The world is full of many substances with properties that indicate they cannot be classified as ionic or metallic substances. Substances such as wax or vegetable oil are neither brittle like ionic substances nor bendable like metallic substances. Could the chemical makeup and bonding in wax and vegetable oil explain these differences? One piece of evidence that can help answer this question is the fact that both wax and vegetable oil consist solely of nonmetal elements.
Covalent Bonds
The nonmetals include hydrogen and elements on the right-hand side of the periodic table. An important characteristic of nonmetals is that they tend to accept electrons from other atoms in order to acquire the electron configuration of the nearest noble gas. Recall that the noble gases have a complete set of valence electrons, which makes them very stable and nonreactive.
Nonmetals are also able to share electrons with other nonmetals. Sharing allows each nonmetal to retain its original electron count and still obtain the additional electrons it needs to complete its valence shell. This type of bonding is called covalent bonding. A compound in which atoms are covalently bonded together is called a covalent compound.
The simplest example to consider is the covalent bond that forms between two hydrogen atoms. Each hydrogen atom has one electron, and each needs a second electron to complete its valence shell to attain the same configuration as the noble gas, helium. When the distance between two hydrogen atoms is small enough, the atoms share their electrons, forming a covalent bond.
Now consider a covalent bond that results from two chlorine atoms coming close enough to share electrons. In this case, each chlorine atom has seven valence electrons and needs only one additional electron to complete its valence shell to attain noble gas configuration of eight electrons. So, each chlorine atom shares just one of its valence electrons with the other. The atoms do not share all of their valence electrons. They only share the number of electrons needed to complete their valence shell. The drawing below illustrates the sharing of two electrons between chlorine atoms.
In a water molecule, formed of two hydrogen atoms and one oxygen atom, each hydrogen atom shares its electron with the oxygen atom. At the same time, the oxygen atom shares one of its valence electrons with each of the hydrogen atoms. In sharing these electrons, each hydrogen atom acquires a complete valence shell with 2 electrons, and the oxygen atom acquires a stable valence shell with 8 electrons. The diagram below illustrates how each atom achieves its noble gas electron configuration.
Fill in the blank.
How many valence electrons did the oxygen atom have before bonding with hydrogen in a water molecule?
valence electrons
6
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| Response: That's the correct answer |
Covalent Bonds - Cont.
Now consider covalent bonding in the compound methane. Methane has a central carbon atom bonded to four hydrogen atoms as shown in Figure 22.
Carbon is a group 14 nonmetal with four valence electrons. Since carbon has four valence electrons, it needs four more electrons to fill its valence shell and attain a noble gas configuration. Each hydrogen atom is able to share only one electron, so the carbon atom needs to share electrons with four hydrogen atoms to reach its noble gas configuration. Therefore, one carbon atom forms four covalent bonds to four hydrogen atoms as shown in Figure 22.
Moreover, you can construct electron dot structures of covalent molecules using the concepts explained above. In the case of an HCl molecule, begin by drawing the electron dot structures for each atom in the molecule.
Next, think about how the atoms can share electrons so that their valence shells can be filled with either 2 or 8 electrons. In the case of the hydrogen atom, one additional electron is needed to reach a total of two electrons in its outer shell. You can draw an arrow to show how chlorine shares this electron with hydrogen. The chlorine atom also needs one more electron to reach a total of eight electrons in its outer shell. Draw an arrow from hydrogen’s one valence electron to show how hydrogen shares this electron with chlorine. Then move the two symbols together to show the shared pair of electrons in between.These shared electrons form the covalent bond in HCl.
Figure 24 Electron dot structure representing a model of an HCl molecule
To verify whether a covalent molecule model is correct or not, it is best to make sure that
all electrons were shared.
all atoms have become stable.
each atom has a full valence shell.
each atom has a noble gas configuration.
answer :
each atom has a full valence shell.
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all atoms have become stable.
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each atom has a noble gas configuration.
Properties of Covalent Compounds
Many covalent compounds are gases or liquids at room temperature, whereas ionic compounds and metals are mostly solids. Even the covalent compounds that are solids at room temperature tend to have much lower melting points than either ionic or metallic substances.
Some covalent compounds form crystal lattices, but they do not separate into cations and anions as do ionic compounds. As a result, less energy is required to pull apart or separate covalent molecules from each other during a change of state. Table sugar is an example of a covalent compound that forms a crystalline solid. Sugar is made up of sucrose molecules composed of carbon, hydrogen, and oxygen atoms. Another covalent compound that forms crystals is water in the solid state.
Most covalent compounds are bad conductors of electricity. Electrons do not move freely between the atoms of a covalent molecule like they do in a metallic substance. Moreover, in covalent compounds that are solids, electrons do not flow freely between the molecules to conduct electricity even when these compounds dissolve in water.
In contrast to most ionic compounds, which can dissolve easily in water, many—not all—covalent compounds are insoluble or only slightly soluble in water. Most covalent compounds that are made of large molecules are insoluble in water, but they are often soluble in other covalently bonded liquids. You can see this anytime you try to mix cooking oil and water: no matter how vigorously you mix the two, they always separate into two distinct layers. However, covalent compounds with small molecules, such as table sugar, are often more soluble in water.
Match.
Your answer :
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