The Bohr Model
An atom* is the smallest particle of an element that retains the characteristics of that element. Each of the known 118 elements has atoms that are different from the atoms of all other elements. This gives each element a unique atomic structure. According to the classical Bohr model, atoms have a planetary type of structure that consists of a central nucleus surrounded by orbiting electrons, as illustrated in Figure 1–1. The nucleusconsists of positively charged particles called protonsand uncharged particles called neutrons.The basic particles of negative charge are called electrons.
Each type of atom has a certain number of electrons and protons that distinguishes it from the atoms of all other elements. For example, the simplest atom is that of hydrogen, which has one proton and one electron, as shown in Figure 1–2(a). As another example, the helium atom, shown in Figure 1–2(b), has two protons and two neutrons in the nucleus and two electrons orbiting the nucleus.
All elements are arranged in the periodic table of the elements in order according to their atomic number. The atomic numberequals the number of protons in the nucleus, which is the same as the number of electrons in an electrically balanced (neutral) atom. For example, hydrogen has an atomic number of 1 and helium has an atomic number of 2. In their normal (or neutral) state, all atoms of a given element have the same number of electrons as protons; the positive charges cancel the negative charges, and the atom has a net charge of zero. *All bold terms are in the end-of-book glossary. The bold terms in color are key terms and are also defined at the end of the chapter.
All matter is composed of atoms; all atoms consist of electrons, protons, and neutrons except normal hydrogen, which does not have a neutron. Each element in the periodic table has a unique atomic structure, and all atoms within a given element have the same number of protons. At first, the atom was thought to be a tiny indivisible sphere. Later it was shown that the atom was not a single particle but was made up of a small dense nucleus around which electrons orbit at great distances from the nucleus, similar to the way planets orbit the sun. Niels Bohr proposed that the electrons in an atom circle the nucleus in different obits, similar to the way planets orbit the sun in our solar system. The Bohr model is often referred to as the planetary model. Another view of the atom called the quantum modelis considered a more accurate representation, but it is difficult to visualize. For most practical purposes in electronics, the Bohr model suffices and is commonly used because it is easy to visualize.
After completing this section, you should be able to
❏ Describe the structure of an atom
◆ Discuss the Bohr model of an atom ◆ Define electron, proton, neutron, and nucleus
❏ Define atomic number
❏ Discuss electron shells and orbits
◆ Explain energy levels
❏ Define valence electron
❏ Discuss ionization
◆ Define free electronand ion
❏ Discuss the basic concept of the quantum model of the atom Niels Henrik David Bohr (October 7,
1885–November 18, 1962) was a
Danish physicist, who made
important contributions to
understanding the structure of the
atom and quantum mechanics by
postulating the “planetary” model
of the atom. He received the Nobel
prize in physics in 1922. Bohr drew
upon the work or collaborated
with scientists such as Dalton,
Thomson, and Rutherford, among
others and has been described as
one of the most influential
physicists of the 20th century.
THEATOM ◆ 3
Electron Proton Neutron
The Bohr model of an atom showing electrons in orbits around the nucleus, which consists of protons and neutrons. The “tails” on the electrons indicate motion. (a) Hydrogen atom (b) Heliumatom
Two simple atoms, hydrogen and helium.
Atomic numbers of...
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