Structure of the atom
Protons, neutrons and electrons
The smallest part of an element that can exist. are very small, they have a radius of around 1Ă10-10 metres.
The modern view of the atom is of a positively-charged The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. The plural of nucleus is nuclei. containing Subatomic particle with a positive charge and a relative mass of 1. The relative charge of a proton is +1. and Uncharged subatomic particle, with a mass of 1 relative to a proton. The relative charge of a neutron is 0. with smaller Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. orbiting outside the nucleus.
Each particle has its own charge and its own mass.
| Relative charge | Relative mass | |
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | -1 | Close to 0 (1/2,000) |
| Proton | |
| Relative charge | +1 |
| Relative mass | 1 |
| Neutron | |
| Relative charge | 0 |
| Relative mass | 1 |
| Electron | |
| Relative charge | -1 |
| Relative mass | Close to 0 (1/2,000) |
Mass number and atomic number
Protons and neutrons are the heaviest particles in an atom and, as a result, they make up most of the The amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). of the atom. The mass of electrons is often not considered to be significant.
The number of protons is what defines the A substance made of one type of atom only., ie an atom with six protons in its nucleus will always be carbon, and uranium will always have 92 protons.
The total number of protons and neutrons is called the The number of protons and neutrons found in the nucleus of an atom. and the number of protons is called the The number of protons in the nucleus of an atom. Also called the proton number..
In a neutral atom, the number of electrons is always the same as the number of protons. If the atom becomes ionised however, the number of electrons will change. An ion is an atom that has lost or gained one or more electrons.
Using atomic symbols
Mass number and atomic number are two important pieces of information about an atom.
An atom can be represented using the symbol notation:
\(_{A}^{Z}\textrm{X}\)
Where:
- Z is the mass number
- A is the atomic number
- X is the symbol
For example, chlorine (Cl) can be shown as:
This symbol shows that chlorine has 35 particles in the nucleus (protons and neutrons), 17 of which are protons. It also tells us that chlorine has 18 neutrons (35 - 17) and, as the number of electrons and protons are equal in a neutral atom, chlorine also has 17 electrons.