Concept of Atom

The concept of matter is that it is formed by some discrete units & can not be divided in to smaller parts has been for milleniums, but it was based on philosophy not on experiments & observations. The concept of atom varied considerably over time between cultures, schools. The basic idea of the atom was adopted by scientists thousands of years later because it elegantly explained new discoveries in the field of chemistry.
The earliest mention of concept of atom was in 6th centuary BCE in ancient INDIA. The Nyaya and Vaisheshika schools developed elaborate theories of how atoms combined into more complex objects (first in pairs, then trios of pairs). The references to atoms in the West emerged a century later from Leucippus whose student, Democritus, systemized his views. In approximately 450 BCE, Democritus coined the term átomos (Greek ἄτομος), which means "uncuttable" or "the smallest indivisible particle of matter", i.e., something that cannot be divided. Although the Indian and Greek concepts of the atom were based purely on philosophy, modern science has retained the name coined by Democritus.
What's inside an atom?
Right in centre of every atom there is a dence core or "nucleus" containing two kinds of tiny particles protons & neutrons.Around nucleus are even tinier particles called electrons, whizzing around at the spped of light.
Atoms are about ten millionth of a millimeter across & wiegh 100 trillionth of a trillionth of a gram. The hydrogen atoms are smallest amogst all.
Protons have positive electrical charge while electrons have negative. They attract each other. Electron-electron or proton-proton repel each other as they have same charges.
Ions are those atoms which gained or lost one or more electrons. When atom looses electrons then it becomes positively charged called cations & when it gains electrons it becomes negatively charge called anions.