Module 06 — Heat and Particles · quiz worth 18 pts
Most particle-count questions become easy when you decide which count is allowed to change.
Fix the element. Proton count is the atomic number Z.
Change the isotope. Find them with A − Z.
Change the charge. Compare them with the proton count.
PhET Build an Atom provides extra proton, neutron, and electron counting practice. Its pictures are models, not literal electron trajectories.
What stays the same in two isotopes of carbon?
The proton count stays 6. Their neutron counts differ.
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A proton has charge +1, an electron has charge −1, and a neutron has charge 0. The atomic number Z is the number of protons and fixes the element's identity. The mass number A is protons plus neutrons. For an ion, the signed charge equals protons minus electrons. These are counts, so they have no SI unit.
Dalton's early-1800s model treated each element as one kind of indivisible atom and helped explain whole-number combining ratios. Thomson's cathode-ray evidence revealed electrons. Rutherford's scattering results showed that positive charge and most atomic mass occupy a tiny nucleus. Bohr's 1913 model used discrete electron energies to explain hydrogen's line spectrum. Each was an important historical model, not the final picture.
Treat each model as an evidence-based step, not as a list of names.
Cathode rays showed a negative particle inside the atom.
Scattering showed a tiny, dense, positive nucleus.
Discrete energies remained, but fixed paths gave way to probability-based orbitals.
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Modern quantum mechanics describes an electron with a quantum state and an orbital: a probability distribution for where a measurement may find it. An orbital is not a little planetary track, and an electron is not a classical bead circling the nucleus. Discrete energy levels remain essential, but the orbit picture is only a limited historical analogy.
The atomic number gives the proton count. Use the periodic table to name the element.
Subtract: neutrons = A − Z.
For a neutral atom, electrons = protons. For a positive ion, electrons were lost; for a negative ion, electrons were gained.
Changing neutrons makes an isotope. Changing electrons makes an ion. Changing protons makes a different element.
Write p, n, and e in three small boxes.
p = Z.
n = A − Z.
Start equal to p, then adjust for the signed charge.
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Find protons, neutrons, and electrons.
Lithium-7 has 3 protons, 4 neutrons, and 3 electrons.
Find protons, neutrons, and electrons.
The ion has 9 protons, 11 neutrons, and 10 electrons.
In a simplified valence model, ionic bonding involves electron transfer followed by electrostatic attraction between oppositely charged ions. Covalent bonding involves atoms sharing electron density. The octet rule is a useful pattern for many main-group compounds, not a universal law; hydrogen follows a two-electron pattern, and many valid compounds are exceptions.
Bohr's fixed-energy model was historically useful. Modern orbitals are probability distributions, not paths.
A is a whole-number count for one isotope. The listed atomic mass is a weighted isotopic average.
Electron count changes charge; proton count determines the element.
Transfer or sharing describes electron behavior, while attraction among charges holds the system together.
Name the evidence or count that decides the answer.
