Atomic number and mass number
Atomic number
The The number of protons in the nucleus of an atom. Also called the proton number. of an The smallest part of an element that can exist. is the number of Subatomic particle with a positive charge and a relative mass of 1. The relative charge of a proton is +1. in its 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.. Remember that:
- all the atoms of a given A substance made of one type of atom only. have the same atomic number
- the atomic number of each element is unique - no two elements have the same atomic number
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 An imbalance of electrons and protons in a material. An excess of electrons results in negative charge, a deficit of electrons results in positive charge., this means that atoms are An atom in which the number of protons and electrons is the same. overall.
For example, the atomic number of chlorine is 17. Every chlorine atom has 17 protons and 17 electrons. It has 17 positive charges and 17 negative charges, meaning that it is neutral overall.
Mass number
Atoms of different elements usually have different mass numbers, 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
An interactive lesson in calculating an atom's mass and atomic number
Question
The atomic number of a chlorine atom is 17 and its mass number is 35. Calculate the number of protons, neutrons and electrons it contains.
- Number of protons = 17
- Number of electrons = 17
- Number of neutrons = (35-17)=18
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