Module 3 · The Particle World: Atoms and MoleculesPractice · 微观世界：原子与分子

Spreading (diffusion) happens because molecules are always moving. At a higher temperature they move faster, so the dye and water molecules mix sooner. Molecules don’t change size with temperature, there are always spaces between them, and the dye only spreads; it doesn’t react.

Evaporation is a physical change. Each water molecule is still a water molecule (2 hydrogen atoms and 1 oxygen atom) of the same size; only the spaces between molecules change, becoming much larger in the vapor.

An atom is a positive nucleus surrounded by negative electrons, and the nucleus is made of protons and neutrons. But the most common hydrogen atom ($\ce{^{1}H}$) has a nucleus of just 1 proton and no neutrons, so B is wrong. An electron is far lighter than a proton, and nearly all of an atom’s mass is in the nucleus, so D is wrong. In an atom, protons = electrons, so the charges cancel and the atom is neutral.

Mass number = protons + neutrons, so protons = 56 − 30 = 26. In an atom, electrons = protons = 26. The element with 26 protons is iron.

The number of protons decides the element: 12 protons means magnesium (Mg). With 2 fewer electrons than protons, the particle carries a 2+ charge, so it’s the magnesium ion $\ce{Mg^2+}$. A magnesium atom would have 12 electrons; a Ne atom also has 10 electrons, but only 10 protons.

Both are chlorine atoms, with 17 protons and 17 electrons each. They have 35 − 17 = 18 and 37 − 17 = 20 neutrons, so their mass numbers differ. They are isotopes. Chemical properties depend mainly on the electrons, so isotopes behave almost the same chemically.

The relative atomic mass is the abundance-weighted average of the isotope masses: 24 × 79% + 25 × 10% + 26 × 11% = 24.32 ≈ 24.3. It’s a little above 24 because the heavier $\ce{^{25}Mg}$ and $\ce{^{26}Mg}$ make up part of the sample. Calculations often round magnesium to 24.

Relative atomic mass = mass of the atom ÷ (1/12 of the mass of a carbon-12 atom), so the atom’s mass = relative atomic mass × (carbon-12 mass ÷ 12):

In other words, a sulfur atom is about 32 ÷ 12 ≈ 2.67 times as heavy as a carbon-12 atom.

A sodium atom’s electrons are arranged 2, 8, 1; it easily loses its single outer electron to become $\ce{Na+}$, arranged 2, 8. The nucleus doesn’t change: still 11 protons and the same neutrons, so it’s still sodium. Only the number of electrons changes (11 → 10).

Even thin gold foil is many layers of atoms thick. That nearly all the alpha particles passed straight through shows that an atom is mostly empty space with a tiny nucleus. It was the few deflected particles, and the very few that bounced back, that showed a small, massive, positive nucleus at the center.