Atoms, Nuclei, and Other Parts

Requirement 2 is

  1. Do the following:
    1. Tell the meaning of the following: atom, nucleus, proton, neutron, electron, quark, isotope; alpha particle, beta particle, gamma ray, X-ray; ionization, radioactivity, radioisotope, and stability.
    2. Choose an element from the periodic table. Construct 3-D models for the atoms of three isotopes of this element, showing neutrons, protons, and electrons. Use the three models to explain the difference between atomic number and mass number and the difference between the atom and nuclear and quark structures of isotopes.

Requirement 2a is simply a lexicon of terms which are important to understand.


The atom is from the Greek word for "indivisible" and was a hypothetical smallest unit of matter that could exist. 19th century chemistry provided hints that atoms might be real in that certain compounds required integer ratios of the reactants in their formation. Or reasonably close to integer ratios given the limitations of their ability to measure. It was Einstein's 1905 paper on Brownian motion which convinced scientists that atoms were real, not hypothetical model parameters.1


The central part of an atom consisting of neutrons and protons. There were several early models of the atom including Dalton's "plum pudding" model where it was assumed that all the parts were mixed together. It was Rutherford's experiments with bombarding atoms with high speed electrons that revealed that the atom must have a small central core where the positive charge was concentrated.2. Although the most commonly presented model of the atom in pre-college classes is the Rutherford-Bohr/planetary model, this is not strictly correct due to quantum mechanics.


The nucleus of an atom is composed of two types of smaller particles: positively charged protons, and neutral (no-charge) neutons. Both are about the same mass (neutrons are slightly more massive). The lowest mass stable hadron.


The electrically neutral particle found in an atomic nucleus. The neutron is unstable in isolation and decays into a proton, an electron, and an antineutrino.3


An electrically negative particle which "orbits" the nucleus of an atom. Electrons are approximately 2000 times lighter than protons or neutrons. The lightest stable lepton.


Experiments in the 1950s lead to the discovery of more and more heavy particles like neutrons and protons (but heavier). Patterns in the mass/charge/spin of those particles lead to the realization that they could all be explained by the existence of "smaller" particles which made up neutrons, protons, and all the rest. These were originally called "partons" but the name "quark" eventually came into common use.4 Quarks are odd particles in that they have never been observed in isolation; their existence is inferred from multiple experiments.


An atom (or nucleus) with the same charge (number of protons) but different number of neutrons is called an isotope. Think "p" for same "proton" number.


An atom (or nucleus) with the same number of neutrons, but different number of protons is called an isotone. Think "n" for the same neutron number. This term is not commonly encountered outside nuclear physics/nuclear chemistry.

Alpha Particle

The same as a helium nucleus. Generally only referred to as a alpha particle in the context of radiation, that is, when it is emitted with high energy. Technically, an ionized helium nucleus is an alpha particle, too.

Beta Particle

The same as an electron. Again, generally only referred to as a beta particle in the context of radiation. Static electricity involves electrons, but these are not generally referred to as beta particles.


High energy/high frequency/short wavelength light. The typical cut-off between ultra-violet and X-ray is taken to be at about 10 nm (nanometers), with the wavelength extending down to about 0.01 nm. These are sometimes broken into "soft" x-rays and "hard" x-rays with an ambiguous cut-off between the two around 0.1-0.3 nm.

Gamma Ray

Like X-rays but with evern higher energy/higher frequency/shorter wavelength. Many nuclear decays involve emission of gamma rays.


This is the process by which an atom loses/gains electrons so it is no longer electrically neutral. Electrically charged atoms are highly reactive which contributes to chemical changes and radiation damage.


When unstable nuclei decay, they emit radiation. This active process of emiting radiation is called radioactivity.


An unstable isotope which emits radiation. Certain isotopes are naturally unstable an decay.


An isotope which does not decay or emit radiation. Note that this is not a matter of it not currently emitting radiation, it is a matter of it not being able to.