GED Science Practice Test: Chemical Bonding
As mentioned above, an element is most stable when it has a full valence shell. However, the vast majority of elements in the periodic table do not have full valence shells. Only the noble gases are stable with full valence shells. Thus the remainder of the elements in the periodic table are unstable, to varying degrees. An atom of an element can become stable by obtaining a full valence shell, which can happen in partnership with another element. Let us again turn to our example of the alkali metals, which have one valence electron. If they can partner with the halogens, group 7, which have seven valence electrons, both elements effectively have eight electrons, and have achieved stability. Here is a diagram showing a partnership between sodium (Na), an alkali metal, and chlorine (Cl), a halogen.
This “partnership” we are talking about is called a chemical bond. Chemical bonds are of many types. In the example above, sodium (Na) gives its electron to chlorine (Cl). When it does, sodium becomes a positively charged ion (12 positively charged protons and 11 negatively charged electrons makes an overall positive charge). Then, chlorine becomes a negatively charged ion (17 positive protons and 18 negative electrons makes an overall negative charge). The electromagnetic force between the positive sodium ion and the negative chlorine ion hold the two atoms together, and is called an ionic bond. The ionic bond between sodium and chlorine produce the compound sodium chloride (NaCl). Remember that a compound is a pure substance, made up of two or more elements that are chemically combined. The properties of the compound differ from the properties of either individual element. Sodium chloride is an excellent example of this change in properties; sodium is a metal that explodes when placed in water, and chlorine is poisonous gas—however, sodium chloride is table salt!
Ionic bonds tend to occur between elements with very large differences between their numbers of valence electrons (sodium had a very small number of valence electrons, 1, compared with chlorine, with a high number of valence electrons, 7). Elements with very large differences between valence electrons are found at far extremes in the periodic table.
Elements with smaller differences between valence electrons are less “willing” to completely give up or take electrons, and are more likely to share electrons. The type of bond in which electrons are shared is called a covalent bond. The following diagram shows a covalent bond between two fluorine atoms:
Fluorine is a halogen, with seven valence electrons. If the two fluorines can share one of their valence electrons with each other, they can each effectively each have eight valence electrons. The formula for the fluorine molecule you see above is F2. The number “2” indicates that there are two fluorine atoms chemically combined.
We have looked at examples of chemical bonds, in which two atoms combine to either transfer valence electrons (ionic bonding), or share valence electrons (covalent bonding). However, some types of bonds are more complex, involving more than two atoms, and a hybrid between transferring and sharing electrons. One example of a more complex compound is water. Observe the following diagram showing a molecule of water:
Oxygen has six valence electrons, while hydrogen has one valence electron. Mathematically, there can be eight valence electrons with a combination of one oxygen (6 valence electrons), and two hydrogens (1 + 1 = 2 total valence electrons). This is why the chemical formula for water is H2O: there are 2 hydrogens for every one oxygen in a molecule of water. It is important to mention a unique property of hydrogen: its valence electron is found in the first energy level or shell. This shell is an exception to the rule that valence shells are full with eight electrons. Hydrogen (and helium) have full valence shells with two electrons, since the first shell can only hold two electrons. If you look back at the first shell of some of the other Bohr models for elements in this lesson, you will notice that the first shell only holds two electrons. Again, this is an exception to the octet rule, which states that atoms combine to get eight electrons in their valence shell to become more stable.
The bonds in water are not entirely ionic, nor are they entirely covalent. The bonds in water are called polar covalent bonds, or polar bonds, which are a hybrid between the two types of chemical bonds. The electrons are shared among the three atoms, but spend more time orbiting the nucleus of the oxygen atom. This unequal sharing results in a partial charge on the water molecule; the hydrogens have a weak positive charge and oxygen has a weak negative charge. You can see this charge on the diagram above.
Use the periodic table below to answer question 1:
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Why do atoms form chemical bonds with each other?