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. Covalent bonds usually occur between nonmetals. For example, in water H 2 O each hydrogen H and oxygen O share a pair of electrons to make a molecule of two hydrogen atoms single bonded to a single oxygen atom.
In general, ionic bonds occur between elements that are far apart on the periodic table. Covalent bonds occur between elements that are close together on the periodic table. Ionic compounds tend to be brittle in their solid form and have very high melting temperatures. Covalent compounds tend to be soft, and have relatively low melting and boiling points.
Water, a liquid composed of covalently bonded molecules, can also be used as a test substance for other ionic and covalently compounds. Ionic compounds tend to dissolve in water e. Properties of ionic and covalent compounds are listed in Table 2. The properties listed in Table 2. Like other ionic compounds, sodium chloride Fig. Chlorine gas Fig. Ionic and covalent compounds also differ in what happens when they are placed in water, a common solvent.
For example, when a crystal of sodium chloride is put into water, it may seem as though the crystal simply disappears. Three things are actually happening. Figure 4. This occurs because as the charge of the nucleus becomes larger relative to the charge of the negative electron cloud around it, the cloud is drawn closer to the nucleus, shrinking ion size.
As electrons are added to atoms to produce more highly charged anions, the anion size increases because more electrons occupy more space. So S 2- ion is larger than Cl - ion. In order to further understand ion formation, several more examples can be considered. Calcium and chlorine react,. This compound is a byproduct of some industrial processes, from which its disposal can be a problem.
It is commonly used as road salt to melt ice and snow on streets and highways. Although calcium chloride is effective in this respect, it is corrosive to automobiles and calcium chloride is a pollutant salt that can contribute to excess salt levels in bodies of water.
Its advantage is that bacteria on soil and in water readily cause biodegradation of the acetate anion as shown by the reaction,. Another example of the formation of an ionic compound is the following reaction of aluminum metal with elemental oxygen,.
This reaction produces aluminum oxide for which the chemical formula is Al 2 O 3. This compound is the source of aluminum in bauxite, the ore from which aluminum is produced and is an important industrial chemical.
Called alumina, aluminum oxide itself has many applications including its use for abrasives and sandpaper, as a raw material for ceramics, and as an ingredient of antacids and antiperspirants. Exercise: Show the ionic products of the reaction of the metal and nonmetal indicated. In addition to ions formed from single atoms losing or gaining electrons, many ions consist of groups of atoms covalently bound together, but having a net electrical charge because of an excessor a deficiency of electrons.
An example of such an ion is the acetate ion shown above in the formula of calcium acetate, Ca C 2 H 3 O 2 2. The structural formula of the acetate anion, C 2 H 3 O 2 - , is shown below in which the two carbon atoms are joined with a single covalent bond consisting of two shared electrons, each of the three H atoms are joined to one of the carbon atoms by a single covalent bond and the other carbon atom is joined to one oxygen with a single covalent bond and to the other by a double covalent bond consisting of 4 shared electrons.
The net charge on the ion is I am not clear on what a molecule is. If water is a molecule, is it also a compound because the hydrogen and oxygen have been chemically combined? If so, how do you determine whether a substance is a compound or a molecule?
Whether something is a molecule or not depends on the type of bond that is formed when its atoms join together. In general, electrons can be shared between atoms a molecular bond or electrons can be completely removed from one atom and given to another an ionic bond.
Molecules have molecular bonds. Whether something is a compound or not depends on how many different kinds of elements make it up.
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