06.Periodicity of Behavior;
       Sodium Through Argon
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       First Three Rows

Atoms with the same outer-shell structure have similar chemical properties. These outer electrons are the most significant features that a neighboring atom sees.

Sodium behaves like lithium, silicon and carbon having many properties in common, and chlorine and fluorine are very much alike. This is the most important single fact in this chapter, and we will develop this theme at length.

Having said this, we must qualify matters by saying that there are important differences in the chemical behavior of second-shell and third-shell elements, which arise because the third-shell atoms are bigger and have a weaker hold on their outer electrons.

Each third-shell atom has a lower first ionization energy than the corresponding atom in the row above it. This property makes every third-shell element less electronegative than its second-shell analogue, and more metallic.

The three rows in the opening diagram, which represent the first three electron shells, are the beginning of a very important means of classifying chemical properties of atoms, the periodic table.


The periodic table is organized such that the successive addition of electrons to the same shell occurs across horizontal periods, and similar outer electron-shell structures and chemical properties are found in vertical groups.

The periodic table was devised a century ago as a memory aid, to make chemistry easier. It eventually became the great statement of chemical reality, against which any theory of atomic structure and properties had to be tested.

The second-shell elements are the elements of living organisms, and the third-shell elements make up the framework of our planet. The crust of the Earth is 60 atomic percent oxygen.

The other 40% is divided among silicon (21%), hydrogen 3%), and third- and fourth-shell metals (16% together): sodium, magnesium, and aluminium in the third shell; and potassium, calcium, and iron in the fourth.

All of the remaining elements taken together make up less than half of one percent of the Earth's crust. The six metals just mentioned occur in combinations with oxygen as silicate minerals, water, and the various metal oxides, carbonates, and nitrates.

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