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What is inside an atom?
The atom was already known to contain tiny particles called "electrons," which carry a negative electric charge. Rutherford found that the region at the center, which we now call the "nucleus," carries a positive charge. The electrons aren't in the nucleus, but instead surround it. So the atom is found to be in two parts: a tiny nucleus at the center, which contains a positive charge, and the surrounding region, which is sparsely populated by negatively charged electrons.The nucleus is populated by two kinds of particles: positively charged "protons," and electrically neutral particles, called "neutrons." These particles are nearly identical to each other, except in electric charge. Protons and neutrons ("nucleons," as they're referred to collectively) are much heavier than electrons, and that's why most of the mass of an atom is in the nucleus. An atom contains protons neutrons  and nucleii.

Atoms are classified according to the number of protons in their nuclei. For example, any atom with six protons in its nucleus is a carbon atom. Any atom with one proton is a hydrogen atom. 26 protons means iron, 92 protons for uranium, and so on.

The number of neutrons in an atom can vary, but is typically close to the number of protons, for light elements (atoms with small numbers of protons)


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