OBJECTIVE: To learn about the internal parts of an atom. To understand Rutherford’s experiment to characterize the atom’s structure.
Dalton’s atomic theory, proposed in about 1808, provided such a convincing explanation for the composition of compounds that it became generally accepted. Scientists came to believe that elements consist of atoms and that compounds are a specific collection of atoms bound together in some way.
A physicist in England named J. J. Thomson showed in the late 1890s that the atoms of any element can be made to emit tiny negative particles. (He knew the particles had a negative charge because he could show that they were repelled by the negative part of an electric field.) Thus he concluded that all types of atoms must contain these negative particles, which are now called electrons. On the basis of his results, Thomson wondered what an atom must be like. Although he knew that atoms contain these tiny negative particles, he also knew that whole atoms are not negatively or positively charged. Thus he concluded that the atom must also contain positive particles that balance exactly the negative charge carried by the electrons, giving the atom a zero overall charge. |
He concluded that the nucleus must have a positive charge to balance the negative charge of the electrons and that it must be small and dense.
By 1919 Rutherford concluded that the nucleus of an atom contained what he called protons. A proton has the same magnitude (size) of charge as the electron, but its charge is positive. We say that the proton has a charge of 1+ and the electron a charge of 1-.
Rutherford reasoned that the hydrogen atom has a single proton at its center and one electron moving through space at a relatively large distance from the proton (the hydrogen nucleus). He also reasoned that other atoms must have nuclei (the plural of nucleus) composed of many protons bound together somehow. In addition, Rutherford and a coworker, James Chadwick, were able to show in 1932 that most nuclei also contain a neutral particle that they named the neutron. A neutron is slightly more massive than a proton but has no charge.
Rutherford reasoned that the hydrogen atom has a single proton at its center and one electron moving through space at a relatively large distance from the proton (the hydrogen nucleus). He also reasoned that other atoms must have nuclei (the plural of nucleus) composed of many protons bound together somehow. In addition, Rutherford and a coworker, James Chadwick, were able to show in 1932 that most nuclei also contain a neutral particle that they named the neutron. A neutron is slightly more massive than a proton but has no charge.