Atoms and Elements

Ordinary matter is made up of protons, neutrons, and electrons and is composed of atoms. An atom consists of a tiny nucleus made up of protons and neutrons, on the order of 20,000 times smaller than the size of the atom. The outer part of the atom consists of a number of electrons equal to the number of protons, making the normal atom electrically neutral.

A chemical element consists of those atoms with a specific number of protons in the nucleus; this number is called the atomic number. The atoms of an element may differ in the number of neutrons; atoms with different neutron numbers are said to be different isotopes of the element.

Elements are represented by a chemical symbol, with the atomic number and mass number sometimes affixed as indicated below. The mass number is the sum of the numbers of neutrons and protons in the nucleus.

The elements can be identified unambiguously by the "spectral fingerprints" of their line spectra and in some cases by the flame colors given off by excited atoms.

Constituents of Atoms

The electrons, protons and neutrons which make up an atom have definite charges and masses. If they are modeled as hard spheres with the same density, they would have the relative sizes shown. While that model should not be taken as reality, it gives us a convenient object to which to attach the definite properties of the particles.

While the charges and masses are precisely known, the sizing is just fun and games. Our best information about the proton and neutron indicates that they are constituent particles, made up of three quarks each. They do however seem to have an effective density which is roughly characteristic of all nuclei, and we can attribute to them a radius of about 1.2 x 10^-15 meters. The electron is a fundamental particle, classified as a lepton, which is apparently not made out of any constituent particles. Down to scales of a thousand times smaller than the proton radius quoted above, they have no apparent structure.