The Bohr Model PDF

The Bohr Model An atom is the smallest particle of an element. According to the classical Bohr model, an atom consists of a central nucleus surrounded by orbiting electrons, The nucleus consists of positively charged particles called protons and uncharged particles called neutrons. The basic particl...

The Bohr Model An atom is the smallest particle of an element. According to the classical Bohr model, an atom consists of a central nucleus surrounded by orbiting electrons, The nucleus consists of positively charged particles called protons and 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. Figure 1—1 an atom Atomic Number The atomic number equals the number of protons in the nucleus, which is the same as the number of electrons in an electrically balanced (neutral) atom. 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. Atomic numbers of all the elements are shown on the periodic table of the elements. Electrons and Shells Electrons orbit at discrete distances from the nucleus. Each orbit corresponds to a certain energy level. Electrons near the nucleus have less energy than those in more distant orbits The orbits are grouped into energy levels known as shells. Each shell has a fixed maximum number of electrons Ex: From the periodic table, The Bohr model of the silicon atom is shown in Figure 1–2. Notice that there are 14 electrons and 14 each of protons and neutrons in the nucleus. The Maximum Number of Electrons in Each Shell The maximum number of electrons (Ne) that can exist in each shell can be calculated by the formula Ne = 2n2 Where n is the number of the shell. Valence Electrons Electrons that are in orbits farther from the nucleus have higher energy and are less tightly bound to the atom than those closer to the nucleus. Electrons with the highest energy exist in the outermost shell of an atom. This outermost shell is known as the valence shell and electrons in this shell are called valence electrons. When a valence electron gains sufficient energy from an external source, it can break free from its atom. Ionization When an atom absorbs energy from a heat source or from light, for example, the energies of the electrons are raised. The valence electrons possess more energy and are more loosely bound to the atom than inner electrons, so they can easily jump to higher energy shells when external energy is absorbed by the atom. If a valence electron acquires a sufficient amount of energy, called ionization energy, it can actually escape from the outer shell and the atom’s influence. The departure of a valence electron leaves a previously neutral atom with an excess of positive charge (more protons than electrons). The process of losing a valence electron is known as ionization, and the resulting positively charged atom is called a positive ion. The escaped valence electron is called a free electron. The reverse process can occur in certain atoms when a free electron collides with the atom and is captured, releasing energy. The atom that has acquired the extra electron is called a negative ion. Electrical Balance When numbers of proton and electron are equal that is known as electrical balance. Unbalance atom will produce electrical charges. Many electrons produce a negative charge. Few electrons will produce a positive charge. Materials Used in Electronic Devices: In terms of their electrical properties, materials can be classified into three groups: Conductors, semiconductors, and insulators Insulators: An insulator is a material that does not conduct electrical current under normal conditions. Conductors: A conductor is a material that easily conducts electrical current. Semiconductors: A semiconductor is a material that is between conductors and insulators in its ability to conduct electrical current. Band Gap In solids, interactions between atoms split the valence shell into a group of energy levels called the valence band. The valence electrons are limited to this band. When an electron gains enough additional energy, it can leave its valence shell, become a free electron, and exist in what is known as the conduction band. The difference in energy between the valence band and the conduction band is called an energy gap or band gap. This is the amount of energy that a valence electron must have in order to jump from the valence band to the conduction band. Once in the conduction band, the electron is free to move throughout the material and is not tied to any given atom. An energy gap or a band gap is the difference between two energy levels and electrons are not allowed to be in this energy gap based on quantum theory. Although an electron may not exist in this region, it can "jump" across it under certain conditions. For insulators, the gap can be crossed only when breakdown conditions occur—as when a very high voltage is applied across the material. In semiconductors the band gap is smaller, allowing an electron in the valence band to jump into the conduction band if it absorbs external energy. The band gap depends on the semiconductor material. In conductors, the conduction band and valence band overlap, so there is no gap. This means that electrons in the valence band move freely into the conduction band, so there are always electrons available as free electrons. Comparison of a Semiconductor Atom to a Conductor Atom Silicon is a semiconductor and copper is a conductor. Bohr diagrams of the silicon atom and the copper atom are shown in Figure 1–8. Notice that the core of the silicon atom has a net charge of +4 (14 protons – 10 electrons) and the core of the copper atom has a net charge of +1 (29 protons – 28 electrons). Recall that the core includes everything except the valence electrons The valence electron in the copper atom “feels” an attractive force of +1 compared to a valence electron in the silicon atom which “feels” an attractive force of +4. Therefore, there is more force trying to hold a valence electron to the atom in silicon than in copper. The copper’s valence electron is in the fourth shell, which is a greater distance from its nucleus than the silicon’s valence electron in the third shell. Recall that, electrons farthest from the nucleus have the most energy. The valence electron in copper has more energy than the valence electron in silicon. This means that it is easier for valence electrons in copper to acquire enough additional energy to escape from their atoms and become free electrons than it is in silicon. In fact, large numbers of valence electrons in copper already have sufficient energy to be free electrons at normal room temperature.