Study Guide

Modern Physics Terms

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Modern Physics Terms


A subunit of a single element that consists of a nucleus and electrons surrounding the nucleus.


Being made of discrete intervals, rather than a continuous spread. Think of packaged (or discrete) food snacks versus an all you can eat buffet.

Planck's Constant

The number correlating the energy of light to its frequency as well as the wavelength of light to its momentum. It appears elsewhere in quantum physics too.


A negatively charged atomic particle.

Alpha particle

An alpha male particle with not 1 but 2 positive charges because it's a Helium nucleus. That thing is unstoppable. Okay, not literally.


The hallmark of a eukaryotic cell, where the majority of the DNA is housed and replication and transcription of this DNA takes place.


A positively charged atomic particle. The number of protons in an atom determines the identity of the atom; for example, carbon atoms always have 6 protons.


A subatomic particle with no charge that resides in the nucleus of an atom. The mass of a neutron is essentially the same as the mass of a proton. Nothing but a chargeless copycat.


Particles that make up the nucleus of an atom. Neutrons and protons are both nucleons. Now, say it three times. Faster!

Mass number

The number of protons and neutrons in an isotope of a chemical element.

Atomic Number

The atomic number is the number of protons in an atomic nucleus.

Binding energy

The amount of energy that ensures the captivity of electrons, protons, and neutrons within an atom or nucleus. It changes with the mass number and proton number.


An atom or molecule that has obtained a charge by either gaining or losing one or more electrons. This means that the proton number does not match the number of electrons giving the atom either a net negative or positive charge.

Ionizing energy

The energy required to create an ion from a neutral atom. In other words, we might say it's a superhero photon that frees an electron from its atomic prison. Or, it's the energy required to force an electron to join a previously neutral atom.

Ground state

Think of it as being grounded. The electrons have their lowest possible energies.

Excited state

An electrons party mode. As opposed to occupying their lowest possible energy (ground state), the electrons occupy higher energy levels usually while dancing.


Variant of a chemical element that has the same number of protons, but different numbers of neutrons.


The letter n stands for neutron. Two atoms of different elements that have the same number of neutrons are isotones.


A light "particle." It's really more of a package with a frequency, wavelength, and energy, but "particle" will do.


It's what it sounds like, but with atomic particles both scattered and doing the scattering.

Back scattering

The scattering of particles in the opposite direction from the one they were traveling: a subatomic (or atomic) about-face.


The study of matter interacting with light. It leads to emission and absorption spectrums, among other things.

Emission spectrum

The "fingerprints" of an element emitting light at specific wavelengths corresponding to the energy released as electrons drop from an excited state into a state with less energy, including but not limited to the ground state.

Absorption spectrum

The "fingerprints," so to speak, of any element, each of which absorbs light at specific wavelengths unique to that element. Yeah, it's just like the emission spectrum for a given element but in reverse.

Nuclear force

The nuclear glue that binds the nucleus together.

Strong force

The strong(er) part of the nuclear force. The Jedi Force.

Weak force

The weak(er) part of the nuclear force. Not the Force but something near it.

Mass defect

We'd think the nucleus mass would equal the mass of its nucleons. It doesn't. This discrepancy is the mass defect. How very deceptive, or even defective.

Stable isotope

An isotope with a stable nucleus is like a stable adult. He (or she) is in a stable relationship, job, income, house, etc. Life is good. No fireworks.


When an adult isotope starts to spend too much money and radiates it away, it will decay into another isotope, with less money but hopefully more stable.

Alpha radiation

One or more alpha particles emitted in nuclear decay of some kind.

Beta radiation

One or more beta particles (high speed electrons or positrons) emitted in some types of radioactive decay .

Gamma radiation

Gamma rays, high energy photons, a result of radioactivity. This radiation is no-one's friend.


Number of decays per second from a radioactive substance.


A unit of radioactivity. Also, fun word to say out loud.

Mean lifetime, τ

The average lifetime of a radioactive particle. It varies by particle: milliseconds to millennia. Actually, it varies beyond those extremes..

Decay constant

The inverse or opposite of a lifetime. If something doesn't live long, it decays quickly.


The time required for a radioactive isotope to decay by half in number. If you have 100% carbon-14 in a substance, the half-life would be the time to 50% carbon-14.


Tiny, quirky particles that make up nucleons, as such they are one of the basic building blocks of matter.

Up quark

A type of quark with a charge.

Down quark

A type of quark with a charge.


A nearly massless neutral elementary particle.


An opposite particle; a particle of the same mass but with an opposite charge. Like a nemesis twin.


Matter made of anti-particles.


The electron's nemesis.

Neutrino flavor

The "type" of neutrino. A very pungent neutrino spice: care to have your neutrino soup flavored with electrons, muons, or taus?

Matter annihilation

Matter destruction, as a particle and its antiparticle confront each other head on. Sparks fly if by sparks we mean photons.

Pair production

The production of a pair of elementary particles.


A family of particles which obey the weak nuclear force while ignoring the strong nuclear force. Like quarks, they're basic building blocks of matter.


A family of particles, including baryons and mesons, which obey the strong nuclear force.

Composite particle

A particle composed of other parts.

Elementary particle

An unbreakable particle.


A composite particle made of 3 quarks.


Type of asexual reproduction where the animal splits in half (or sometimes more parts).


An explosion reaction that results from two particles having a melt down, well, melding into one particle.

Thermonuclear weapon

An atomic weapon that blows up, causes great woe of unimaginable size, and uses both fusion and fission. Whoa.

Radiation fall-out

A toxic and radiative cloud of dust after a nuclear weapon blows up. It's dangerous, poisonous, cancerous, and any other bad adjective you can think of.

Electron capture

The capture of an electron by a nucleus.

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