Subatomic particles are the building blocks of atoms, the smallest units of matter that cannot be divided by any chemical means. But, they also exist separate from atoms.
Subatomic Particles of an Atom
By definition, subatomic particles are smaller than an atom. Atoms consist of three subatomic particles: protons, neutrons, and electrons. Each atom has a nucleus that contains protons and neutrons and has a positive electrical charge. Protons and neutrons are roughly the same mass as one another (neutrons are slightly more massive). Protons carry a positive charge, while neutrons are neutral. Electrons orbit outside the nucleus and carry a negative electrical charge. Electrons are very light and do not appreciably contribute to the mass of an atom. The electrical charge of a proton and an electron are equal but opposite (+1 and -1, respectively).
- Protons:
- Discovery: Discovered by Ernest Rutherford in 1919.
- Location: Reside in the nucleus of an atom.
- Mass: Approximately 1 atomic mass unit (amu).
- Electrical Charge: Positive (+1e).
- Neutrons:
- Discovery: Discovered by James Chadwick in 1932.
- Location: Also located in the nucleus.
- Mass: Slightly more than a proton, roughly 1 amu.
- Electrical Charge: Neutral (0e).
- Electrons:
- Discovery: Identified by J.J. Thomson in 1897.
- Location: Orbit around the nucleus in electron shells.
- Mass: Significantly lighter than protons and neutrons, roughly 1/1836 of an amu.
- Electrical Charge: Negative (-1e).
| Particle | Electric Charge | Atomic Charge | Mass (amu) | Atomic Mass | Spin |
|---|---|---|---|---|---|
| Proton | +1 | +1e | 1 | 1 | 1/2 |
| Neutron | 0 | 0e | 1.00866 | 1.00866 | 1/2 |
| Electron | -1 | -1e | 0.00054858 | 0.00054858 | 1/2 |
Shorthand Notation
Scientists often represent atoms of elements using shorthand notation that includes the element symbol or name, mass number, and sometimes the atomic number or proton number. The atomic number identifies the element. This notation allows for quick identification of the number of protons, neutrons, and electrons in an atom.
Isotopes
- Example: Carbon-14 (C-14)
- Proton number (Z) = 6 (from the periodic table for carbon).
- Electron number = Proton number = 6 (in a neutral atom).
- Neutron number = Mass number (14) – Proton number (6) = 8.
- Example: 146C
- Proton number (Z) = 6 (either from the lower number or from the periodic table for carbon).
- Electron number = Proton number = 6 (in a neutral atom).
- Neutron number = Mass number (14) – Proton number (6) = 8.
- Example: 14C (most common notation)
- Proton number (Z) = 6 (from the periodic table for C or carbon).
- Electron number = Proton number = 6 (in a neutral atom).
- Neutron number = Mass number (14) – Proton number (6) = 8.
Find the number of protons from the element symbol or atomic number (if given). Look to the left of the element name or symbol for determining the number of neutrons.
Ions
If the number of electrons differs from the number of protons, either a positive or negative charge appears as a superscript to the right of the element symbol. Atoms of the same element with different numbers of electrons are different ions. Note that if the charge is +1 or -1, the number “1” is omitted, leaving just + or -.
- Example: 146C–
- Proton number (Z) = 6 (either from the lower number or from the periodic table for carbon).
- Electron number is one more than the proton number = 6 + 1 = 7.
- Neutron number = Mass number (14) – Proton number (6) = 8.
- Example: 14C2+
- Proton number (Z) = 6 (from the periodic table for C or carbon).
- Electron number is two less than the proton number = 6 – 2 = 4.
- Neutron number = Mass number (14) – Proton number (6) = 8.
Atoms of the same element with different numbers of neutrons are different isotopes. Except when talking about pure isotopes, the neutron number is an average from samples taking in the Earth’s crust. For example, if you see Ca2+, the proton number (from the periodic table) for calcium is 20. The charge is 2+, so there are two more protons than electrons. So, the number of electrons is 18. The atomic mass (from the periodic table) for calcium is 40.08, which is the sum of protons and neutrons. So. the average neutron mass is 40.08 – 20 = 20.08. In other words, usually there are 20 neutrons in a calcium atom. Having an equal number of protons and neutrons is not necessarily the norm for all elements.
Other Basic Subatomic Particles
Other subatomic particles include antimatter particles (which can form antimatter atoms), alpha particles, and beta particles. Alpha and
- Antimatter:
- There are antimatter equivalents to protons, neutrons, and electrons.
- They are the antiproton, antineutron, and positron.
- Alpha Particles (α):
- Composition: 2 protons and 2 neutrons.
- Properties: Highly ionizing, relatively heavy.
- Notation: 42He or α.
- Beta Particles (β):
- Types: β- (electron) and β+ (positron).
- Properties: High energy, lighter than alpha particles.
- Notation: β- or β+.
Composite vs. Elementary Particles
Some subatomic particles are individual particles, while others consist of still smaller parts.
- Composite Particles: Made up of other particles.
- Examples: Protons and neutrons are composite particles, made of quarks.
- Elementary Particles: Fundamental particles that are not made up of smaller particles.
- Example: Electrons are elementary particles.
Classification of Subatomic Particles
Another way of classifying subatomic particles is according to their properties or functions:
- Bosons: Particles that carry forces. Examples include photons and gluons.
- Fermions: Particles that make up matter. Examples include electrons, quarks.
- Hadrons: Hadrons are composite particles made of quarks. Examples include protons and neutrons. Each hadron is either a boson or a fermion. The two types of hadrons are baryons and mesons. Baryons consist of three quarks, while mesons consist of one quark and one anti-quark.
Protons and neutrons are types of baryons, and thus are hadrons. Electrons are fermions. While protons and neutrons are subatomic particles that consist of smaller elementary particles, an electron is itself an elementary particle.
Elementary Particles of the Standard Model
The Standard Model of physics recognizes two general classes of particles: fermions and bosons. Fermions have half-integer spin and are either quarks or leptons. Quarks participate in all four interactions (strong, weak, electromagnetic, gravitation), while leptons are not involved in strong interactions. Bosons, on the other hand, have integer spin. Like quarks, they are involved in all four interactions.
- Quarks: Up, Down, Charm, Strange, Top, Bottom. There are also antimatter equivalents for each of these.
- Leptons: Electron, Muon, Tau, and their corresponding neutrinos. There are antimatter equivalents for each of these.
- Bosons: Photon, W and Z bosons, Gluon, Higgs boson are examples. Even bosons have antiparticles. However, the difference is mainly a matter of spin. For example, an anti-photon is a photon with -1 spin.
References
- Braibant, Sylvie; Giacomelli, Giorgio; Spurio, Maurizio (2012). Particles and Fundamental Interactions: An Introduction to Particle Physics (2nd ed.). Springer. ISBN 978-94-007-2463-1.
- Cottingham, W.N.; Greenwood, D.A. (2007). An Introduction to the Standard Model of Particle Physics. Cambridge University Press. ISBN 978-0-521-85249-4.
- Fritzsch, Harald (2005). Elementary Particles. World Scientific. ISBN 978-981-256-141-1.
- Rutherford, Sir Ernest (1920). “The Stability of Atoms”. Proceedings of the Physical Society of London. 33 (1): 389–394. doi: 10.1088/1478-7814/33/1/337