How do we know that the nucleus is there and that it has a (+) charge?

An atom consists of a positively charged nucleus, surrounded by one or more negatively charged particles called electrons. The positive charges equal the negative charges, so the atom has no overall charge; it is electrically neutral.

Explanation:

Because the atom is neutral (it has no electrical charge), there must be equal numbers of positive and negative charges inside it, or equal numbers of protons and electrons.  The protons of an atom are all crammed together inside the nucleus. Each proton carries a positive charge, and like charges repel each other.

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