We have now developed the concept that matter was built from a basic unit called the atom, and we have discussed the nature of the atom. We know, however, that very little matter exists as free elements. Most of the things around us are combinations of elements. Logically, the next step is to consider how things combine.
Valence. The valence of an element can be defined as a measure of its combining power or the number of electrons an atom must gain, lose, or share to have a full or stable outer electron shell. The reason atoms combine is contained in this definition. There are certain electron configurations in nature that are unusually stable (unreactive). The elements that have these configurations are in Group VIII A of the periodic table. They are sometimes referred to as the inert or noble gasses because they are found in very few combinations in nature. Other elements, by gaining, losing, or sharing electrons, can try to make their outer electron shells resemble the shells of the noble gases and hence become very stable. We can see how this works by considering the two simplest elements, hydrogen, and helium. Hydrogen has one electron in the K shell since it has only one proton. Therefore, hydrogen is a very reactive element, occurring naturally in many compounds. Helium, a noble gas, has two electrons in the K shell since it has two protons. Helium is very unreactive. Note that helium, by having two electrons, has a completed outer shell, since the K shell can hold only two electrons. Hydrogen would like to be as stable as helium and could be if it could gain or share one more electron to give it a completed outer shell. Hydrogen seeks this electron in nature by combining with other elements.
Octet Rule. If you examine the noble (inert) gases (like helium), you will see that not all have a completed (full) electron shell. Except for helium, the noble gases have eight electrons in their outer shell, yet they are still very stable. Chemists have observed that other elements sometimes gain, lose, or share electrons in order to have eight electrons in their outer shell. This observation led to the development of the octet rule, which states that outer electron shells prefer to have eight electrons even though the shell may not be full. (Octet means a group of eight.) On the next page are some examples of the electron configurations for various elements which indicate to us how many electrons they can gain, lose, or share to fit the octet rule or have a completed outer shell.
Positive Valence. An atom that must give up electrons to become stable will have more protons than electrons in its stable configuration and will not be electrically neutral. It will be positively charged since there are more positive charges than negative charges. This is indicated by a + sign. The number of electrons it gives up is the numerical value of its valence. Consider, for example, the element sodium, which has 11 protons and 11 electrons in its free state. It has one electron in the M shell, which it loses easily to become stable. After it loses the electron (that is, gives up a negative charge), it will have a positive one charge and its valence will be +1.
Negative Valence. An atom that must gain electrons to become stable will have more electrons than protons in its stable configuration and will not be electrically neutral. It will be negatively charged since there are more negative than positive charges. This is indicated by a "-" sign. The number of electrons it gains is the numerical value of its valence. Consider, for example, the element chlorine, which has 17 protons and 17 electrons in its free state. It is one electron short of fitting the octet rule in the M shell as that shell contains 7 electrons. After it gains the electron, it will have a negative one charge and its valence will be -1.
Important Symbols and Valences. Since it is very tedious to continually write complete names for elements, chemists developed the symbols for the elements which you observed on the periodic table. It will not be necessary for you to know all the symbols for your work but a number of them appear frequently enough that they should be memorized. Table 1-3 lists important elements with their symbols and valences. These should be committed to memory. (Please note that most, but not all, valences conform either to the completed shell or octet rules.)
Ions. Any atom that gains or loses electrons becomes charged (electrical charge) and is called an ion. An ion can be defined as any charged atom or group of atoms. If the ion is positively charged, it is called a cation. If it is negatively charged, it is called an anion. A group of atoms that has a charge and goes through a reaction unchanged is called a radical. Whenever we write the symbol for an element and wish to indicate it is an ion, we write the charge as a superscript to the symbol, for example, Cl-1 or Na+1.
Chemical Bonding. When elements combine to form chemical compounds, the electrons in the outer shell may be transferred from one atom to another or there may be a mutual sharing of the electrons. In either case, a chemical bond is produced. This means the two atoms do not travel or react independently of one another but are held together by the exchange or sharing of the electrons. Both atoms involved in the reaction attain a completed outer orbit, and stability results. There are three types of chemical bonds--electrovalent, covalent, and coordinate covalent.
Covalent bond. If two atoms each donate an electron that is shared with the other atom, the bond is a covalent bond. An example of this is the bond between two H (hydrogen) atoms. Double and triple covalent bonds are also possible.
Table 1-3. Valences.
|Antimony||Sb||-3, +3, +5|
|Arsenic||As||-3, +3, +5|
|Bromine||Br||-1, +1, +3, +5, +7|
|Carbon||C||+2, +4, -4|
|Chlorine||Cl||-1, +1, +3, +5, +7|
|Iodine||I||-1, +1, +3, +5, +7|
|Manganese||Mn||+2, +3, +4, +6, +7|
|Nitrogen||N||+1, -3, +3, +5|
|Phosphorus||P||+1, -3, +3, +5|
|Sulfur||S||-2, +2, +4, +6|
|NOTE: The most common valences are shown in red where there may be more than one valence.|