When Ernest Rutherford shot a beam of alpha particles (helium nuclei) at layers of thin, gold leaf, the majority of these alpha particles made it through and some were deflected. Due to these findings, he concluded that most of the mass of atoms was found in a positively charged region called the nucleus and was surrounded by negative electrons. He deduced this because a small number of alpha particles were repelled by the positively charged nucleus. The nucleus was believed to be small, due to the small number of repelled alpha particles. He did this work in 1911 and pioneered the Rutherford model of the atom, based on his Rutherford scattering discovery.