Atoms & Chemistry

What this part of the map covers

All matter is made of atoms, and atoms can join with other atoms to form molecules.^[1] This page looks at what atoms are made of, how elements and isotopes differ, how atoms bond, the states that matter takes, what happens to mass and energy in a reaction, and what catalysts do.

Inside the atom

An atom is a cloud of electrons around a dense nucleus of protons and neutrons, and the nucleus is 100,000 times smaller than the atom.^[2] Protons carry a positive electric charge and neutrons carry none.^{[4, 3]} The strong force, the strongest fundamental force known, holds the nucleus together.^[4]

Electrons are different: they are elementary particles, not made of anything smaller, and they sit around the nucleus rather than inside it.^[5] An atom with more or fewer electrons than protons is an ion: a negative ion is an anion and a positive one a cation.^[5]

The nucleus

In 1911, Ernest Rutherford found that every atom has a nucleus at its core.^[4] The nucleus takes up much less than 0.01% of an atom’s volume, yet typically holds more than 99.9% of its mass.^[4]

Electrons and shells

Scientists once pictured electrons circling the nucleus the way the Moon orbits Earth.^[5] Today they describe a cloud divided into shells, where different regions have different chances of holding an electron.^[5] An electron that gains energy can become excited, and when it drops back the atom gives off a photon, a particle of light, with wavelengths that are specific to each element.^[5]

In 1913, Niels Bohr published three important papers on the structure of atoms.^[13] Drawing on quantum ideas from Planck and Einstein, he proposed that an atom can exist only in a set of separate, stable energy states.^[13] The story of that quantum revolution, and of how mass and energy relate, is on the Modern Physics page.

Elements and the periodic table

The number of protons, the atomic number (Z), decides which element an atom is.^{[2, 3]} The electrons, meanwhile, determine the element’s chemical properties.^{[2, 4]} An element whose two outer shells are full is much less reactive with other elements.^[5]

For more than a century, the periodic table has guided people through the elements by showing where their chemical properties are alike and where they differ.^[10] IUPAC, the International Union of Pure and Applied Chemistry, publishes a Periodic Table of the Elements and Isotopes to show students, teachers and the public that isotopes exist and matter.^[9]

Isotopes

Isotopes are atoms of one element that share the same number of protons but have different numbers of neutrons, so their mass number (A) differs.^{[2, 6]} Carbon occurs naturally as carbon-12 (6 neutrons), carbon-13 (7) and carbon-14 (8).^[6]

Isotopes of one element behave almost identically in chemistry, but their nuclei behave differently.^[2] Some are stable and some are unstable, or radioactive, meaning the nucleus may change on its own over time, turning a “parent” into a “daughter” isotope that may belong to another element.^{[6, 2]} The half-life is the time it takes for half of a radioactive material to decay.^[2]

Chemical bonds

Gerald Bergtrom of the University of Wisconsin-Milwaukee, in an open university textbook, puts it simply: atoms form bonds to make molecules.^[15] The electromagnetic pull that keeps electrons around a nucleus is also what lets bonds form.^[14] Atoms are most stable when their outer shell of electrons is filled, and they can get there by sharing electrons or by gaining or losing them.^[15]

In a covalent bond, atoms share electrons.^[15] In hydrogen gas (H₂), one pair of electrons, shared equally, bonds two hydrogen atoms into a molecule.^[15] In methane, a carbon atom shares the four electrons of its outer shell with four hydrogen atoms, so the outer shells of all of them are filled at least some of the time.^[15] Equal sharing makes a non-polar covalent bond and unequal sharing a polar one.^[15] Organic molecules with the same formula but different shapes are called isomers, and one cannot be turned into another without breaking covalent bonds.^[17]

Water’s bonds are polar: oxygen’s larger nucleus pulls the shared electrons more strongly than hydrogen does, so the oxygen carries a partial negative charge and each hydrogen a partial positive one.^[15] Those charges let water molecules attract one another through hydrogen bonds, a kind of attraction between opposite charges.^[16] That polarity also makes water a good solvent, while non-polar fats and oils do not dissolve in it.^[16]

Atoms that fill their outer shell by gaining or losing electrons become stable ions.^[15] Sodium can give one electron to chlorine, making Na⁺ and Cl⁻ ions, and the attraction of their opposite charges, an ionic bond, holds them together in crystals of salt.^[15] Ionic bonds are weaker than covalent ones.^[15]

States of matter

Matter normally exists as a solid, a liquid or a gas.^[1] A solid keeps its own shape and volume, while a gas fills whatever container holds it; liquids and gases can flow, so both are called fluids.^[1] If atoms gain enough energy, electrons break free and mix with ions to form a fourth state, plasma.^{[5, 1]}

Conservation of mass

In a chemical reaction the bonds between atoms are broken and rebuilt, joining the atoms into new molecules.^[7] The total number of atoms stays the same, and so mass is conserved.^{[11, 12]}

Energy in chemical reactions

Chemistry is a matter of electrons: in a reaction, atoms can share, gain or lose electrons, but they never gain or lose protons or neutrons.^[15]

Bergtrom sorts energy into two broad kinds: potential (stored) and kinetic, which includes heat, electricity, sound and light.^[18] Chemical energy is a form of potential energy.^[18]

A reaction that releases heat is exothermic; one that absorbs heat is endothermic.^[19] Either way, no energy is created or destroyed: it can only be moved from one place to another or converted from one form to another.^[19]

Catalysts

A catalyst speeds up a chemical reaction, or lowers the temperature or pressure needed to start it, without being used up.^[7] It does this by lowering the activation energy, the energy barrier a reaction has to get over.^[7] People have used them for thousands of years, and newer catalysts have led to biodegradable plastics, new medicines and cleaner fuels and fertilizers.^[7]

Even without a catalyst, adding more reactants or raising the temperature speeds a reaction up, because the molecules collide more often.^[20]

The math (optional)

Isotopes are written as the element’s name or symbol, a hyphen and the mass number, as in carbon-14 or C-14; a trailing “m”, as in technetium-99m, marks a metastable state of the nucleus.^[2]

Electron arrangements are written as configurations. NIST lists hydrogen as 1s¹ and carbon as [He] 2s² 2p², where [He] means every subshell belonging to helium is full.^[8]

In a chemical formula, subscripts count the atoms of each element in a molecule, and in a reaction, coefficients count the molecules.^[11]

Common misconceptions

“Electrons orbit the nucleus like tiny planets.” That was the older picture; scientists now describe shells in which electrons are found with different probabilities.^[5]

“Every isotope is radioactive.” Isotopes come in two main types, stable and radioactive, and carbon-12, for one, never decays.^[6]

“An element’s atomic weight is a fixed constant.” IUPAC stresses that atomic weights are not constants of nature, and for elements whose atomic weight varies in a well-documented way, the standard value is given as an interval.^{[10, 9]}

“Boiling or freezing water makes a new substance.” Changes of state are physical changes, not chemical ones: water vapour, liquid water and ice are all H₂O.^[1]

What this page does not cover yet

We have not yet found sources good enough for these subjects, so this page leaves them out for now: metallic bonds, electronegativity, double and triple bonds, bond energies, worked examples of balancing chemical equations, types of reaction such as combustion or decomposition, reaction rates in numbers, acids, bases and pH, and the history of the periodic table and the periodic law.

Going further

Start with Electric Charge, then see Modern Physics for the quantum atom and mass-energy, Heat & Thermodynamics and Light & Waves.