# Atom and Its Structure

A basic understanding of the fundamental concepts 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 Fig.1(a).
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 Fig. 1(b). In all neutral atoms the number of electrons is equal to the number of protons. The mass of the electron is $9.11 \times 10^{-28} g$, and that of the proton and neutron is $1.672 \times 10^{-24} g$. 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 $2 \times 10^{-15} m$.
Fig. 1: The Hydrogen and Helium Atom