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The element with atomic number 3 is lithium (Li), atomic mass 6.941. The most abundant lithium isotope is

having 4 neutrons in its nucleus. A few percent of helium atoms are the
isotope, which has only 3 neutrons. The third electron in lithium cannot fit in the lowest energy shell, which, as noted above, is full with only 2 electrons. Therefore, the third electron in lithium goes into a second shell, that is, an outer shell.

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As a consequence of its electronic structure, lithium is the lowest atomic number element that is a metal. In a general sense, metals are elements that normally have only 1–3 electrons in their outer shells. These electrons can be lost from metals to produce positively charged cations with charges of +1, +2, or +3. In the pure elemental state metals often have a characteristic luster (shine), they are malleable (can be flattened or pushed into various shapes without breaking) and they conduct electricity. Although some metals, notably lead and mercury, are very dense, lithium is the least dense metal at only 0.531 g/cm3.

Two of lithium’s 3 electrons are inner electrons contained in an inner shell as in the immediately preceding noble gas helium. Inner shell electrons such as these stay on average relatively close to the nucleus, are very tightly held, and are not exchanged or shared in starrkingschool.netical bonds. As mentioned above, the third electron in lithium is an outer electron farther from, and less strongly attracted to, the nucleus. The outer electron is said to be in the atom’s outer shell. These concepts are illustrated in Figure 3.4.

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Figure 3.4. An atom of lithium, Li, has 2 inner electrons and 1 outer electron. The latter can be lost to another atom to produce the \(\ce{Li^{+}}\) ion, which is present in ionic compounds (see Section 2.12).

The Lewis symbol for atoms such as lithium that have both inner shell and outer shell electrons normally shows just the latter. (Inner shell electrons can be shown on symbols to illustrate a point, but normally this takes too much space and can be confusing.) Since lithium has only one outer shell electron, its Lewis symbol is


Consider that the lithium atom has an inner shell of 2 electrons, just like helium. Being only 1 electron away from the helium noble gas structure, lithium has a tendency to lose its extra electron so it can be like helium as shown by the following: