Electrons Negatively charged particles valence, 7, 8 Table

The addition of an electron to a neutral atom is described quantitatively by its electron affinity. This process produces a negatively charged particle called an anion, whose chemical properties bear no relationship to those of the neutral atom. In adding an electron to a neutral atom the principal force which operates is the attractive force of the positive nucleus. The attractive force of the positive nucleus decreases as the periods increase, because as additional electron shells are added to the atom, the outer electrons become more insulated from the nucleus by the intervening electron shells. It is easier to add an electron to the valence shell of fluorine (Period 2) than to that of chlorine (Period 3), and easier yet to add an electron to the valence shell of chlorine (Period 3) than to that of bromine (Period 4). The ease of forming anions within Group VII of the Periodic Table is F > Cl > Br > I. 

Brittle materials such as inorganic salts are held together by electrostatic forces. In this case, the valence electrons from the metal transfer to the nonmetal, and the two resulting positively and negatively charged particles are held together electrostatically (ex., table salt, NaCl,... 

A periodic table of the elements siunmarizes a number of important characteristics of neutral atoms. The iron atom (Fig. 8.1) has 26 planetary electrons in 4 shells and 26 protons plus 26 neutrons in the nucleus. This electrically neutral particle is about 0.01 microinches (pin.) in diameter. The outermost planetary electrons (valence electrons) are special. They determine the chemical properties of the element. Metals have few valence electrons but the nonmetals have several. When a metal loses its valence electrons, it becomes an ion. A positively charged ion is called a cation. When a nonmetal takes on valence electrons to bring the total munber to eight, it too becomes an ion (in this case a negative particle called m anion). Cations (positively charged particles) are considerably smaller than neutral atoms, while anions are relatively larger than neutral atoms. 

A semiconductor is a material having electrical conductivity between a conductor and a nonconductor. Silicon, which is in the fourth column of the periodic table (valence = 4), is normally a nonconductor. It may be converted to a semiconductor by diffusing a small amount ( 1 part in 10 ) of boron (valence = 3) or phosphorous (valence 5) throughout its structure. This is called doping. When boron is the dopant, it is called a positive type (p-type) semiconductor but when phosphorous is the dopant it is called an n-type semiconductor. When a potential difference (voltage) is applied across an n-type semiconductor, the unattached electrons where phosphorous atoms are located move toward the positive terminal. When a boron atom is in a p-type semiconductor, there is an unfilled bond site called a hole. Holes tend to act as positively charged particles and move toward the negative terminal when a potential difference is applied. 

---