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Exceptions to the expected behavior involve elements toward the bottom of the groups. In addition to the expected ions Tl 3+ , Sn 4+ , Pb 4+ , and Bi 5+ , a partial loss of these atoms’ valence shell electrons can also lead to the formation of Tl + , Sn 2+ , Pb 2+ , and Bi 3+ ions. The formation of these 1+, 2+, and 3+ cations is ascribed to the inert pair effect    , which reflects the relatively low energy of the valence s -electron pair for atoms of the heavy elements of groups 13, 14, and 15. Mercury (group 12) also exhibits an unexpected behavior: it forms a diatomic ion, Hg 2 2+ (an ion formed from two mercury atoms, with an Hg-Hg bond), in addition to the expected monatomic ion Hg 2+ (formed from only one mercury atom).

Transition and inner transition metal elements behave differently than main group elements. Most transition metal cations have 2+ or 3+ charges that result from the loss of their outermost s electron(s) first, sometimes followed by the loss of one or two d electrons from the next-to-outermost shell. For example, iron (1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 3 d 6 4 s 2 ) forms the ion Fe 2+ (1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 3 d 6 ) by the loss of the 4 s electron and the ion Fe 3+ (1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 3 d 5 ) by the loss of the 4 s electron and one of the 3 d electrons. Although the d orbitals of the transition elements are—according to the Aufbau principle—the last to fill when building up electron configurations, the outermost s electrons are the first to be lost when these atoms ionize. When the inner transition metals form ions, they usually have a 3+ charge, resulting from the loss of their outermost s electrons and a d or f electron.

Determining the electronic structures of cations

There are at least 14 elements categorized as “essential trace elements” for the human body. They are called “essential” because they are required for healthy bodily functions, “trace” because they are required only in small amounts, and “elements” in spite of the fact that they are really ions. Two of these essential trace elements, chromium and zinc, are required as Cr 3+ and Zn 2+ . Write the electron configurations of these cations.

Solution

First, write the electron configuration for the neutral atoms:

Zn: [Ar]3 d 10 4 s 2

Cr: [Ar]3 d 5 4 s 1

Next, remove electrons from the highest energy orbital. For the transition metals, electrons are removed from the s orbital first and then from the d orbital. For the p -block elements, electrons are removed from the p orbitals and then from the s orbital. Zinc is a member of group 12, so it should have a charge of 2+, and thus loses only the two electrons in its s orbital. Chromium is a transition element and should lose its s electrons and then its d electrons when forming a cation. Thus, we find the following electron configurations of the ions:

Zn 2+ : [Ar]3 d 10

Cr 3+ : [Ar]3 d 3

Check your learning

Potassium and magnesium are required in our diet. Write the electron configurations of the ions expected from these elements.

Answer:

K + : [Ar], Mg 2+ : [Ne]

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Electronic structures of anions

Most monatomic anions form when a neutral nonmetal atom gains enough electrons to completely fill its outer s and p orbitals, thereby reaching the electron configuration of the next noble gas. Thus, it is simple to determine the charge on such a negative ion: The charge is equal to the number of electrons that must be gained to fill the s and p orbitals of the parent atom. Oxygen, for example, has the electron configuration 1 s 2 2 s 2 2 p 4 , whereas the oxygen anion has the electron configuration of the noble gas neon (Ne), 1 s 2 2 s 2 2 p 6 . The two additional electrons required to fill the valence orbitals give the oxide ion the charge of 2– (O 2– ).

Determining the electronic structure of anions

Selenium and iodine are two essential trace elements that form anions. Write the electron configurations of the anions.

Solution

Se 2– : [Ar]3 d 10 4 s 2 4 p 6

I : [Kr]4 d 10 5 s 2 5 p 6

Check your learning

Write the electron configurations of a phosphorus atom and its negative ion. Give the charge on the anion.

Answer:

P: [Ne]3 s 2 3 p 3 ; P 3– : [Ne]3 s 2 3 p 6

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Key concepts and summary

Atoms gain or lose electrons to form ions with particularly stable electron configurations. The charges of cations formed by the representative metals may be determined readily because, with few exceptions, the electronic structures of these ions have either a noble gas configuration or a completely filled electron shell. The charges of anions formed by the nonmetals may also be readily determined because these ions form when nonmetal atoms gain enough electrons to fill their valence shells.

Chemistry end of chapter exercises

Does a cation gain protons to form a positive charge or does it lose electrons?

The protons in the nucleus do not change during normal chemical reactions. Only the outer electrons move. Positive charges form when electrons are lost.

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Iron(III) sulfate [Fe 2 (SO 4 ) 3 ] is composed of Fe 3+ and SO 4 2− ions. Explain why a sample of iron(III) sulfate is uncharged.

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Which of the following atoms would be expected to form negative ions in binary ionic compounds and which would be expected to form positive ions: P, I, Mg, Cl, In, Cs, O, Pb, Co?

P, I, Cl, and O would form anions because they are nonmetals. Mg, In, Cs, Pb, and Co would form cations because they are metals.

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Which of the following atoms would be expected to form negative ions in binary ionic compounds and which would be expected to form positive ions: Br, Ca, Na, N, F, Al, Sn, S, Cd?

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Predict the charge on the monatomic ions formed from the following atoms in binary ionic compounds:

(a) P

(b) Mg

(c) Al

(d) O

(e) Cl

(f) Cs

(a) P 3– ; (b) Mg 2+ ; (c) Al 3+ ; (d) O 2– ; (e) Cl ; (f) Cs +

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Predict the charge on the monatomic ions formed from the following atoms in binary ionic compounds:

(a) I

(b) Sr

(c) K

(d) N

(e) S

(f) In

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Write the electron configuration for each of the following ions:

(a) As 3–

(b) I

(c) Be 2+

(d) Cd 2+

(e) O 2–

(f) Ga 3+

(g) Li +

(h) N 3–

(i) Sn 2+

(j) Co 2+

(k) Fe 2+

(l) As 3+

(a) [Ar]4 s 2 3 d 10 4 p 6 ; (b) [Kr]4 d 10 5 s 2 5 p 6 (c) 1 s 2 (d) [Kr]4 d 10 ; (e) [He]2 s 2 2 p 6 ; (f) [Ar]3 d 10 ; (g) 1 s 2 (h) [He]2 s 2 2 p 6 (i) [Kr]4 d 10 5 s 2 (j) [Ar]3 d 7 (k) [Ar]3 d 6 , (l) [Ar]3 d 10 4 s 2

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Write the electron configuration for the monatomic ions formed from the following elements (which form the greatest concentration of monatomic ions in seawater):

(a) Cl

(b) Na

(c) Mg

(d) Ca

(e) K

(f) Br

(g) Sr

(h) F

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Write out the full electron configuration for each of the following atoms and for the monatomic ion found in binary ionic compounds containing the element:

(a) Al

(b) Br

(c) Sr

(d) Li

(e) As

(f) S

(a) 1 s 2 2 s 2 2 p 6 3 s 2 3 p 1 ; Al 3+ : 1 s 2 2 s 2 2 p 6 ; (b) 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 3 d 10 4 s 2 4 p 5 ; 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 3 d 10 4 s 2 4 p 6 ; (c) 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 3 d 10 4 s 2 4 p 6 5 s 2 ; Sr 2+ : 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 3 d 10 4 s 2 4 p 6 ; (d) 1 s 2 2 s 1 ; Li + : 1 s 2 ; (e) 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 3 d 10 4 s 2 4 p 3 ; 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 3 d 10 4 s 2 4 p 6 ; (f) 1 s 2 2 s 2 2 p 6 3 s 2 3 p 4 ; 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6

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From the labels of several commercial products, prepare a list of six ionic compounds in the products. For each compound, write the formula. (You may need to look up some formulas in a suitable reference.)

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Practice Key Terms 2

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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