valence electrons

valence electrons and reactivity

The table salt pictured in the Figure 1.3 contains two elements that are so reactive they are rarely found alone in nature. Instead, they undergo chemical reactions with other elements and form compounds. Table salt is the compound named sodium chloride (NaCl). It forms when an atom of sodium (Na) gives up an electron and an atom of chlorine (Cl) accepts it. When this happens, sodium becomes a positively charged ion (Na+ ), and chlorine becomes a negatively charged ion (Cl ). The two ions are attracted to each and join a matrix of interlocking sodium and chloride ions, forming a crystal of salt. Q: Why does sodium give up an electron? A: An atom of a group 1 element such as sodium has just one valence electron. It is eager to give up this electron in order to have a full outer energy level, because this will give it the most stable arrangement of electrons. You can see how this happens in the animation at the following URL and in the Figure 1.4. Group 2 elements with two valence electrons are almost as reactive as elements in group 1 for the same reason. Q: Why does chlorine accept the electron from sodium? A: An atom of a group 17 element such as chlorine has seven valence electrons. It is eager to gain an extra electron to fill its outer energy level and gain stability. Group 16 elements with six valence electrons are almost as reactive for the same reason. Atoms of group 18 elements have eight valence electrons (or two in the case of helium). These elements already have a full outer energy level, so they are very stable. As a result, they rarely if ever react with other elements. Elements in other groups vary in their reactivity but are generally less reactive than elements in groups 1, 2, 16, or 17. Q: Find calcium (Ca) in the periodic table (see Figure 1.1). Based on its position in the table, how reactive do you think calcium is? Name another element with which calcium might react. A: Calcium is a group 2 element with two valence electrons. Therefore, it is very reactive and gives up electrons in chemical reactions. It is likely to react with an element with six valence electrons that wants to gain two electrons. This would be an element in group 6, such as oxygen. Table salt (sodium chloride).





valence electrons and the periodic table

The number of valence electrons in an atom is reflected by its position in the periodic table of the elements (see the periodic table in the Figure 1.1). Across each row, or period, of the periodic table, the number of valence electrons in groups 1-2 and 13-18 increases by one from one element to the next. Within each column, or group, of the table, all the elements have the same number of valence electrons. This explains why all the elements in the same group have very similar chemical properties. For elements in groups 1-2 and 13-18, the number of valence electrons is easy to tell directly from the periodic table. This is illustrated in the simplified periodic table in the Figure 1.2. It shows just the numbers of valence electrons in each of these groups. For elements in groups 3-12, determining the number of valence electrons is more complicated. Q: Based on both periodic tables above (Figures 1.1 and 1.2), what are examples of elements that have just one valence electron? What are examples of elements that have eight valence electrons? How many valence electrons does oxygen (O) have? A: Any element in group 1 has just one valence electron. Examples include hydrogen (H), lithium (Li), and sodium (Na). Any element in group 18 has eight valence electrons (except for helium, which has a total of just two electrons). Examples include neon (Ne), argon (Ar), and krypton (Kr). Oxygen, like all the other elements in group 16, has six valence electrons.

electron dot diagrams

Because valence electrons are so important, atoms are often represented by simple diagrams that show only their valence electrons. These are called electron dot diagrams, and three are shown below. In this type of diagram, an elements chemical symbol is surrounded by dots that represent the valence electrons. Typically, the dots are drawn as if there is a square surrounding the element symbol with up to two dots per side. An element never has more than eight valence electrons, so there cant be more than eight dots per atom. Q: Carbon (C) has four valence electrons. What does an electron dot diagram for this element look like? A: An electron dot diagram for carbon looks like this:

what are valence electrons

Valence electrons are the electrons in the outer energy level of an atom that can participate in interactions with other atoms. Valence electrons are generally the electrons that are farthest from the nucleus. As a result, they may be attracted as much or more by the nucleus of another atom than they are by their own nucleus.

valence electrons and electricity

Valence electrons also determine how wellif at allthe atoms of an element conduct electricity. The copper wires in the cable in the Figure 1.5 are coated with plastic. Copper is an excellent conductor of electricity, so it is used for wires that carry electric current. Plastic contains mainly carbon, which cannot conduct electricity, so it is used as insulation on the wires. Q: Why do copper and carbon differ in their ability to conduct electricity? A: Atoms of metals such as copper easily give up valence electrons. Their electrons can move freely and carry electric current. Atoms of nonmetals such as the carbon, on the other hand, hold onto their electrons. Their electrons cant move freely and carry current. A few elements, called metalloids, can conduct electricity, but not as well as metals. Examples include silicon and germanium in group 14. Both become better conductors at higher temperatures. These elements are called semiconductors. Q: How many valence electrons do atoms of silicon and germanium have? What happens to their valence electrons when the atoms are exposed to an electric field? A: Atoms of these two elements have four valence electrons. When the atoms are exposed to an electric field, the valence electrons move away from the atoms and allow current to flow.


instructional diagrams

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valence electrons tend to be attracted by the nuclei of other atoms as much as, or more than, the nucleus of their own atom.

-->  a. true

b. false

all the elements in the same period of the periodic table have the same number of valence electrons.

a. true

-->  b. false

what is the maximum number of valence electrons an element may have?

a) 6

-->  b) 8

c) 10

d) 12

when a group 1 element reacts it

a) gains two electrons.

b) loses two electrons.

c) gains one electron.

-->  d) loses one electron.

which element would you expect to gain one electron in a chemical reaction?

-->  a) chlorine

b) oxygen

c) nitrogen

d) carbon

diagram questions

No diagram questions associated with this lesson