Chemistry · Structure 1: Models of the particulate nature of matter. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
An atom has a very small, dense, positively charged nucleus made of protons and neutrons. Almost all the mass is there. The electrons move around the nucleus in a space that is mostly empty.
The atomic number Z is the number of protons and decides which element it is. The mass number A is protons plus neutrons. A neutral atom has as many electrons as protons; an ion has lost or gained electrons.
Isotopes are atoms of the same element with different numbers of neutrons. They have the same chemical properties, because the electrons are the same, but different physical properties such as mass and density. The relative atomic mass is the weighted average of the isotopes.
Changing the number of protons when an atom becomes an ion. Only the electrons change: a positive ion has lost electrons, a negative ion has gained them.
Answer: Ar = 63.6 (no unit)
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