Atomic number and mass number
The number of Subatomic particle with a positive charge and a relative mass of 1. The relative charge of a proton is +1. in the nucleus of an All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. is called its atomic number:
- the atoms of a particular A substance made of one type of atom only. all have the same number of protons
- the atoms of different elements have different numbers of protons
The total number of protons and Uncharged subatomic particle, with a mass of 1 relative to a proton. The relative charge of a neutron is 0. in an atom is called its mass number.
The atomic number is shown below the chemical symbol, and the mass number is shown above. In this example the atomic number is 17 and the mass number is 35. This means that each of these atoms has:
- 17 protons
- 17 electrons
- 35 – 17 = 18 neutrons
Isotopes
Isotopes are atoms of an element with the normal number of protons (and Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons.), but different numbers of neutrons. Isotopes have the same atomic number, but different mass numbers.
Three isotopes of hydrogen
Most hydrogen atoms consist of just one proton and one electron, but some also have one or two neutrons.