Anatomy of the Atom: Protons, Neutrons, Electrons
An atom consists of a central nucleus containing positively charged protons and neutral neutrons, surrounded by negatively charged electrons that occupy the space around the nucleus. In brief: a proton has a charge of +1 and a relative mass of about 1 u, a neutron has no charge and a mass of about 1 u, and an electron has a charge of −1 and a mass of about 1/1,840 u; a neutral atom has equal numbers of protons and electrons.
The number of each subatomic particle in an atom determines the identity of the element, its mass and its chemical behavior. This article describes the three particles, compares their properties in a table, explains how to find the number of each particle from an element’s data, mentions what protons and neutrons are made of, works through several examples and gives the history of each discovery.
What are the three subatomic particles?
Protons are found in the nucleus and carry a positive charge. The number of protons defines the element: every atom with 6 protons is carbon. Neutrons are also found in the nucleus, have no electric charge and contribute to the mass and the stability of the nucleus. Electrons are much lighter and move in the space around the nucleus. They carry a negative charge and determine how atoms bond and react.
Properties of protons, neutrons and electrons
| Particle | Symbol | Relative charge | Charge (C) | Mass (kg) | Mass (u) |
|---|---|---|---|---|---|
| Proton | p or p+ | +1 | +1.602 × 10−19 | 1.673 × 10−27 | 1.0073 |
| Neutron | n or n0 | 0 | 0 | 1.675 × 10−27 | 1.0087 |
| Electron | e− | −1 | −1.602 × 10−19 | 9.109 × 10−31 | 0.000549 |
The atomic mass unit (u) is defined as one twelfth of the mass of a carbon-12 atom. Protons and neutrons are each about 1 u, and the electron’s mass is so small that it is usually ignored in mass number calculations.
How do you find the number of each particle?
- Protons = atomic number (Z), which is found on the periodic table.
- Electrons in a neutral atom = protons. In an ion, the number of electrons differs from the number of protons by the charge.
- Neutrons = mass number (A) − atomic number (Z). The mass number is the sum of protons and neutrons in a particular atom, and it is a whole number.
Worked examples
- Carbon-12. Z = 6, A = 12. Protons = 6, electrons = 6, neutrons = 12 − 6 = 6.
- Sodium-23. Z = 11, A = 23. Protons = 11, electrons = 11, neutrons = 12.
- Uranium-235. Z = 92, A = 235. Protons = 92, electrons = 92, neutrons = 235 − 92 = 143.
- From counts to the element. An atom has 17 protons, 18 neutrons and 17 electrons. Z = 17 identifies chlorine, and A = 17 + 18 = 35, so the atom is chlorine-35.
- Mass of an atom. The approximate mass of an atom of copper-63 is 63 u, and 63 × 1.661 × 10−27 kg = 1.05 × 10−25 kg. Electron mass is only 29 × 0.000549 = 0.016 u, or 0.025% of the total.
- Total charge. A neutral oxygen atom has 8 protons (+8) and 8 electrons (−8), so the net charge is 0.
| Atom | Atomic number Z | Mass number A | Protons | Neutrons | Electrons |
|---|---|---|---|---|---|
| Hydrogen-1 | 1 | 1 | 1 | 0 | 1 |
| Helium-4 | 2 | 4 | 2 | 2 | 2 |
| Oxygen-16 | 8 | 16 | 8 | 8 | 8 |
| Iron-56 | 26 | 56 | 26 | 30 | 26 |
| Gold-197 | 79 | 197 | 79 | 118 | 79 |
What holds the atom together?
The positively charged nucleus attracts the negative electrons by the electromagnetic force. Within the nucleus, protons repel each other, because they have the same charge. The strong nuclear force is a very strong attraction between nucleons at distances of about 1 to 3 femtometers, and it overcomes the electric repulsion. Neutrons add strong-force attraction without adding repulsion, which is why heavier nuclei need more neutrons than protons to be stable.
Are protons and neutrons made of smaller particles?
Yes. In the 1960s, experiments and theory showed that protons and neutrons are made of quarks. A proton contains two up quarks and one down quark, and a neutron contains one up quark and two down quarks. Up quarks have a charge of +2/3 and down quarks a charge of −1/3, which gives the proton +2/3 + 2/3 − 1/3 = +1 and the neutron +2/3 − 1/3 − 1/3 = 0. Electrons, as far as is known, are fundamental particles with no internal structure.
How were the particles discovered?
J. J. Thomson discovered the electron in 1897. The proton was identified by Ernest Rutherford in 1919, when he observed hydrogen nuclei emitted in a nuclear reaction, though positive rays had been studied earlier by Eugen Goldstein in 1886. James Chadwick discovered the neutron in 1932, because neutral particles are hard to detect, and received the Nobel Prize in Physics in 1935. The existence of quarks was supported by deep inelastic scattering experiments at SLAC in 1968 and the following years.
Which mistakes are common?
Mistake 1: Thinking that the number of neutrons equals the number of protons
This is true for light elements such as carbon-12, but heavier elements have more neutrons than protons.
Mistake 2: Using the atomic mass as the mass number
The average atomic mass on the periodic table is a decimal number. The mass number of a particular atom is a whole number.
Mistake 3: Counting electrons in the mass number
The mass number counts only protons and neutrons.
Mistake 4: Assuming that the number of protons changes in chemical reactions
Chemical reactions involve only the electrons. The number of protons defines the element and stays constant.