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An atom is the smallest particle of an element which may or may not have an independent existence. It is the building blocks of elements. All atoms combine to form molecules. There are numerous atoms in elements. 
An atom consists of protons,neutrons and electrons. There is a nucleus further inside the atom which contains the protons and neutrons. 
Protons carry positive charge and have unit mass,i.e., 1 a.m.u. (atomic mass unit). Neutrons carry no charge/neutral and have unit mass.
The atom is surrounded by orbits or shells around which electrons revolve. Electrons have negative charge and have unit mass.
In a neutral atom, the number of protons is equal to number of electrons. But it happens not in all the atoms of elements.
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The atom is the smallest unit that defines the chemical elements and their isotopes. Most matter on Earth is made up of atoms. The size of atoms is measured in picometers (trillionths of a meter).
For an example, a piece of pulse just contains millions of not see them but they are there in it.

Every atom has a nucleus, the protons and neutrons are present inside the nucleus and the electrons are present outside in shells having different capacity of having an electron.

the atom's mass is present in its nucleus because the electrons have negligible mass, therefore the mass of an atom is made by its protons and neutrons well known as nucleoids. 
the protons have positive charge and the electrons have negative charge whereas the neutrons have no charges.
the number of protons and electrons are equal in every atom compared to its neutrons!.
Atoms can attach themselves to each other by chemical bonds to form molecules...

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