Representing Valence Electrons with Dots

The Lewis structure of any element is the symbol for the element with the valence electrons represented as dots drawn aroxmd the element. The number of valence electrons is equal to the group number of the element (for main-group elements). 

Because valence electrons are so important to the behavior of an atom, it is useful to represent them with symbols. A Lewis dot diagram illustrates valence electrons as dots (or other small symbols) aroimd the chemical symbol of an element. Each dot represents one valence electron. In the dot diagram, the element s symbol represents the core of the atom—the nucleus plus aU the inner electrons. The Lewis dot diagrams for several elements are shown in Figure 2.26. 

The splitting of a Cl2 molecule is an initiation step that produces two highly reactive chlorine atoms. A chlorine atom is an example of a reactive intermediate, a short-lived species that is never present in high concentration because it reacts as quickly as it is formed. Each Cl- atom has an odd number of valence electrons (seven), one of which is unpaired. The unpaired electron is called the odd electron or the radical electron. Species with unpaired electrons are called radicals or free radicals. Radicals are electron-deficient because they lack an octet. The odd electron readily combines with an electron in another atom to complete an octet and form a bond. Figure 4-1 shows the Lewis structures of some free radicals. Radicals are often represented by a structure with a single dot representing the unpaired odd electron. 

It is time-consuming to draw electron arrangements using Bohr-Rutherford diagrams. It is much simpler to use Lewis structures to represent elements and the valence electrons of their atoms. To draw a Lewis structure, you replace the nucleus and inner energy levels of an atom with its atomic symbol. Then you place dots around the atomic symbol to represent the valence electrons. The order in which you place the first four dots is up to you. You may find it simplest to start at the top and proceed clockwise right, then bottom, then left. 

Because the hydrogen and chlorine atoms share a common pair of electrons, the two dots can be replaced with a dash to represent that a bond has been made. The valence dot diagrams can be rewritten as H—H and Cl—Cl. The bond that is represented by the dash is called a single bond because there is one pair of electrons being shared between the two atoms. 

Both ionic and covalent bonds involve valence electrons, the electrons in the outermost energy level of an atom. In 1920, G. N. Lewis, the American chemist shown in Figure 9, came up with a system to represent the valence electrons of an atom. This system—known as electron-dot diagrams or Lewis structures —uses dots to represent valence electrons. Lewis s system is a valuable model for covalent bonding. However, these diagrams do not show the actual locations of the valence electrons. They are models that help you to keep track of valence electrons. 

A Lewis structure shows only the valence electrons in an atom or molecule. The nuclei and the electrons of the inner energy levels (if any) of an atom are represented by the symbol of the element. With only one valence electron, a hydrogen atom has the electron configuration When drawing hydrogen s Lewis structure, you represent the nucleus by the element s symbol, H. The lone valence electron is represented by a dot. 

The two 5s and the two 5p electrons (the electrons in the orbitals related to the atom s highest principal energy level) represent tin s four valence electrons. Draw tin s electron-dot structure by representing its four valence electrons with dots, arranged one at a time, around the four sides of tin s chemical symbol (Sn). 

The American chemist G. N. Lewis introduced a useful model that describes the electronic structure of the atom and provides a starting point for describing chemical bonds. The Lewis model represents the valence electrons as dots arranged around the chemical symbol for the atom the core electrons are not shown. The first four dots are displayed singly around the four sides of the elemental symbol. If the atom has more than four valence electrons, their dots are then paired with those already present. The result is a Lewis dot symbol for that atom. The Lewis notation for the elements of the first two periods is... 

The Lewis model for covalent bonding starts with the recognition that electrons are not transferred from one atom to another in a nonionic compound, but rather are shared between atoms to form covalent bonds. Hydrogen and chlorine combine, for example, to form the covalent compound hydrogen chloride. This result can be indicated with a Lewis diagram for the molecule of the product, in which the valence electrons from each atom are redistributed so that one electron from the hydrogen atom and one from the chlorine atom are now shared by the two atoms. The two dots that represent this electron pair are placed between the symbols for the two elements ... 

The key to solving this type of problem is to look at the electron configuration of the element. Beryllium, with an atomic number of 4, has 4 electrons, giving it an electron configuration of Is2 2s2. The nucleus and the two electrons in the first energy level (Is2) make up the kernel of the atom, and they are represented by the elemental symbol in our Lewis dot notation. The two electrons in the second energy level (2s2) represent the valence electrons, which are represented by dots in the Lewis dot notation. So, to construct the proper notation, we write the elemental symbol and two dots to the left of the symbol, as shown here. 

Remember The kernel, which, in this case, is the nucleus and all of the electrons in the first two energy levels, will simply be represented by the elemental symbol (P). The five valence electrons (3s2 3p3) will be shown as dots around the symbol, with no more than two dots per side. 

Molecular structure in which the valence electrons are shown as dots placed between the bonded atoms, with one pair of dots representing two electrons or one (single) covalent bond, for example... 

Two sets of atomic symbols are shown on the facing page the gray symbols, with dots representing electrons, and the red symbols, with dashes representing valence bonds. Both sets are commonly used by chemists, the choice being made as indicated by convenience or habit. Often the two sets are combined, bonds being represented by dashes and unshared electrons by dots. 

Each valence electron is represented by one dot, thus, a lone atom of hydrogen would be drawn as an //with one dot, whereas a lone atom of Helium would be drawn as an He with two dots, and so forth. 

The covalent compound hydrogen fluoride, for example, would be represented by the symbol H joined to the symbol / by a single line, with three pairs (six more dots) surrounding the symbol F. The line represents the two electrons shared by both hydrogen and fluorine, whereas the six paired dots represent fluorine s remaining six valence electrons. 

Lewis structures (Lewis formulas or electron-dot formulas) are two-dimensional pictures of covalent species that show how the atoms are joined together with covalent bonds. A bond is shown as a pair of dots (2 dots = 2 electrons) or a dash (-), which represents a bonding or shared pair of electrons. A single dash represents two shared electrons two dashes (=) represent four shared electrons and so forth. In addition, Lewis structures also show the location of electron pairs not used in bonds, the nonbonding or unshared pairs of electrons. In a correct Lewis structure, all the valence electrons from every atom in the molecule or polyatomic ion must be accounted for, either in bonds or as nonbonding pairs (nb-pairs). 

Lewis dot symbol. The symbol of an element with one or more dots that represent the number of valence electrons in an atom of the element. (9.1)... 

Note particularly that the number of dots corresponds to the number of valence electrons in the outermost shell of the atoms of the element. Each unpaired dot (representing an unpaired electron) is available to form a chemical bond with another element, producing a compound. Figure 4.1 depicts the Lewis dot structures for the representative elements. 

Valence electrons Recall from Chapter 5 that an electron-dot structure is a type of diagram used to keep track of valence electrons. Electron-dot structures are especially helpful when used to illustrate the formation of chemical bonds. Table 7.1 shows several examples of electron-dot structures. For example, carbon, with an electron configuration of ls 2s 2p2, has four valence electrons in the second energy level. These valence electrons are represented by the four dots around the symbol C in the table. 

The symbol of the element represents the nucleus and all of the electrons except the outer valence shell. The valence electrons are represented by dots, and sometimes by crosses or circles. A key concept in building Lewis structural formulas is to satisfy the octet rule which states that many elements achieve stability by forming covalent bonds in order to fill their outer shell with eight electrons. 

The reacting atoms are represented with numbers of valence electrons equal to their group numbers. In the fluorine molecule, each atom is surrounded by a completed octet. The electron dot picture of the molecule, or Lewis formula, can be simplified by representing the bonding pair of electrons by a line between atoms, and the other pairs as dots surrounding the atoms f F—Ft. The pairs of electrons not shared in the covalent bond are called nonbonded electrons or lone pairs. 

---