Atomic number and mass number
Atomic 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 an atom of an element is its The number of protons in the nucleus of an atom. Also called the proton number.. Remember that:
- all atoms of a given A substance made of one type of atom only. have the same number of protons
- atoms of different elements have different numbers of protons
An atom contains equal numbers of protons and Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons.. Since protons and electrons have equal and opposite charges, this means that atoms have no overall electrical charge.
For example, the atomic number of sodium is 11. Every sodium atom has 11 protons and 11 electrons. It has 11 positive charges and 11 negative charges.
Mass number
Atoms of different elements usually have different The number of protons and neutrons found in the nucleus of an atom., but they can be the same. For example, the mass number of argon atoms and calcium atoms can both be 40.
Calculating numbers of subatomic particles
The symbol for an atom can be written to show its mass number at the top, and its atomic number at the bottom.
To calculate the numbers of Objects found in atoms, including protons, neutrons and electrons. in an atom, use its atomic number and mass number:
- number of protons = atomic number
- number of electrons = atomic number
- number of neutrons = mass number - atomic number
Question
The atomic number of a sodium atom is 11 and its mass number is 23. Calculate the number of protons, neutrons and electrons it contains.
Number of protons = 11
Number of electrons = 11
Number of neutrons (mass number - atomic number) = 23 - 11 = 12