An atom is the smallest particle of an element that retains the characteristics of that element. Each of the known 110 elements has atoms that are different from the atoms of all other elements.
This gives each element a unique atomic structure. According to the classic Bohr model, an atom is visualized as having a planetary type of structure that consists of a central nucleus surrounded by orbiting electrons,
Every atom has a nucleus. The nucleus is located at the center of the atom. It contains positively charged particles called protons and uncharged particles called neutrons. Negatively charged particles called electrons orbit around the nucleus.
The number of protons in the nucleus of the atom is called the element’s atomic number. Atomic numbers distinguish one element from another.
Electron Shells and Orbits
Electrons orbit the nucleus of an atom at certain distances from the nucleus. Electrons near the nucleus have less energy than those in more distant orbits. It is known that only discrete (separate and distinct) values of electron energies exist within atomic structures. Therefore, electrons must orbit only at discrete distances from the nucleus.
The outer shell is called the valence shell, and the number of electrons it contains is the valence. The farther the valence shell is from the nucleus, the less attraction the nucleus has on each valence electron.
Thus the potential for the atom to gain or lose electrons increases if the valence shell is not full and is located far enough away from the nucleus.
Electrons in the valence shell can gain energy. If these electrons gain enough energy from an external force, they can leave the atom and become free electrons, moving randomly from atom to atom. Materials that contain a large number of free electrons are called conductors. below Figure compares the conductivity of various metals used as conductors.
Insulators, the opposite of conductors, prevent the flow of electricity. Insulators are stabilized by absorbing valence electrons from other atoms to fill their valence shells, thus eliminating free electrons. Materials classified as insulators are compared in below Figure . Mica is the best insulator because it has the fewest free electrons in its valence shell. A perfect insulator has atoms with full valence shells. This means it cannot gain electrons.
Halfway between conductors and insulators are semiconductors. Semiconductors are neither good conductors nor good insulators but are important because they can be altered to function as conductors or insulators. Silicon and germanium are two semiconductor materials.
An atom that has the same number of electrons and protons is identified as an electrically balanced atom. A balanced atom that receives one or more electrons is no longer balanced. It is said to be negatively charged and is called a negative ion. A balanced atom that loses one or more electrons is said to be positively charged and is called a positive ion. The process of gaining or losing electrons is called ionization. Ionization is significant in current flow.
- Introduction to Basic Electricity and Electronics Technology By Earl Gates
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