Atoms and Molecules

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The mass number of an atom is 15, and its atomic number is 7. The atom probably has…

A) 8 neutrons in the nucleus.
B) 7 units of negative charge in the nucleus.
C) at least 15 electrons.
D) about as much mass in electrons as in protons.
E) 7 electrons in the nucleus.

8 neutrons in the nucleus

Which statement is true of atoms?

A) Electrons determine the atom’s size.
B) Most of an atom’s volume is filled with matter.
C) Protons attract other protons.
D) Protons repel electrons.
E) All of the above.

Electrons determine the atom’s size

Dr. Jones says an atom has 3 electrons in the first shell and four electrons in the second shell. Someone should tell Dr. Jones that …

A) the first shell must fill before the second shell can have electrons.
B) the second shell can’t have 4 electrons.
C) no shell can hold more than 2 electrons.
D) the first shell shouldn’t have 3 electrons.
E) the second shell should have 8 electrons.

the first shell shouldn’t have 3 electrons

Which statement is true of the energy levels of electrons in shells?

A) All the electrons in an atom have similar amounts of energy.
B) Electrons must lose energy to move from the first to the second shell.
C) The valence shell has higher energy than other occupied shells.
D) All of the above.
E) None of the above.

The valence shell has higher energy than other occupied shells

Two atoms always represent the same element if they have …

A) the same number of electrons.
B) the same mass number.
C) the same number of particles in the nucleus.
D) the same number of shells.
E) the same number of protons.

the same number of protons

An atom has 6 electrons, 6 protons, and 6 neutrons. You can tell that this atom belongs to the element _____ because _____________________.

A) C; it has 6 protons.
B) O; its mass number is 12.
C) C; it has 6 electrons.
D) N; it has 6 protons.
E) N; it has 6 electrons.

C; it has 6 protons

An atom has 8 protons, 8 neutrons, and 8 electrons. Another isotope of the same element might have …

A) 7 electrons.
B) mass number 16, atomic number 7.
C) 10 neutrons.
D) 9 protons.
E) All of the above.

10 neutrons

Radioactive decay is likely to occur when …

A) an atom has too many electrons.
B) an electron hits the nucleus.
C) atoms collide with one another.
D) an atom has too many neutrons.
E) protons break into neutrons and electrons.

an atom has too many neutrons

Which statement is most useful in explaining why chemists assign atoms to chemical elements by counting protons?

A) Protons at the atom’s surface determine the atom’s behavior.
B) The nucleus doesn’t change in stable isotopes.
C) 99% of the atom’s mass consists of protons.
D) The proton’s negative charge holds electrons in the atom.
E) None of these. Elements are defined by the number of protons.

None of these. Elements are defined by the number of protons

Covalent bonds hold atoms together because they …

A) (a) fill shells without giving atoms much charge.
B) (b) bring electrons closer to protons.
C) (c) use forces between nuclei as well as forces between electrons.
D) do all of the above.
E) do both (a) and (b).

do both (a) and (b)

In molecules, C, H, O, and N atoms usually make __, __, __, and __ bonds respectively.

A) 4, 1, 2, 3
B) 4, 1, 3, 2
C) 3, 2, 1, 4
D) 2, 1, 3, 4
E) 3, 2, 4, 2

4, 1, 2, 3

An atom’s atomic number is 7. Its valence is most likely …

A) 5
B) 7
C) 2
D) 3
E) 1


By making two covalent bonds, an O atom (with 8 protons) fills its valence shell. Why does the atom’s charge stay close to zero?

A) The atom lost electrons from other shells.
B) Shared electrons aren’t always near oxygen.
C) The atom has 8 electrons.
D) The charge isn’t near zero; it’s -2.
E) The valence shell has 6 electrons.

Shared electrons aren’t always near oxygen

In a double covalent bond, a carbon atom shares …

A) electrons in two orbitals.
B) two electrons.
C) both valence and nonvalence electrons.
D) electrons in two of its shells.
E) None of the above.

electrons in two orbitals

Two C atoms form a double bond. Each C is bound to two H atoms. Which statement is true?

A) The bonds orient in a tetrahedral fashion.
B) All the atoms lie in a line.
C) The bonds orient like tripods or pyramids.
D) All the atoms lie in a plane.
E) The groups rotate around the C=C bond.

All the atoms lie in a plane

Partial charges occur when …

A) (a) a covalent bond links atoms of two kinds.
B) (b) atoms share electrons unequally.
C) (c) two ions are close together.
D) any of the above occur.
E) both (a) and (b).

(b) atoms share electrons unequally

To fill the valence shell, an electrically neutral, unbonded atom with atomic number 8 must add …

A) 3 electrons.
B) 2 electrons.
C) 1 electron.
D) 8 electrons.
E) Can’t tell without knowing which element it is.

2 electrons

Which answer helps to explain why carbon atoms tend to make 4 covalent bonds?

A) The carbon nucleus has 4 protons.
B) The first electron shell has 4 orbitals.
C) The valence shell needs 8 electrons.
D) All of the above.
E) None of the above; carbon makes 3 covalent bonds.

The valence shell needs 8 electrons

An electrically neutral molecule has the formula C3H4O2N. If the carbon atoms form the usual number of bonds, how many covalent bonds will each hydrogen atom have with other atoms in the molecule?

A) 4
B) 3
C) 1
D) 5
E) 2


A phrase that applies to covalent bonding and not other kinds of bonds is …

A) paired electrons.
B) electron-sharing.
C) charge attraction.
D) great strength.
E) All of the above.


When chemists and biologists want to show how atoms are bonded in a molecule, they usually use a …

A) space-filling model.
B) molecular formula.
C) ball-and-stick model.
D) structural formula.

structural formula

Polar attractions are …

A) (a) forces between atoms with partial charges.
B) (b) weaker than covalent bonds.
C) (c) important because they are numerous.
D) (a) and (b) are true. (c) is false.
E) (a), (b), and (c) are correct.

(a), (b), and (c) are correct

Hydrogen bonds …

A) can form between H and N.
B) occur within a water molecule.
C) share electrons between the two bonded atoms.
D) are covalent bonds.
E) All of the above.

can form between H and N

Compared to covalent bonds, ionic bonds …

A) have less bond energy.
B) rely much less on electron-sharing.
C) are stronger.
D) are more abundant in biological molelcules.
E) are more stable in water.

rely much less on electron-sharing

A cation with two units of charge has 10 neutrons and 8 protons. The ion also has …

A) 8 electrons.
B) 16 electrons.
C) 6 electrons.
D) 10 electrons.
E) 12 electrons.

6 electrons

True or false: Ions may form by transferring an atomic nucleus (with or without electrons) to another molecule.

A) False
B) True


Which answer correctly ranks the atoms in terms of decreasing electronegativity (the highest electronegativity first)?

A) N, O, C, H
B) O, N, H, C
C) O, N, C, H
D) C, H, O, N
E) N, O, H, C

O, N, C, H

Dr. Haxton says the O-O bond is polar and the C-C bond is nonpolar. A good student would say …

A) Right! O is electronegative, so O2 is polar.
B) No way. C is more electronegative than O.
C) Wrong again, Ralph. Both bonds are nonpolar.
D) No, both bonds are highly polar.
E) Yes. O attracts electrons more strongly than C.

Wrong again, Ralph. Both bonds are nonpolar

Use your knowledge of electronegativity to pick out the most polar bond in the list below.

A) C-H
B) N-H
C) O-H
D) O-C
E) O-N


If you want a molecule that is highly polar, look for one that contains:

A) S-H
B) O-N
C) H-N
D) C-H
E) C-S


Which biological activity does NOT directly involve proteins?

A) Sensing light.
B) Defending cells against viruses.
C) Breaking food polymers into smaller molecules.
D) Changing the shape of a cell.
E) None of the above; proteins are involved in all of them.

None of the above; proteins are involved in all of them

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