When positively charged particles were radiated onto a gold atom, a few of the particles bounced back. which of the following caused this? positively charged protons in the nucleus of the gold atom positively charged electrons in the nucleus of the gold atom negatively charged protons around the nucleus of the gold atom negatively charged electrons around the nucleus of the gold atom

Answer is: Positively charged protons in the nucleus of the gold atom.

This was famous Rutherford's Gold Foil Experiment: he bombarded thin foil of gold with positive alpha particles (helium atom particles, consist of two protons and two neutrons).  

Rutherford observed the deflection of alpha particles on the photographic film and notice that most of alpha particles passed straight through foil.

Rutherford theorized that atoms have their charge concentrated in a very small nucleus.

radioactive decay occurs in unstable atomic nuclei – that is, ones that don't have enough binding energy to hold

the nucleus together due to an excess of either protons or neutrons.