Kaplan MCAT General Chemistry Review

CONCEPTS TO REMEMBER

The periodic table of the elements organizes the elements according to their atomic numbers and reveals a repeating pattern of similar chemical and physical properties. Elements in the same row are in a period, while those elements in a column are in a group. Elements in the same period have the same principal energy level, n. Elements in the same group have the same valence shell electron configuration.

The valence electrons are those located in the outer shell and/or are available for interaction (bonding) with other atoms. The representative elements have their valence electrons in either s- or s- and p-orbitals. The nonrepresentative elements (the transition elements) have their valence electrons either in s- and d- or in s-, d-, and f-orbitals.

Effective nuclear charge (Z_eff) is the net positive charge experienced by electrons in the valence shell. Z_eff increases from left to right across a period, with little change in value from top to bottom in a group. Valence electrons become increasingly separated from the nucleus as the principal energy level, n, increases from top to bottom in a group. These two trends are the basis for all the other trends exhibited by the elements in the periodic table.

Atomic radius decreases from left to right across a period and increases from top to bottom in a group.

Ionization energy (IE) is the amount of energy necessary to remove an electron from the valence shell. It increases from left to right across a period and decreases from top to bottom in a group.

Electron affinity is the amount of energy released when an atom gains an electron in its valence shell. It increases from left to right across a period and decreases from top to bottom in a group.

Electronegativity is a measure of the attractive force that an atom in a chemical bond will exert on the electron pair of the bond. It increases from left to right across a period and decreases from top to bottom in a group.

There are three general classes of elements:

—The metals, located on the left and middle of the periodic table, including the active metals and the transition metals

—The nonmetals, located in the upper right side of the periodic table, including hydrogen, carbon, oxygen, nitrogen, and phosphorus, among others

—The metalloids or semimetals, located in a staircase formation between the metals and nonmetals, with qualities and behaviors that are combinations of those of the metals and nonmetals

The alkali and alkaline earth metals are the most reactive of all metals; they exist only in their ionic forms, having given up one or two electrons, respectively, in order to achieve the electronic configuration of the prior noble gas. The transition metals are less reactive, and many can have two or more oxidation states.

The halogens are very reactive nonmetals and are highly electronegative. They need only one electron to complete their octets and are naturally found only in the anionic state. The noble gases are the least reactive of all the elements because they have the stable octet in their valence shell. They have very high ionization energies and virtually nonexistent electronegativities.

Practice Questions

1. Lithium and sodium have similar chemical properties. For example, both can form ionic bonds with chloride. Which of the following best explains this similarity?

A. Both lithium and sodium ions are positively charged.

B. Lithium and sodium are in the same group of the periodic table.

C. Lithium and sodium are in the same period of the periodic table.

D. Both lithium and sodium have low atomic weights.

2. Carbon and silicon, elements used as the basis of biological life and synthetic computing, respectively, have some similar chemical properties. Which of the following describes a difference between the two elements?

A. Carbon has a smaller atomic radius than silicon.

B. Silicon has a smaller atomic radius than carbon.

C. Carbon has fewer valence electrons than silicon.

D. Silicon has fewer valence electrons than carbon.

3. What determines the length of an element’s atomic radius?

I. The number of valence electrons

II. The number of electron shells

III. The number of neutrons in the nucleus

A. I only

B. II only

C. I and II only

D. I, II, and III

4. Ionization energy contributes to an atom’s chemical reactivity. An accurate ordering of ionization energies, from lowest ionization energy to highest ionization energy, would be

A. Be, first ionization energy < Be, second ionization energy < Li, first ionization energy.

B. Be, second ionization energy < Be, first ionization energy < Li, first ionization energy.

C. Li, first ionization energy < Be, first ionization energy < Be, second ionization energy.

D. Li, first ionization energy < Be, second ionization energy < Be, first ionization energy.

5. Selenium is often an active component of treatments for scalp dermatitis. What type of element is selenium?

A. Metal

B. Metalloid

C. Halogen

D. Nonmetal

6. The properties of atoms can be predicted, to some extent, by their location within the periodic table. Which property or properties increase in the direction of the arrows shown?

I. Electronegativity

II. Atomic radius

III. First ionization energy

A. I only

B. II only

C. I and III only

D. II and III only

7. Metals are often used for making wires that conduct electricity. Which of the following properties of metals explains why?

A. Metals are malleable.

B. Metals have high electronegativities.

C. Metals have valence electrons that can move freely.

D. Metals have high melting points.

8. Which of the following is an important property of the set of elements shaded in the periodic table shown?

A. These elements are the best electrical conductors in the periodic table.

B. These elements form divalent cations.

C. The second ionization energy for these elements is lower than the first ionization energy.

D. The atomic radii of these elements decrease as one moves down the column.

9. When dissolved in water, which of the following ions is most likely to form a complex ion with H₂O?

A. Na⁺

B. Fe²⁺

C. Cl^-

D. S^2-

10. How many valence electrons are present in elements in the third period?

A. 2

B. 3