a periodic-table box, in isotope notation (mass number top left, atomic number bottom left, charge top right) or by name. Enter the number of protons, neutrons and electrons; the drawing and readout update as you type. Mission: 6 cards right on the first check, including at least one ion and one isotope. Want to build atoms freely? Use the free Build an Atom tool on the periodic table page.
Atomic numbers you may need: H 1 · C 6 · O 8 · F 9 · Na 11 · Al 13 · S 16 · Cl 17 · Ca 20 · Au 79 (or look them up on the periodic table).
Too many electrons to draw: shells are shown for Z ≤ 20.
Most elements are a mix of isotopes: atoms with the same number of protons but different numbers of neutrons. A mass spectrometer sorts atoms by mass and counts them. Pick an element, collect a sample, then work out the average atomic mass from the real isotope masses and abundances. Mission: 2 averages right (one element with 3 isotopes) and 1 mystery spectrum right on the first pick.
For each isotope: abundance as a decimal × isotope mass. Then add the rows. Keep at least 3 decimal places.
| Isotope | Protons | Neutrons | Mass (u) | Natural abundance |
|---|
Masses and abundances: IUPAC / NIST isotopic compositions (u = unified atomic mass unit, 1/12 of the mass of a carbon-12 atom). Small samples give averages that wander; large samples come close to the periodic-table value. Carbon also has a trace of radioactive carbon-14 (about 1 atom in a trillion), too little to change the average.
In the Bohr model, electrons are found at certain energy levels (shells). The first level holds up to 2, the second up to 8 and, for the first 20 elements, the third up to 8; then the 4th level starts (potassium and calcium). Pick an element, predict its number of valence (outer-level) electrons, then place its electrons by pressing a shell (or the + buttons). Mission: 5 elements right (first prediction and arrangement), including one from period 3 (Na to Ar) and K or Ca.
Shells are energy levels, not tracks that electrons follow like planets around the Sun. In the modern model each electron is spread out in a cloud (an orbital) that shows where it is likely to be found; the rule 2, 8, 8, 2 is a school rule that works for elements 1 to 20. For orbitals and the full filling order, see electron configuration (Aufbau); for how valence electrons form bonds, see chemical bonding.
Each card changes one atom. Decide what you get: an isotope (neutrons change), an ion (electrons change) or a new element (protons change). Only your first choice counts. Mission: at least 8 of 10 right.
In chemical reactions atoms gain, lose or share electrons, but the nucleus does not change, so they never become other elements. Changing the protons needs a nuclear reaction.
10 questions from a bank of 20. Score 7 or more to complete the mission. Each new round picks new questions in a new order.
Everything is made of atoms, and they are small beyond imagining: about half a million carbon atoms fit across one human hair. Even stranger, an atom is almost all empty space. Its nucleus is only about 1/10,000 to 1/100,000 of its width, like a bead on the center spot of a stadium, yet it holds more than 99.9 % of the mass. We know this from evidence: in the gold foil experiment almost every alpha particle passed straight through the gold, and only about 1 in several thousand was pushed back by the tiny nucleus.
| Particle | Charge | Mass (u) | Relative mass | Where |
|---|---|---|---|---|
| Proton (p) | +1 | 1.00728 | 1 | nucleus |
| Neutron (n) | 0 | 1.00866 | 1 | nucleus |
| Electron (e⁻) | −1 | 0.000549 | 1/1836 | around the nucleus |
One charge unit is 1.602 × 10⁻¹⁹ coulombs. A neutral atom has as many electrons as protons, so the charges cancel. The positive nucleus and the negative electrons attract each other; that electrical attraction holds the atom together.
In the notation ¹⁴₆C, the bottom number is the atomic number (Z = 6 protons) and the top number is the mass number (A = 14). Neutrons = A − Z = 8. A charge in the top right counts electrons: ²³Na⁺ has 11 protons, 12 neutrons and 10 electrons, because charge = protons − electrons. Ions form when atoms gain or lose electrons; the nucleus does not change in a chemical reaction.
All chlorine atoms have 17 protons, but 75.76 % have 18 neutrons (chlorine-35) and 24.24 % have 20 (chlorine-37). Isotopes of an element react the same way, because they have the same electrons. The periodic table gives the weighted average: 0.7576 × 34.969 + 0.2424 × 36.966 = 35.45 u. No single chlorine atom has a mass of 35.45 u. Most isotopes are stable; only some, such as carbon-14, are radioactive.
Bohr (1913) proposed that electrons can only have certain energies, the energy levels or shells. For the first 20 elements they fill as 2, 8, 8, 2: sodium is 2, 8, 1 and potassium 2, 8, 8, 1. Both have one valence electron in the outer level, which is why they react in the same way and sit in the same group. When an electron drops from a higher level to a lower one, the atom gives off light of one exact energy: that is why each element has its own flame color and line spectrum. The modern model replaces the shells with electron clouds (orbitals) that show where an electron is likely to be found.
In Mission 2, fire 8,000 particles with each model and compare the “bounced back” column. In Mission 4, enter 8 protons with 8 and then 10 neutrons: the element stays oxygen. In Mission 5, collect 10 atoms of chlorine several times and watch the average jump around; then collect 10,000. In Mission 6, compare Na and K, or F and Cl.
| Find | Formula | Example (³²S²⁻, Z = 16) |
|---|---|---|
| Neutrons | n = A − Z | 32 − 16 = 16 |
| Electrons | e = Z − charge | 16 − (−2) = 18 |
| Average atomic mass | Σ (abundance as a decimal × isotope mass) | Cl: 0.7576 × 34.969 + 0.2424 × 36.966 = 35.45 u |
| Energy level (shell) | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Holds up to (school rule, Z ≤ 20) | 2 | 8 | 8 | 1–2 for K, Ca (it holds more in bigger atoms) |
| Examples | H 1, He 2 | C 2, 4 · Ne 2, 8 | Na 2, 8, 1 · Cl 2, 8, 7 | K 2, 8, 8, 1 · Ca 2, 8, 8, 2 |
Fill from the inside out: each level is full before the next one starts. Valence electrons = electrons in the outer level = the group number for groups 1–2 and the group number minus 10 for groups 13–18 (helium: 2). Related pages: periodic table, chemical bonding, Bohr atom, electron configuration, periodic trends, states of matter.
Heating a metal salt in a flame lifts electrons to higher energy levels. When an electron falls back down, the atom gives off a photon whose energy equals the gap between the levels: E = hc/λ (h = 6.626 × 10⁻³⁴ J·s, c = 2.998 × 10⁸ m/s). Each element has its own gaps, so its own colors. Pick a salt:
For calcium, strontium and copper much of the flame color comes from small molecules formed in the flame (such as CaOH or CuCl), but the idea is the same: electrons drop between energy levels. Real-lab safety: flame tests are a teacher demonstration with goggles; strontium and copper salts are toxic. Line spectra and the Bohr energy formula are explored on the Bohr atom page.
Protons repel each other strongly at nuclear distances. They stay together because of the strong nuclear force, which attracts protons and neutrons to each other but acts only over about the width of a nucleus (a few femtometers). Neutrons add this attraction without adding repulsion, which is why larger nuclei need more neutrons than protons: gold-197 has 79 protons and 118 neutrons. (Advanced: a nucleus has slightly less mass than its separate protons and neutrons; this “mass defect” corresponds to the binding energy through E = mc².)
The Bohr model works well for hydrogen but not for atoms with many electrons. Schrödinger’s wave model (1926) describes each electron by an orbital, a 3D cloud of probability, grouped into energy levels and sublevels (s, p, d, f). The shell numbers 2, 8, 8 come from these sublevels: 2 = 1s; 8 = 2s + 2p; the third shell has room for 18, but the 4s sublevel fills before 3d, which is why potassium starts a 4th shell. See the shapes on hydrogen orbitals and the filling order on electron configuration (Aufbau).
Carbon-14 dating: living things take in a tiny, steady share of radioactive carbon-14; after death it decays with a half-life of about 5,730 years, so the share left gives the age of wood, charcoal or shells up to about 50,000 years old. Medical tracers: small amounts of short-lived isotopes, such as technetium-99m, are given to patients so that scanners can follow where they go in the body. Mass spectrometry of stable isotopes, such as oxygen-18 in ice cores, records past temperatures.
Standards: NGSS MS-PS1-1 (models of atomic composition: Missions 1, 2, 4), HS-PS1-1 (patterns of outer electrons and the periodic table: Mission 6), HS-PS1-8 introduction (isotopes and nuclear change: Missions 4, 5, 7), HS-PS4-3 link (light from energy levels: flame lab). NGSS places subatomic particles mainly in high school, and the MS-PS1-1 assessment boundary excludes valence electrons; many state standards teach protons, neutrons and electrons in grade 8 (e.g., Florida SC.8.P.8.7); Bohr models are a common grade 8–9 extension. So in grade 7 use Missions 1–3 and 7 as the core; in grade 8 add Missions 4–6 (Mission 6 is enrichment beyond MS-PS1-1); in high school all 8 are core. Objective: students use evidence from the gold foil experiment to describe the nuclear atom, count protons, neutrons and electrons from notation, calculate average atomic mass from isotope data, and draw Bohr arrangements for elements 1–20.
Differentiation. Support: start with the cards that show Z (carbon-14, fluoride), use the calculator rows in Mission 5 one at a time, and give the sentence frame for the CER. Extension: E = hc/λ in the flame lab, the mass-defect note, orbitals on Aufbau. Language support: the vocabulary list below, and the particle table in “The Idea”. Misconceptions to target: electrons orbiting like planets; atoms as solid balls; the decimal mass as the mass number; isotopes as different elements or always radioactive; ions made by changing protons. Missions are saved on the student’s device; the certificate prints as proof of completion.
Atoms are made of protons, neutrons and electrons. Protons (charge +1) and neutrons (no charge) sit in the tiny nucleus at the center and each has a mass of about 1 u. Electrons (charge −1) are found around the nucleus and have only about 1/1836 of a proton’s mass, so almost all of an atom’s mass is in its nucleus.
Key takeaway: protons (+) and neutrons (0) in the nucleus, electrons (−) around it.The atomic number (Z) is the number of protons; it decides the element, so every oxygen atom has Z = 8. The mass number (A) is the number of protons plus neutrons in one nucleus. Oxygen-16 has A = 16 and oxygen-18 has A = 18; both have Z = 8.
Key takeaway: Z = protons; A = protons + neutrons.Subtract the atomic number from the mass number: neutrons = A − Z. Carbon-14 has A = 14 and Z = 6, so it has 8 neutrons. Gold-197 has 197 − 79 = 118 neutrons. Do not use the decimal atomic mass from the periodic table; it is an average of several isotopes.
Key takeaway: neutrons = mass number − atomic number.Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons. Carbon-12, carbon-13 and carbon-14 all have 6 protons but 6, 7 and 8 neutrons. Isotopes of an element have the same electrons and react the same way. Most isotopes are stable; some, such as carbon-14, are radioactive.
Key takeaway: same protons, different neutrons.An isotope has a different number of neutrons than another atom of the same element, so its mass changes but its charge does not. An ion has gained or lost electrons, so it has a charge: Na⁺ has lost one electron and Cl⁻ has gained one. If the number of protons changes, the atom becomes a different element.
Key takeaway: neutrons change → isotope; electrons change → ion; protons change → new element.Because it is an average of an element’s isotopes, weighted by how common each one is. Chlorine is 75.76 % chlorine-35 (34.969 u) and 24.24 % chlorine-37 (36.966 u), so the average is 35.45 u. No single chlorine atom has that mass.
Key takeaway: it is a weighted average of the isotopes, not the mass of one atom.Almost all alpha particles passed straight through thin metal foil (gold foil about 1,400 atoms thick), but only about 1 in several thousand bounced back by more than 90° (about 1 in 8,000 from platinum in 1909; about 1 in 20,000 for thin gold foil). Rutherford concluded in 1911 that an atom is mostly empty space with a tiny, dense, positively charged nucleus. Thomson’s plum-pudding model, with positive charge spread through the atom, could not explain the bounce-backs.
Key takeaway: atoms are mostly empty space around a tiny, dense, positive nucleus.For the first 20 elements, the school rule is 2 in the first shell, then 8, then 8, then the 4th shell starts: sodium is 2, 8, 1 and calcium is 2, 8, 8, 2. Fill from the inside out. The electrons in the outer shell are the valence electrons. (The third shell can hold 18 in heavier atoms.)
Key takeaway: 2, 8, 8, 2 for elements 1–20, filled from the inside out.No. Bohr’s shells are energy levels, not paths. In the modern model an electron does not follow a track at all; it is described by an orbital, a cloud that shows where the electron is likely to be found. The Bohr model is still useful for counting electrons at each energy level.
Key takeaway: shells are energy levels; real electrons are clouds, not planets.A typical atom is about 0.1 to 0.3 nanometers across (a gold atom is 0.29 nm, about 3 × 10⁻¹⁰ m). About half a million carbon atoms fit across a human hair. The nucleus is about 10,000 to 100,000 times smaller than the atom: if the atom were a 100 m stadium, the nucleus would be a bead a few millimeters wide.
Key takeaway: about 10⁻¹⁰ m; the nucleus is 10,000–100,000 times smaller still.