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8.4 The exclusion principle and the periodic table  (Page 3/11)

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First, the periodic table is arranged into columns and rows. The table is read left to right and top to bottom in the order of increasing atomic number Z . Atoms that belong to the same column or chemical group    share many of the same chemical properties. For example, the Li and Na atoms (in the first column) bond to other atoms in a similar way. The first row of the table corresponds to the 1 s ( l = 0 ) shell of an atom.

Consider the hypothetical procedure of adding electrons, one by one, to an atom. For hydrogen (H) (upper left), the 1 s shell is filled with either a spin up or down electron ( ↑ or ↓ ). This lone electron is easily shared with other atoms, so hydrogen is chemically active. For helium (He) (upper right), the 1 s shell is filled with both a spin up and a spin down ( ↑ ↓ ) electron. This “fills” the 1 s shell, so a helium atom tends not to share electrons with other atoms. The helium atom is said to be chemically inactive, inert, or noble; likewise, helium gas is said to be an inert gas or noble gas.

Build an atom by adding and subtracting protons, neutrons, and electrons. How does the element, charge, and mass change? Visit PhET Explorations: Build an Atom to explore the answers to these questions.

The Periodic Table of Elements, showing the structure of shells and subshells, is shown. The 18 columns are numbered labeled “Group” and the 7 rows are numbered and labeled “Period.” Groups 1 and 2 are shaded purple. Groups 3 through 12 are shaded yellow. Groups 13 through 18, are shaded red, with the exception of period 1, group 18, which is purple. The period 6 and 7, group 3 boxes are outlined and an arrow points from them to an additional section of two rows and 14 columns that is shaded green. The period 6 group 3 box has an asterisk, which also appears to the left of the first row of the additional section. The period 7 group 3 box has two asterisks, which also appear to the left of the second row of the additional section. Below the table to the left is an enlarged picture of the upper-left most box on the table. The letter “H” is in its upper-left hand corner and is labeled “Symbol.” The number 1 is in its upper-right hand corner and is labeled “Electrons.” In its center the entry “1 s” is labeled “subshell.” The box is shaded purple. Every element has its symbol and electrons indicated in the box. The subshells are indicated as a group for contiguous sections of a row. Beginning at the top left of the table, period 1, group 1, is shaded purple and contains symbol H, electrons 1, subshell 1 s. The only other element box in period 1 is in the last column, group 18, which is shaded purple and contains “H e, 1, 1 s”. Period 2, group 1 contains “L i, 1” Group 2 contains “B e, 2.” Period 2 groups 1 and 2 both have subshell 2 s. Groups 3 through 12 are skipped. Group 13 contains “B, 1.” Group 14 contains “C, 2.” Group 15 contains “N, 3.” Group 16 contains “O, 4.” Group 17 contains “F, 5.” Group 18 contains “N e, 6.” Period 2 group 13 through 18 have subshell 2 p. Period 3, group 1 contains “N a,1.” Group 2 contains “M g, 2.” These two have subshell 3 s. Groups 3 through 12 are skipped again in period 3 and group 13 contains “A l, 1.” Group 14 contains “S I, 2.” Group 15 contains “P, 3.” Group 16 contains “S, 4.” Group 17 contains “C l, 5.” Group 18 contains “A r, 6.” These 6 have subshell 3 p. Period 4, group 1 contains “K, 1.” Group 2 contains “C a, 2.” These two have subshell 4 s. Group 3 contains “S, 1.” Group 4 contains “T i, 2.” Group 5 contains “V, 3.” Group 6 contains “C r, 4.” Group 7 contains “M n, 5.” Group 8 contains “F e, 6.” Group 9 contains “C o, 7.” Group 10 contains “N i, 8.” Group 11 contains “C u, 9.” Group 12 contains “Z n, 10.” These 10 have subshell 3 d. Group 13 contains “G a, 1.” Group 14 contains “G e, 2.” Group 15 contains “A s, 3.” Group 16 contains “S e, 4.” Group 17 contains “B r, 5.” Group 18 contains “K r, 6.” These six have subshell 4 p. Period 5, group 1 contains “R b, 1.” Group 2 contains “S r, 2.” These 2 have subshell 5 s. Group 3 contains “Y, 1.” Group 4 contains “Z r, 2.” Group 5 contains “N b, 3.” Group 6 contains “M o, 4.” Group 7 contains “T c, 5 “R u, 6.” Group 9 contains “R h, 7.” Group 10 contains “P d, 8.” Group 11 contains “A g, 9.” Group 12 contains “C d, 10.” These ten have subshell 4 d. Group 13 contains “I n, 1.” Group 14 contains “S n, 2.” Group 15 contains “S b, 3.” Group 16 contains “T e, 4.” Group 17 contains “I, 5.” Group 18 contains “X e, 6.” These six have subshell 5 p. Period 6, group 1 contains “C s, 1.” Group 2 contains “B a, 2.” These two have subshell 6 s. Group 3 contains “L a, 1,” and has an additional asterisk. Group 4 contains “H f, 2.” Group 5 contains “T a, 3.” Group 6 contains “W, 4.” Group 7 contains “R e, 5.” Group 8 contains “O s, 6.” Group 9 contains “I r, 7.” Group 10 contains “P t, 8.” Group 11 contains “A u, 9.” Group 12 contains “H g, 10.” These 10 have subshell 5 d. Group 13 contains “T l, 1.” Group 14 contains “P b, 2.” Group 15 contains “B i, 3.” Group 16 contains “P o, 4.” Group 17 contains “A t, 5.” Group 18 contains “R n, 6.” These six have subshell 6 p. Period 7, group 1 contains “F r, 1.” Group 2 contains “R a, 2.” These two have subshell 7 s. Group 3 contains “A c, 1,” and has an additional double asterisk. Group 4 contains “R f, 2.” Group 5 contains “D b, 3.” Group 6 contains “S g, 4.” Group 7 contains “B h, 5.” Group 8 contains “H s, 6.” Group 9 contains “M t, 7.” Group 10 contains “D s, 8.” Group 11 contains “R g, 9.” Group 12 contains “C n, 10.” These 10 have subshell 6 d. Group 13 contains “U u t, 1.” Group 14 contains “F l, 2.” Group 15 contains “U u p, 3.” Group 16 contains “L v, 4.” Group 17 is missing. Group 18 contains “U u o, 6.” These five have subshell 7 p. An arrow links the period 6 and 7, group 3 to an additional section with two rows, each with 14 columns. The columns are not numbered. The first row is labeled with an asterisk and all the elements in it have subshell 4 f. The boxes in this row contain, in order: C e, 1, P r, 2, N d, 3, P m, 4, S m, 5, E u, 6, G d, 7, T b, 8, D y, 9, H o, 10, E r, 11, T m, 12, Y b, 13, L u, 14. The second row is labeled with a double asterisk and all the elements in it have subshell 5 f. The boxes in this row contain, in order: T h 1, P a, 2, U, 3, N p, 4, P u, 5, A m, 6, C m, 7, B k, 8, C f, 9, E s, 10, F m, 11, M d, 12, N o, 13, L r, 14.
The periodic table of elements, showing the structure of shells and subshells.

The second row corresponds to the 2 s and 2 p subshells. For lithium (Li) (upper left), the 1 s shell is filled with a spin-up and spin-down electron ( ↑ ↓ ) and the 2 s shell is filled with either a spin-up or -down electron ( ↑ or ↓ ). Its electron configuration is therefore 1 s 2 2 s 1 or [He]2 s , where [He] indicates a helium core. Like hydrogen, the lone electron in the outermost shell is easily shared with other atoms. For beryllium (Be), the 2 s shell is filled with a spin-up and -down electron ( ↑ ↓ ), and has the electron configuration [He] 2 s 2 .

Next, we look at the right side of the table. For boron (B), the 1 s and 2 s shells are filled and the 2 p ( l = 1 ) shell contains either a spin up or down electron ( ↑ or ↓ ). From carbon (C) to neon (N), we the fill the 2 p shell. The maximum number of electrons in the 2 p shells is 4 l + 2 = 4 ( 2 ) + 2 = 6 . For neon (Ne), the 1 s shell is filled with a spin-up and spin-down electron ( ↑ ↓ ), and the 2 p shell is filled with six electrons ( ↑ ↓ ↑ ↓ ↑ ↓ ) . This “fills” the 1 s , 2 s , and 2 p subshells, so like helium, the neon atom tends not to share electrons with other atoms.

The process of electron filling repeats in the third row. However, beginning in the fourth row, the pattern is broken. The actual order of order of electron filling is given by

1 s , 2 s , 2 p , 3 s , 3 p , 4 s , 3 d , 4 p , 5 s , 4 d , 5 p , 6 s , 4 f , 5 d , 6 p , 7 s ,...

Notice that the 3 d , 4 d , 4 f , and 5 d subshells (in bold) are filled out of order; this occurs because of interactions between electrons in the atom, which so far we have neglected. The transition metal     s are elements in the gap between the first two columns and the last six columns that contain electrons that fill the d ( l = 1 ) subshell. As expected, these atoms are arranged in 4 l + 2 = 4 ( 2 ) + 2 = 10 columns. The structure of the periodic table can be understood in terms of the quantization of the total energy ( n ), orbital angular momentum ( l ), and spin ( s ). The first two columns correspond to the s ( l = 0 ) subshell, the next six columns correspond to the p ( l = 1 ) subshell, and the gap between these columns corresponds to the d ( l = 2 ) subshell.
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Source:  OpenStax, University physics volume 3. OpenStax CNX. Nov 04, 2016 Download for free at http://cnx.org/content/col12067/1.4
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