Chapter map — Atoms and Molecules

The three changes

Learn this before anything else Change the protons → a different element.
Change the neutrons → an isotope of the same element.
Change the electrons → the charge changes (never happens in a neutral atom, where electrons always equal protons).

Definitions

AtomThe simplest unit of an element. From the Greek atomos — "cannot be cut". Each element consists of only one type of atom.

MoleculeTwo or more atoms that are chemically combined. Of an element if all atoms are the same type; of a compound if two or more types.

IsotopesAtoms of the same element with the same number of protons and electrons, but a different number of neutrons.

Chemical formulaIdentifies each constituent element by its chemical symbol and indicates the number of atoms of that element. A structural formula also shows how the atoms are chemically combined.

Sub-atomic particles

Particle	Relative charge	Relative mass	Position
proton (p)	+1	1	within the nucleus
neutron (n)	0	1	within the nucleus
electron (e)	−1	1/1840	around the nucleus

An atom is electrically neutral because protons = electrons, so +1s and −1s cancel. Neutrons have no charge and never affect it.
The nucleus is positively charged — it holds protons (+) and neutrons (0) only.
The mass of an atom is mainly the mass of its nucleus. Electron mass is negligible (1/1840).
The atom is mostly empty space — atom : nucleus is like a football stadium : a marble in the middle.

Models of the atom, in order

Model	What it says, and its limit
Billiard ball John Dalton, 1800s	A hard solid sphere; wooden balls joined by hooks showed how atoms combine. No sub-atomic particles at all.
Plum pudding J. J. Thomson, early 1900s	Electrons (plums) within a mass of protons (pudding). No nucleus.
Planetary the textbook's model	Nucleus of protons and neutrons; electrons move around it at high speed. Still a simplification — no one knows exactly what an atom looks like.

Both earlier models are now considered inaccurate because they cannot explain many properties of atoms. Science involves constant revision so that it stays current and reliable — what is accepted today may be changed or replaced.

Proton number and nucleon number

Proton number — the number above the symbol in the periodic table. The number of protons in one atom.
Elements are arranged in order of increasing proton number. No two elements share one. It is what differentiates one element from another.
Nucleon number = protons + neutrons, so neutrons = nucleon number − proton number.
118 elements known at present. No naturally occurring element has more than 92 protons on Earth.

Making new elements: collide atoms of two elements at high speed in a particle accelerator; the masses should add to the new element's. Probability of success is about one in a trillion, the atom lasts about a thousandth of a second, and trillions of collisions over many years may be needed. Element 113 came from zinc (30) + bismuth (83) — Kosuke Morita's team, over four trillion collisions in nine years — named nihonium (Nh), the first element discovered in Asia.

Mendeleev — father of the periodic table. Devised one with the 63 known elements and left gaps for undiscovered ones. His iodine/tellurium inconsistency was resolved decades later by ordering on proton number, which is why tellurium now comes before iodine. Over a thousand versions since; AI is now being used to explore alternatives.

Relative atomic mass from isotopes

The method Multiply each isotope's nucleon number by its % abundance ÷ 100, then add.

( ⁷⁵⁄₁₀₀ × 35 ) + ( ²⁵⁄₁₀₀ × 37 ) = 26.25 + 9.25 = 35.5

Two checks: abundances add to 100%, and the answer lies between the two nucleon numbers, nearer the more abundant one. This is why relative atomic masses are often not whole numbers.

Isotope examples: carbon 6 protons with 6, 7 or 8 neutrons · nitrogen 7 protons with 7 or 8 · uranium 92 protons with 143 or 146 (uranium-235 and uranium-238).

Molecules and formulae

Molecules of elements	Molecules of compounds
hydrogen H₂ · oxygen O₂ · nitrogen N₂ · chlorine Cl₂ · iodine I₂ · sulfur S₈	water H₂O · ammonia NH₃ · carbon monoxide CO · carbon dioxide CO₂ · hydrogen chloride HCl · nitrogen dioxide NO₂

One chemical symbol in the formula → element. Two or more → compound. The number of atoms is irrelevant (S₈ is an element; CO is a compound).
A subscript counts the atoms of the symbol immediately before it. No subscript means one atom.
Limitation of the labelled-circle model: different colours and sizes are used so we can tell atoms apart, but atoms have no colour. Models are simplified and not always real and accurate.

Atomic technologies (8.4)

Field	Applications	Issues
New materials	Gold-plated orchids (living plant coated with gold atoms) · nanotechnology and nanorobots that deliver drugs into the bloodstream and repair damaged cells	Nanoscale objects need special instruments — the atomic force microscope and scanning tunnelling microscope
Healthcare	Radiation therapy to cure disease · medical imaging: MRI and X-ray · a radiographer operates the equipment; the radiation is given off by atoms with large amounts of energy	Radiation that penetrates skin and bone can also harm, so exposure must be controlled
Food	Molecular gastronomy — the study of molecules in the physical and chemical processes in food during cooking; new tastes, textures and appearances	—
Nuclear energy	Controlled release of heat from uranium and plutonium → steam → turbines → electricity. An alternative to oil, especially where there is no access to oil	Waste disposal · accidents causing loss of life and pollution · atomic bombs

The closing argument Social and ethical issues often arise from scientific discoveries. Resolving them requires a good understanding of scientific applications and sound moral values from everyone. We need to actively understand and debate these issues to make wise decisions that benefit mankind.