AP Chemistry : Ions

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

Example Question #1 : Ions

Which of the following molecules has an O atom with the most negative formal charge?

Possible Answers:

(CO3)2–

H2O

O3 

O2

Correct answer:

(CO3)2–

Explanation:

For the carbonate ion, there are 3 resonance structures, and each of the 3 oxygen atoms carries a formal charge of –1 in two out of the three structures. This means that there is a –2/3 charge on each O atom. There are no formal charges in H2O or O2. and in O3, the central O has a positive charge and each outer O has a negative charge, so there is an average charge of –1/2 on the two outer ones.

Example Question #2 : Ions

What is the formal charge on the N atom in the [NH4]+ ion?

Possible Answers:

2

0

-1

1

Correct answer:

1

Explanation:

N has 5 valence electrons. In the ion, N has 4 bonds, and thus 8 bonding electrons and 0 nonbonding electrons. Thus, formal charge = 5- (1/2)*8 - 0 = 1.

Example Question #3 : Ions

Which of the following ions or atoms is isoelectronic with neon?

Possible Answers:

O2-

Cl-

Mg

Na

Correct answer:

O2-

Explanation:

Neon has an electron configuration of 1s22s22p6, just like O2-. Sodium has one extra electron, while magnesium has two extra electrons, beyond the neon configuration. Finally, the electron configuration for Cl- is 1s22s22p63s23p6. It is one period below flourine; F- would be isoelectric with neon.

Example Question #4 : Ions

Which of the following orders the given elements in terms of increasing electronegativity?

Possible Answers:

Correct answer:

Explanation:

Electronegativity increases moving from the left side of the periodic table to the right, and decreases moving down through a group (Excluding the Group 18 noble gases).

Thus the correct answer is  (the furthest left), followed by , , , and  (the most electronegative element).

 

Example Question #5 : Ions

Which of the following ranks the acids from strongest to weakest?

Possible Answers:

Correct answer:

Explanation:

Charge stability of halide ions increases with increasing atomic size ( is more stable than , which is more stable than , which is more stable than ).

This is not to be confused with electronegativity trends, which would predict the ordering of acids opposite the correct answer.

Atomic size (and thus the propensity to release protons () into solution forming anionic halides) increases moving down Group 7, so the correct ordering is: .

This trend is tue, however, keep in mind that  and  are all considered strong acids. This means that they dissociate completely in solution and have negative pKa values.

Example Question #274 : Ap Chemistry

Anionic atoms are __________ than their respective neutral atoms because they have __________ electron(s) than their neutral atoms.

Possible Answers:

larger . . . more

smaller . . .  more

smaller . . .  less

larger . . .  less

Correct answer:

larger . . . more

Explanation:

Anions have at least one more electron than their neutral atoms. Adding an electron creates a shielding effect on other electrons in the same shell, decreasing the nuclear charge felt by each valence electron. As a result, the electrons will lie slightly further from the nucleus, corresponding to an increase in atomic radius. If there are no other electrons than the newly added electron in the valence shell, then the added electron was added to the now highest shell, which would significantly increase the size of the atom.

Example Question #6 : Ions

Which of the following is a cation?

Possible Answers:

Correct answer:

Explanation:

A cation is a positively charged ion; calcium is the only species of the answer choices that has a positive charge. Negatively charged ions, such as the fluoride ion are known as anions.

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