Atomic structure

Three particles, three different roles

A neutral atom has the same number of electrons as protons. Changing electrons gives an ion; changing neutrons gives an isotope; changing protons gives a different element.

Atomic number, mass number and isotope notation

The atomic number Z is the number of protons. The mass number A counts protons plus neutrons, so the neutron number is N = A − Z.

The periodic-table atomic mass is usually not an integer because it is a weighted average over naturally occurring isotopes, not the mass number of one particular atom.

From Rutherford to the quantum atom

Rutherford scattering established the nucleus, but a classical electron orbiting a positive nucleus would radiate energy and collapse. Bohr introduced discrete energy levels for hydrogen; quantum mechanics replaced fixed orbits with wavefunctions and orbitals.

An orbital is not a path. It is a quantum state whose wavefunction determines a probability distribution for finding an electron. Shells and subshells organize these states by quantum numbers and energy.

This distinction matters because chemical trends—bonding, ionization, size, spectra—come from the energies and spatial distributions of electrons, not from planets circling the nucleus.

Build an atom: charge and mass bookkeeping

The tool deliberately separates the three counts. Nuclear identity follows proton number; charge follows the proton–electron imbalance.

Scale, forces and what holds the atom together

Electrons are bound to the nucleus by electromagnetic attraction. Protons repel one another electrically inside the nucleus, but at nuclear distances the strong interaction can bind protons and neutrons together.

The two scales should not be mixed: chemistry mostly reorganizes electrons and typically involves electronvolts per particle; nuclear reactions change the nucleus and can release energies millions of times larger per event.

Even the phrase “size of an atom” needs care. Electron density has no sharp wall, so atomic radii are inferred from distances between atoms in molecules or solids.

Evidence that built the modern atom

The model changed because experiments ruled out earlier pictures. A scientifically useful atomic diagram should therefore be read as a model tied to specific evidence, not as a literal photograph.

Mass, isotopes and the atomic-mass scale

Proton and neutron masses are close to 1 unified atomic mass unit, while an electron contributes only about 0.00055 u. This is why mass number A is a good whole-number count of nucleons but not an exact atomic mass.

Nuclear binding also changes the mass. A bound nucleus weighs slightly less than its separated protons and neutrons; the missing mass corresponds to binding energy through E = mc². Chemical atomic masses additionally refer to neutral atoms and include electron masses.

The value printed in a periodic table is usually an isotopic abundance-weighted mean. For chlorine, the naturally abundant ³⁵Cl and ³⁷Cl isotopes produce an average near 35.45 u. No ordinary chlorine atom has “35.45 nucleons.”

Position, probability and the electron cloud

Quantum mechanics does not assign an electron a classical orbit with a known position and velocity at every instant. A wavefunction describes the state, and |ψ|² gives a probability density for position measurements.

Orbital drawings usually enclose a chosen fraction—often about 90%—of the probability. The boundary is therefore a visualization convention. Electron density extends beyond the drawn surface.

The uncertainty principle is deeper than poor measurement technique: quantum states cannot possess arbitrarily precise position and momentum simultaneously. That is one reason the planetary-orbit picture eventually fails.

Exercises

Magnesium ion

²⁴Mg²⁺ has Z = 12. Count protons, neutrons and electrons.

Isotope or ion?

Two atoms have 17 protons; one has 18 neutrons and the other 20. What is their relationship?

Rutherford evidence

What observation rules out a uniformly spread positive charge?