Atomic Structure
During Rutherford's experiment, most alpha particles passed directly through the gold foil. Which conclusion is best supported by this observation?
A. Atoms contain mostly empty space
B. Electrons occupy most of the atom's mass
C. Positive charge is evenly distributed
D. Neutrons surround the nucleus
A — Atoms contain mostly empty space
Why: Most particles passed through without major deflection, showing that most of the atom is empty space.
The total number of protons and neutrons in an atom is its __________ number.
Mass
Why: Mass number equals protons plus neutrons.
Why is the atomic mass of many elements listed as a decimal on the periodic table?
A. It represents a weighted average
B. Atoms contain partial protons
C. Mass numbers are decimals
D. Electrons have variable charges
A — It represents a weighted average
Why: The listed atomic mass reflects the masses and natural abundances of isotopes.
An atom has six valence electrons. Which change would move it closer to a full outer energy level?
A. Gain two electrons
B. Lose two electrons
C. Gain six protons
D. Lose six neutrons
A — Gain two electrons
Why: Six valence electrons require two more electrons to reach eight.
Two atoms form a bond by sharing electrons unequally. What type of bond is this?
A. Polar covalent
B. Ionic
C. Nonpolar ionic
D. Hydrogen
A — Polar covalent
Why: Polar covalent bonds involve unequal sharing of electrons.
Thomson's cathode ray experiments provided evidence that atoms contain negatively charged __________.
Electrons
Why: Thomson's experiments led to the discovery of the electron.
Two atoms have identical numbers of protons but different numbers of neutrons. Which statement must be true?
A. They are isotopes
B. They are different elements
C. They have different atomic numbers
D. They have different proton charges
A — They are isotopes
Why: Isotopes are atoms of the same element with different neutron numbers.
To calculate an isotope's contribution to average atomic mass, multiply its mass by its __________ abundance.
Decimal
Why: Percent abundance must be converted to a decimal before multiplying.
Elements in the same group generally have similar chemical properties because they have similar valence-electron arrangements. (T/F)
True
Why: Valence electrons strongly influence chemical behavior.
An ionic bond forms when electrons are __________ between atoms.
Transferred
Why: Electron transfer creates oppositely charged ions that attract.
Rutherford's results supported a model in which nearly all of an atom's mass is concentrated in a tiny region. (T/F)
True
Why: The nucleus contains protons and neutrons and therefore most of the atom's mass.
An atom contains 23 protons and 28 neutrons. What is its mass number?
A. 5
B. 23
C. 28
D. 51
D — 51
Why: 23 + 28 = 51.
An element has two isotopes: 20 amu at 80% abundance and 22 amu at 20% abundance. Which value is closest to its average atomic mass?
A. 20.2 amu
B. 20.4 amu
C. 21.0 amu
D. 21.6 amu
B — 20.4 amu
Why: (20 × 0.80) + (22 × 0.20) = 20.4 amu.
An electron absorbs energy and moves farther from the nucleus. What happens to its energy level?
A. It increases
B. It decreases
C. It becomes zero
D. It becomes negative
A — It increases
Why: Electrons farther from the nucleus occupy higher energy levels.
Hydrogen bonds are stronger than covalent bonds within a water molecule. (T/F)
False
Why: Hydrogen bonds are weaker attractions between molecules, while covalent bonds hold atoms together within a molecule.
Which discovery most directly challenged the idea that atoms were indivisible?
A. Electrons were discovered
B. Isotopes were named
C. Atomic masses were averaged
D. Chemical reactions were observed
A — Electrons were discovered
Why: The discovery of electrons showed that atoms contain smaller particles.
An atom contains 11 protons, 12 neutrons, and 10 electrons. Which statement correctly describes it?
A. A 1+ sodium ion
B. A 1− sodium ion
C. A neutral sodium atom
D. A 2+ magnesium ion
A — A 1+ sodium ion
Eleven protons identify sodium, and having one fewer electron produces a 1+ charge.
An element has isotopes of 35 amu and 37 amu. Its average atomic mass is 36.8 amu. Which isotope is more abundant?
A. 35 amu
B. 37 amu
C. Both equally
D. Cannot be determined
B — 37 amu
Why: The average is much closer to 37, indicating that isotope contributes more heavily.
An atom's tendency to participate in chemical changes is strongly influenced by its __________ electrons.
valence
Why: Valence electrons are involved in bonding and chemical interactions.
Water molecules attract one another because oxygen in one molecule can attract which region of another water molecule?
A. Partially positive hydrogen
B. Negatively charged neutron
C. Positive proton
D. Neutral electron cloud
A — Partially positive hydrogen
Why: The partial charges produced by water's polar bonds allow hydrogen bonding.
A particle has approximately 1 amu of mass but no electrical charge. Where would it most likely be located?
A. Inside the nucleus
B. Within the electron cloud
C. Outside the atom
D. Between energy levels
A — Inside the nucleus
Why: Neutrons have about 1 amu of mass, no charge, and are located in the nucleus.
A neutral atom containing 18 protons must contain __________ electrons.
18
Why: Neutral atoms have equal numbers of protons and electrons.
Why can average atomic mass be decimal?
It is a weighted average.
Why: It combines isotope masses according to their natural abundances.
Why do atoms gain, lose, or share electrons?
To achieve greater stability.
Why: Atoms tend toward more stable outer electron arrangements.
A reaction reaches equilibrium. Describe what is happening.
A — Forward and reverse rates are equal
Why: Dynamic equilibrium occurs when the forward and reverse reactions proceed at equal rates.