5.4 Defining the Nuclear Atom
Learning Objectives
By the end of this section, you will be able to:
- Summarize the structural characteristics of the nuclear atom
- Illustrate a simple model of the nuclear atom; locate its subatomic particles and their charges
- Explain why atoms have no overall charge
- Define ion and use the number of electrons lost or gained from an atom to calculate the overall charge of the ion and write the corresponding ion symbol.
The idea that matter is composed of tiny particles called atoms is at least 25 centuries old. It took until the twentieth century, however, for scientists to invent instruments that permitted them to probe inside an atom and find that it is not, as had been thought, hard and indivisible as Dalton theorized. Instead, experiments by Thomson, Rutherford, Chadwick, and other scientists revealed the atom is a complex structure composed of still smaller subatomic particles – electrons, protons, and neutrons.
Probing the Nuclear Atom
You have learned in the previous section that the first of these smaller particles were discovered by British physicist James (J. J.) Thomson in 1897. Named the electron, this particle is negatively charged. (It is the flow of these particles that produces currents of electricity, whether in lightning bolts or in the wires leading to your lamp.) Because an atom in its normal state is electrically neutral, each electron in an atom must be balanced by the same amount of positive charge.
The next step was to determine where in the atom the positive and negative charges were located. British physicist Ernest Rutherford devised the alpha-particle scattering, gold foil experiment that provided part of the answer to this question. The only way to account for the alpha particles that reversed direction when they hit the gold foil was to assume that nearly all of the mass, as well as all of the positive charge in each individual gold atom, is concentrated in a tiny centre or nucleus. When a positively charged alpha particle strikes a nucleus, it reverses direction, much as a cue ball reverses direction when it strikes another billiard ball. He termed this positive charge – a proton. Rutherford’s model also placed the other type of charge—the negative electrons—in orbit around this nucleus.
Rutherford’s model required that the electrons be in motion. Positive and negative charges attract each other, so stationary electrons would fall into the positive nucleus. Also, because both the electrons and the nucleus are extremely small, most of the atom is empty, which is why nearly all of Rutherford’s particles were able to pass right through the gold foil without colliding with anything. Rutherford’s model was a very successful explanation of the experiments he conducted, although eventually, scientists would discover that even the nucleus itself has structure.
Chadwick identified the neutron (n0). It has neither a positive nor a negative charge, so it is considered neutral. It was determi