Question

The first subatomic particle discovered by scientists was the negatively charged electron. Why did scientists conclude that the atom also contained positively charged particles?

Answer 1

It had long been known that the atom was electrically neutral. Thus, if one part was negative, some other aspect had to carry positive charge.

Explanation:

In fact, Thomson's first "guess" was that the bulk of the atom was a positively-charged ball of fluid-like matter, with just enough positive charge to cancel the negative charge on the electrons that he envisioned as being stuck into the fluid "like raisins in a pudding".

The awareness of positive particles within the atoms came about ten years later, thanks the Rutherford's famous alpha-particle experiment.