Atoms and Their Structure

A basic understanding of the fundamental concept of current and voltage requires a degree of familiarity with the atom and its structure. The simplest of all atoms is the hydrogen atom, made up of two basic particles, the proton and the electron, in the relative positions shown in given screenshot.

Hydrogen atom

The nucleus of the hydrogen atom is the proton, a positively charged particle. The orbiting electron carries a negative charge that is equal in magnitude to the positive charge of the proton. In all other elements, the nucleus also contains neutrons, which are slightly heavier than protons and have no electrical charge. The helium atom, for example, has two neutrons in addition to two electrons and two protons, as shown in given picture.

Helium atom

In all neutral atoms the number of electrons is equal to the number of protons. The mass of the electron is 9.11x10^-28 g, and that of the proton and neutron is 1.672x10^-24 g. The mass of the proton (or neutron) is therefore approximately 1836 times that of the electron. The radii of the proton, neutron and electron are all of the order of magnitude of 2x10^-15 m

For the hydrogen atom, the radius of the smallest orbit followed by the electron is about 5x10¹¹ m. The radius of this orbit is approximately
25,000 times that of the radius of the electron, proton, or neutron. This is approximately equivalent to a sphere the size of a dime revolving about another sphere of the same size more than a quarter of a mile away.

Different atoms will have various numbers of electrons in the concentric shells about the nucleus. The first shell, which is closest to the nucleus, can contain only two electrons. If an atom should have three electrons, the third must go to the next shell. The second shell can contain a maximum of eight electrons; the third, 18; and the fourth, 32; as determined by the equation 2n², where n is the shell number. These shells are usually denoted by a number (n 1, 2, 3, . . .) or letter (n k, l, m, . . .).

Each shell is then broken down into subshells, where the first subshell can contain a maximum of two electrons; the second subshell, six electrons; the third, 10 electrons; and the fourth, 14; as shown in fig. given below.

Shells and subshells of the atomic structure

The subshells are usually denoted by the letters s, p, d, and f, in that order, outward from the nucleus. It has been determined by experimentation that unlike charges attract, and like charges repel. The force of attraction or repulsion between two charged bodies Q1 and Q2 can be determined by Coulomb’s law:

Eq: 2.1

Coulomb’s law

(newtons, N) where F is in newtons, k = a constant ϭ 9.0 x 10⁹ N⋅m² /C² , Q1 and Q2 are the charges in coulombs, and r is the distance in meters between the two charges. In particular, note the squared r term in the denominator, resulting in rapidly decreasing levels of F for increasing values of r.

In the atom, therefore, electrons will repel each other, and proton sand electrons will attract each other. Since the nucleus consists of many positive charges (protons), a strong attractive force exists for the electrons in orbits close to the nucleus [note the effects of a large charge Q and a small distance r
in Eq. (2.1)]. As the distance between the nucleus and the orbital electrons increases, the binding force diminishes until it reaches its lowest level at the outermost subshell (largest r). Due to the weaker binding forces, less energy must be expended to remove an electron from an outer subshell than from an inner subshell. Also, it is generally true that electrons are more readily removed from atoms having outer subshells that are incomplete and, in addition, possess fewelectrons. These properties of the atom that permit the removal of elec-trons under certain conditions are essential if motion of charge is to becreated. Without this motion, this text could venture no further—our basic quantities rely on it.

Copper is the most commonly used metal in the electrical/electronics industry. An examination of its atomic structure will help identify why it has such wide spread applications. The copper atom (Screenshot Below)has one more electron than needed to complete the first three shells. This incomplete outermost subshell, possessing only one electron, and the distance between this electron and the nucleus reveal that the twenty-ninth electron is loosely bound to the copper atom. If this twenty-ninth electron gains sufficient energy from the surrounding medium to leave its parent atom, it is called a free electron. In one cubic inch of copper at room temperature, there are approximately 1.4x10²⁴ free electrons. Other metals that exhibit the same properties as copper, but to a different degree, are silver, gold, aluminum, andtungsten. Additional discussion of conductors and their characteristics can be found in next article.

The copper atom