Carbon is represented with four unpaired electrons see Fig. If carbon can share four electrons with other atoms, its valence shell will be full. Most elements involved in covalent bonding need eight electrons to have a complete valence shell.
One notable exception is hydrogen H. Hydrogen can be considered to be in Group 1 or Group 17 because it has properties similar to both groups. Hydrogen can participate in both ionic and covalent bonding. When participating in covalent bonding, hydrogen only needs two electrons to have a full valence shell. As it has only one electron to start with, it can only make one bond. Hydrogen is shown in Fig 2. In the formation of a covalent hydrogen molecule, therefore, each hydrogen atom forms a single bond, producing a molecule with the formula H 2.
A single bond is defined as one covalent bond, or two shared electrons, between two atoms. A molecule can have multiple single bonds. For example, water, H 2 O, has two single bonds, one between each hydrogen atom and the oxygen atom Fig. Figure 2. Sometimes two covalent bonds are formed between two atoms by each atom sharing two electrons, for a total of four shared electrons. For example, in the formation of the oxygen molecule, each atom of oxygen forms two bonds to the other oxygen atom, producing the molecule O 2.
Similarly, in carbon dioxide CO 2 , two double bonds are formed between the carbon and each of the two oxygen atoms Fig. In some cases, three covalent bonds can be formed between two atoms. The most common gas in the atmosphere, nitrogen, is made of two nitrogen atoms bonded by a triple bond.
Each nitrogen atom is able to share three electrons for a total of six shared electrons in the N 2 molecule Fig. In addition to elemental ions, there are polyatomic ions.
Polyatomic ions are ions that are made up of two or more atoms held together by covalent bonds. Polyatomic ions can join with other polyatomic ions or elemental ions to form ionic compounds. It is not easy to predict the name or charge of a polyatomic ion by looking at the formula. Polyatomic ions found in seawater are given in Table 2. Polyatomic ions bond with other ions in the same way that elemental ions bond, with electrostatic forces caused by oppositely charged ions holding the ions together in an ionic compound bond.
Charges must still be balanced. For example, in Fig. In Figure 2. P olyatomic ions can bond with monatomic ions or with other polyatomic ions to form compounds. In order to form neutral compounds, the total charges must be balanced. A molecule or compound is made when two or more atoms form a chemical bond that links them together.
As we have seen, there are two types of bonds: ionic bonds and covalent bonds. In an ionic bond, the atoms are bound together by the electrostatic forces in the attraction between ions of opposite charge. Ionic bonds usually occur between metal and nonmetal ions. For example, sodium Na , a metal, and chloride Cl , a nonmetal, form an ionic bond to make NaCl. In a covalent bond, the atoms bond by sharing electrons. Cations are positively charged ions and anions are negatively charged ions.
Compounds consisting of ions are ionic compounds and the bonds holding them together are ionic bonds. Ionic bonds depend upon the mutual attraction between positive cations and negative anions for their bond strength oppositely charged bodies attract each other, whereas negatively charged bodies repel each other.
The formation of ions based upon the octet rule is readily seen for the well-known ionic compound, sodium chloride, NaCl, as illustrated in Figure 4. Chlorine attains a stable octet of 8 outer-shell electrons by gaining 1 electron per atom to produce Cl - ion. Sodium chloride is a very stable compound because of the mutual attraction of oppositely charged ions. But the ions have to be arranged in an optimum manner for this attraction to be effective. Since oppositely charged ions attract each other, but ions with the same charge are mutually repulsive, the ions in an ionic compound such as sodium chloride have to be packed to maximize attraction and minimize repulsion.
The arrangement that does this for NaCl is shown by a ball and stick model in Figure 4. In reality, ions are more accurately represented in an ionic structure as spheres that touch. But the imperfect ball and stick model shown in Figure 4. It illustrates the relative positions of the ions. These positions, combined with ionic charge and size, determine the crystal structure of the solid crystal of which the ionic compound is composed. So, although the chemical formula NaCl accurately represents the relative numbers of Na and Cl atoms in sodium chloride it does not imply that there are discrete molecules consisting of 1 Na and 1 Cl.
For this reason it is not correct to refer to a molecule of sodium chloride because distinct molecules of ionic compounds do not exist as such. The stabilities of chemical compounds are all about energy. In general, the more energy released when a compound forms, the more stable the compound is. Sodium chloride could be formed by reacting elemental solid sodium with elemental Cl 2 gas,. This reaction releases a large amount of energy and elemental sodium burns explosively in chlorine gas.
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