Chemical Bonding: How Do Atoms Combine? What Are the Forces That Bind the Atoms Together?
To understand how elements are combined to form compounds, it is necessary to understand the structure of atoms. Atoms consist mainly of electrically charged particles called electrons and protons. Each electron has negative charge and each proton has a positive charge. Neutrons, which are also present in atoms, have no charge. Normally, an atom contains as many electrons as protons. The negative charges and the positive charges balance each other and the atom is neutral (uncharged). If the balance between electrons and protons is upset, the atom becomes an electrically charged unit called anion. An atom becomes positive ion if it loses one or more electrons and they are called cation. For example, when a hydrogen atom loses its single electron. It becomes a positive hydrogen ion (H+). A negative chlorine ion (Cl-) is a chlorine atom with one additional electron.
Electrons revolve in various distances from the nucleus of an atom. The path of the electron forms a series of shells with the nucleus at the center. Each succeeding shell is farther from the nucleus from the one below it. Scientist has found that each shell can contain no more than a certain number of electrons. The first shell holds no more than 2 electrons. The second can hold 8; the third, no more than 18 and so on. Most interactions among atoms take place in the outermost shell of each atom. The number of each electron in this shell determines how an atom combines with other atoms to form compounds. When atoms combine they gain, lose or share electrons in such a way that the outer shells become chemically complete.
Valence is the property related to the electrons in an atom’s outer shell. The valence of an element is the number of electrons the elements gain or loses when it forms compounds with other elements. Electrons in the outermost shell are called valence electron.
What is chemical bonding?
Atoms, in a sense, are tied together to form molecules. The atoms of molecules are linked together through a reaction known as chemical bonding. A chemical bond is a force that holds atom together. How do atoms combine? What are the forces that bind them? These questions are fundamental in the study of chemistry since a chemical reaction is essentially an alteration of chemical bonds. An important clue to the understanding of the driving force for chemical bonding was the discovery of the noble gases and their apparently inert chemical behavior. Elements tend to attain this configuration of completely filled outer shells in order to gain stability.
The transfer or sharing of electrons of the atoms in a compound forms a linkage between them which chemists call the chemical bond. There are two types of chemical bonds, (1) ionic bond and (2) covalent bond.
In order to acquire an inert gas configuration, there is a need for 8 electrons to occupy the s p distribution in the highest energy level of an atom.
Consider the individual elements Na and Cl. Sodium has the electronic configuration:
Na = 1s2 2s2 2p6 3s1
And its outer shell configuration is 3s
Cl = 1s2 2s2 2p6 3s2 3p5
And its outer shell configuration is 3p5
How could Na and Cl attain the outer-shell octet?
There are three possible ways for any atom to take in pursuit of an octet:
1. Electrons could be given up to some other atoms or group of atoms.
2. Electrons could be gained from some other atoms.
3. Electrons could be shared between two atoms.
The three choices are depicted in the figure below. Apply these choices to sodium and chlorine.
Let us consider first Sodium and apply each of these choices:
In the first choice, if the 3s1 is lost, the second shell becomes the outer-shell, with a configuration of 2s2 2p6, an outer-shell octet. The sodium now has 11 protons and 10 electrons, giving it a net charge of +1 (Na +1).
For the second possibility, a total of 7 electrons would have to be gained to produce the outer-shell octet3s2 3p6. Each time an electron is gained, the Na atom acquires one unit of negative electrical charge, therefore, a gain of seven electrons produces a net charge of -7, which is noted as Na -7.
If the third choice is taken and electrons are shared, Sodium could provide one electron (the 3s1) and the other atom(s), would have to provide a total of seven more.
Now which of the three possibilities will Na choose?
In general, atoms will follow the "course of action" which results in the most stable situation - the lowest energy state. It is difficult for any atom to find other atoms, which will give up a total of 7 electrons.
Also, the Na -7 is not stable, because the 11 protons of Sodium could not be able to exert a strong force of attraction to hold on to the 18 electrons. And in an attempt to share electrons, Sodium will have the trouble finding atoms, which have the trouble finding atoms, which must provide the majority of electrons shared. Figure 6-2 illustrates these points.
Therefore, the best possibility for Na to achieve an outer-shell octet is the loss of one electron to form Na +1.
Apply the same type of reasoning to chlorine atom. Because there are seven electrons in the outer energy level, Chlorine needs only one electron to complete an octet in the third energy level. Therefore, the possibility that Cl will follow most likely is by gaining an electron from some other atom, forming Cl-1. Since an electron has been gained, the configuration of Chlorine ion is:
Cl - 1s2 2s2 2p6 3s2 3p6
Outer-shell octet structures of Na and Cl
Ionic or electrovalent bond
An ionic bond is formed in a compound when the electrons from the outermost shell of an atom are actually transferred to the outermost shell of the combining atom.
This transfer occurs from one who has a lesser attraction to one who has a greater attraction for electrons. After the transfer has occurred, the atom, which gained the electron(s), now contains more electrons than protons thus, it is negatively charge.
That one from which the electron(s) have been removed has more protons than electrons and is therefore positively charged. These charged particles are called ions. A positively charged ion is called cation, and a negatively charged ion is called an anion. Since these ions have opposite charges, there is an attractive force between them. This attractive force constitutes the ionic bond otherwise called electrovalent bond. However, the ions are free and exist as separate particles whether they are in dissolved or solid form. A typical example of an ionic or electrovalent bond is the bond formed between sodium and chlorine atoms when they enter into chemical combination.
An illustration of Ionic Bonding
Illustrations of covalent bonds
Some compounds are formed when electrons are shared between two atoms to fill up the incomplete outer shell of both in order to attain the stable configuration of an inert gas. This usually occurs when reaction takes place between atoms of Group IV, V, and VII. The chemical bond in which two atoms share a pair of electron and form molecules is called covalent bond. The atoms of covalent compounds are not free like those in ionic compounds. They are tightly linked to one another by the covalent bond. Hence each independent particle is a combination of the atoms.
What is the nature of the bond formed between H and F in the molecule HF?
The electron configurations:
Hydrogen (H) 1s1
Fluorine (F) 1s2 2s2 2p5
Make clear that H needs one electron to attain a stable 1s2 outer shell configuration, and F needs one electron to attain an octet. Since neither can easily lose electron, sharing occurs and a covalent bond is formed.
Covalent bond is the bond formed in which two atoms share a pair of electrons and form molecules. The bond that results whenever unequal sharing occurs is called polar covalent bond while equal sharing of electrons is called non polar covalent bond.
Chemical bonds are produced when the outer-shell electrons are either transferred or shared from one atom to another. The formation of chemical bonds usually enables an atom to acquire a chemically stable outer-shell consisting of an octet of electrons. There are two types of chemical bonds. (1) Ionic bond, in which electrons are actually transferred from the outer-shell of one atom to the second atom. The resulting particles are ion – atom or groups of atoms with an unbalance electrostatic charge. (2) Covalent bond, in which two atoms share a pair of electrons and form molecules. The bond that results whenever unequal sharing occurs is called a polar covalent bond. Equal sharing of electrons is called non polar covalent bond.
This two minute animation describes the Octet Rule and explains the difference between ionic and covalent bonds.
Questions for Study and Review
A. Classify the bond formed by the following pairs of atoms as ionic or covalent
- Silicon and Fluorine
- Boron and Carbon
- Lithium and Chlorine
- Hydrogen and Oxygen
- Aluminum and Chlorine
- Magnesium and Nitrogen
- Cesium and Bromine
- Hydrogen and Iodine
B. Draw the Lewis Dot Structure of the following compounds: