Consider the 90 or so elements in the periodic table.
Are these 90 different and distinct elements the building blocks of the universe?
Or are there some simpler building blocks which are common to all the elements?
William Prout, physician and chemist of Edinburgh and London suggested in 1816 that all atoms are built of hydrogen, believing that all atomic weights were multiples of the atomic weight of hydrogen. At the time, values were rough, and his hypothesis seemed reasonable.
More accurate measurements showed chlorine to have atomic weight about 35.5 and boron to have atomic weight of about 10.8, so the hypothesis was discounted. When it was realised that the atomic weights were an average of the weights of isotopes, e.g. Cl = 35.5 is made up of Cl-35 and Cl-37, it became apparent that Prout's hypothesis contained an element of truth.
We now believe that
all atoms have nuclei consisting of protons and neutrons;
electrons occupy the space around the nucleus of each atom;
the universe probably consisted originally of hydrogen;
all the other elements have been synthesised in the stars.
What is the foundation of these beliefs?
Identifying the electron
The photoelectric effect
Three types of radiation
Atoms emit electromagnetic radiation
Electric charges in the atom