The work of Thompson, Rutherford, and Millikan established the modern picture of the atom, consisting of a nucleus with Z protons and N neutrons. The atom consists of the nucleus, which contains most of the mass and positive charge, surrounded by a cloud of Z electrons, which is negatively charged.
Atoms are thus neutral.
The Curies and Becquerel established that atoms were not always stable. Certain kinds of atomic nuclei can decay, as indicated in the text. The scattering of a moving particle off of a stationary particle may be viewed quantum mechanically as a diffraction process. Thus, the spreading angle of the diffracted wave is related to the ratio of the wavelength to twice the diameter of the stationary particle as for the hole in the screen. Thus, the smaller the hole, the larger the spreading angle for a given incident particle wavelength.