http://www.ck12.org Chapter 2. Matter
FIGURE 2.97
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 theFigure2.100. Group 2 elements with two
valence electrons are almost as reactive as elements in group 1 for the same reason.
http://www.visionlearning.com/img/app/library/objects/Flash/VLObject-1349-030213040210.swf
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