0.4 Electron shell model of an atom  (Page 3/6)

It should seem strange that elements with very different numbers of charged particles should behave alike chemically, but elements with very similar numbers of charged particles should behave very differently chemically. There must be more to learn about what determines the chemistry of individual atoms.

One clue is found in looking at the pattern of elements which have similar chemical properties. We can go to [link] , which lists the elements by relative atomic mass and, as we now know, by atomic number. Let’s pick out three elements with similar properties: lithium, sodium, and potassium. All three of these elements are soft metals with low melting points, all three react violently with water, and all three form salts with chlorine with the similar molecular formula, LiCl, NaCl, KCl. Because these elements are so similar, they can be regarded as a “group,” and a name has been given to this group, the “alkali metals.” Surprisingly, immediately before each of these elements in the list are three elements which are also very similar to each other but very different from the alkali metals. These are the “noble gases,” helium, neon, and argon. These elements are all gases to very, very low temperatures, and they are all very unreactive, sometimes called “inert.” The fact that each alkali metal is always preceded by a noble gas suggests that there is a pattern to the properties of the elements.

We can see this again by looking at the elements immediately after each alkali metal. These are beryllium, magnesium, and calcium, and again these three have very similar properties. They are all soft metals with higher melting points than the alkali metals, and they all form salts with chlorine with similar molecular formulas, BeCl ₂ , MgCl ₂ , and CaCl ₂ . Because they are similar, we place them in a group together, which has been called the “alkaline earth metals.”

In fact, we can keep this up with other groups. The elements that are just before the noble gases are fluorine and chlorine, which again have very similar chemical properties as we discussed before. Later on in the list of elements, the noble gas krypton is immediately preceded by the element bromine, which also has similar properties to chlorine and fluorine. We place fluorine, chlorine, and bromine in a group together called the “halogens.”