Structure of atom

Models of the Atom

Experimental data has been the impetus behind the creation and dismissal of physical models of the atom. Rutherford's model, in which electrons move around a tightly packed, positively charged nucleus, successfully explained the results of scattering experiments, but was unable to explain discrete atomic emission—that is, why atoms emit only certain wavelengths of light. Bohr began with Rutherford’s model, but then postulated further that electrons can only move in certain quantized orbits; this model was able to explain certain qualities of discrete emission for hydrogen, but failed completely for other elements. Schrödinger’s model, in which electrons are described not by the paths they take but by the regions where they are most likely to be found, can explain certain qualities of emission spectra for all elements; however, further refinements of the model, made throughout the 20th century, have been needed to explain all observable spectral phenomenon.

The present picture, or model, of atoms is as follows. An atom has a central nucleus, which is very small compared with the rest of the atom and contains most of the atomic mass (or weight). The nucleus carries a positive electric charge and is surrounded by a diffuse shell, or cloud, of negatively charged particles called electrons. The diameter of the atom is determined by the size of this electron cloud and is about 10^-8 cm (3.94 x 10^-9 in), whereas the nucleus is about 10^-12 cm (3.94 x 10^-13 in) in diameter. The size ratio of the atom to the nucleus is 10,000 to 1.

The simplest atom of all, hydrogen, has one particle—called a proton—in its nucleus. The mass of a proton is 1836 times the mass of an electron. A proton carries a positive electric charge with an assigned value of +1, and an electron carries a negative electric charge with an assigned value of -1. The atoms of other elements have more than one proton in their nucleus, and, in addition, other elements have another kind of nuclear particle called a neutron. The neutron has nearly the same mass as the proton, but the neutron has no electric charge.

The number of protons in the nucleus of an atom of a certain element determines the element’s atomic number. The number of protons in the nucleus can be determined by measuring the positive charge on the nucleus. For example, an atom with a nuclear charge of +26 has 26 protons in its nucleus and must be iron. To be electrically neutral, an atom of iron must have 26 electrons surrounding the nucleus.