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Ionization energy (IE) is the amount of energy required to remove an electron from its electron cloud. Ionization energies are periodic as illustrated for 20 elements in Table B2.1.

The first ionization energy is the amount of energy required to remove one electron from the electron cloud; the second ionization energy is the amount required to remove a second electron and so forth. Ionization energies are lowest for electrons that are weakly held by the nucleus and higher for electrons that are strongly held by the nucleus or are in stable configurations. Ionization energies decrease down the periodic table because the most weakly held outer electrons are shielded from the positively charged nucleus by a progressively larger number of intervening electrons. Elements with relatively low first ionization energies are called electropositive elements because they tend to lose one or more electrons and become positively charged cations. Most elements with high first ionization energies are electronegative elements because they tend to add electrons to their electron clouds and become negatively charged anions. Since opposite charges tend to attract, you can imagine the potential such ions have for combining to produce other Earth materials. The arrangement of elements into vertical columns or groups within the periodic table helps us to comprehend the tendency of specific atoms to lose, gain or share electrons. For example, on the periodic table (see Table 2.3), column 2 (IIA) elements commonly exist as divalent (+2) cations because the first and second ionization energies are fairly similar and much lower than the third and higher ionization energies. This permits two electrons to be removed fairly easily from the electron cloud, but makes the removal of additional electrons much more difficult. Column 13 (IIIA) elements commonly exist as trivalent (+3) cations (see Table 2.3). These elements have somewhat similar first, second and third ionization energies, which are much smaller than the fourth and higher ionization energies. The transfer of electrons is fundamentally important in the understanding of chemical bonds and the development of mineral crystals.

Table B2.1 Ionization energies for hydrogen through calcium (units in kJ/mole).

Element	Ionization energy
	First		Second		Third		Fourth		Fifth		Sixth		Seventh		Eighth
H		1312
He		2372		5250
Li		520		7297		11 810
Be		899		1757		14 845		21 000
B		800		2426		3 659		25 020		32 820
C		1086		2352		4 619		6 221		37 820		47 260
N		1402		2855		4 576		7 473		9 452		53 250		64 340
O		1314		3388		5 296		7 467		10 987		13 320		71 320		84 070
F		1680		3375		6 045		8 408		11 020		15 150		17 860		91 010
Ne		2080		3963		6 130		9 361		12 180		15 240		—		—
Na		496		4563		6 913		9 541		13 353		16 610		20 114		26 660
Mg		737		1451		7 733		10 540		13 630		17 995		21 703		25 662
Al		578		1817		2 745		11 575		14 830		18 376		23 292		—
Si		787		1577		3 231		4 356		16 091		19 784		23 783		—
P		1012		1903		2 912		4 956		6 273		22 233		25 397		—
S		1000		2251		3 361		4 564		7 012		8 495		27 105		—
Cl		1251		2297		3 822		5 160		6 540		7 458		11 020		—
Ar		1520		2665		3 931		5 570		7 238		8 781		11 995		—
K		418		3052		4 220		5 877		7 975		9 590		11 343		14 944
Ca		590		1145		4 912		6 491		8 153		10 496		12 270		14 206