Atoms, isotopes, and the periodic table
Everything around you is built from about a hundred kinds of atoms. Their inner structure explains why elements behave as they do.
Inside the atom
An atom has a tiny, dense nucleus of protons (charge \(+1\)) and neutrons (no charge), surrounded by electrons (charge \(-1\)). The nucleus holds nearly all the mass but occupies about one hundred-thousandth of the atom's width; atoms are mostly empty space. Rutherford's 1911 gold-foil experiment revealed this structure.
Atomic number and mass number
The atomic number \(Z\) is the number of protons and defines the element: every carbon atom has 6. The mass number \(A\) is protons plus neutrons. A neutral atom has as many electrons as protons. Carbon-14 has \(Z = 6\) and \(A = 14\), so it has 8 neutrons.
A = Z + NIsotopes and atomic mass
Isotopes are atoms of the same element with different numbers of neutrons, such as carbon-12 and carbon-14. The atomic mass listed on the periodic table is a weighted average of the isotopes' masses. Chlorine is about 75% chlorine-35 and 25% chlorine-37, so its average mass is about \(0.75(35) + 0.25(37) = 35.5\) u.
Ions
Atoms that gain or lose electrons become ions. Losing electrons gives a positive cation (\(\text{Na}^+\)); gaining electrons gives a negative anion (\(\text{Cl}^-\)). The number of protons never changes in a chemical reaction; only electrons are transferred or shared.
The periodic table
Elements are arranged by atomic number into rows (periods) and columns (groups). Elements in the same group have similar chemistry because they have similar outer-electron arrangements: the alkali metals of group 1 all react vigorously with water, and the noble gases of group 18 are nearly inert. Metals lie to the left, nonmetals to the upper right.
Periodic trends
Atomic radius decreases across a period (more protons pull electrons closer) and increases down a group (more electron shells). Ionisation energy, the energy to remove an electron, increases across a period and decreases down a group. Electronegativity, an atom's pull on shared electrons, is highest for fluorine at the upper right.
Worked example
How many protons, neutrons, and electrons are in \(^{56}\text{Fe}^{3+}\)? Iron has \(Z = 26\), so 26 protons. Neutrons: \(56 - 26 = 30\). The \(3+\) charge means 3 electrons were lost: \(26 - 3 = 23\) electrons.
Why structure matters
The number of protons fixes the element; the arrangement of electrons, especially the outermost ones, decides how it bonds and reacts. Nearly all of chemistry is the behaviour of electrons around nuclei.
Key takeaways
Atoms have a dense nucleus of protons and neutrons surrounded by electrons. \(Z\) defines the element; \(A = Z + N\). Isotopes differ in neutrons, and atomic mass is a weighted average. Ions form by gaining or losing electrons. Periodic trends in size, ionisation energy, and electronegativity follow from atomic structure.
Flashcard
Positive particle in the nucleus
Check your understanding
How many neutrons are in an atom of oxygen-18 (Z = 8)?