What is the relationship between atoms and molecules?

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Atoms are the smallest form of matter that still retain their properties. Atoms consist of protons, neutrons and electrons, with neutrons and protons forming their nucleus and electrons orbiting around it. For example, sodium (Na) is an atom and consists of 11 protons and electrons each, and 12 neutrons. 

Atoms combine with each other to form molecules.The combinations are in a particular ratio and can be with atoms of the same type or different types. For example, O2 is a molecule formed by the combination of two oxygen atoms. Water, H2O is a molecule formed by the combination of two different atoms- hydrogen (H) and oxygen (O) in a fixed 2:1 ratio.

When atoms combine with each other, they can do so by forming ionic bonds or covalent bonds. In the former, electrons are lost and gained by different atoms, as in sodium chloride, NaCl. In the case of a covalent bond, atoms share their electrons, as in the case of oxygen gas (O2).

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