MCAT - Chem/Phys Foundations:: General Chemistry

The atomic number of an element represents the number of:

- **A)** Neutrons in the nucleus
- **B)** Protons in the nucleus (which also equals the number of electrons in a neutral, uncharged atom of that element)
- **C)** Total nucleons (protons plus neutrons)
- **D)** Valence electrons only

Isotopes of a given element are atoms that share the same number of protons but differ in their number of:

- **A)** Electrons, while having identical protons and neutrons
- **B)** Protons, contradicting the definition of isotopes of the same element
- **C)** Neutrons - since atomic mass number (protons plus neutrons) varies between isotopes while the atomic number (proton count, defining the element's identity) stays constant, isotopes of the same element have different atomic masses
- **D)** Valence shell configuration exclusively, with identical neutron count

The modern quantum mechanical model of the atom describes electrons as occupying:

- **A)** Fixed, precisely defined circular orbits at exact distances from the nucleus, identical to the older Bohr model
- **B)** Random positions with absolutely no describable pattern or structure whatsoever
- **C)** A single, undifferentiated 'electron cloud' with no further internal structure or organization
- **D)** Orbitals - three-dimensional regions of space around the nucleus describing the probability distribution of where a given electron is likely to be found, each orbital characterized by a specific set of quantum numbers describing its energy, shape, and orientation

The Pauli exclusion principle states that:

- **A)** No two electrons within the same atom can have the identical set of all four quantum numbers - practically, this means that any single atomic orbital can hold a maximum of two electrons, and if two electrons do occupy the same orbital, they must have opposite (paired) spin states
- **B)** All electrons within an atom must occupy the exact same orbital simultaneously
- **C)** Electrons are repelled by protons, contradicting basic electrostatic attraction
- **D)** An atom can have an unlimited number of electrons within a single orbital, with no capacity restriction

Hund's rule describes how electrons are distributed among a set of orbitals with EQUAL energy (degenerate orbitals, e.g., the three p orbitals within a given p subshell); it states that electrons will:

- **A)** Always pair up within a single orbital of the set before occupying any other orbital in that set
- **B)** Never occupy more than one orbital within the same subshell under any circumstances
- **C)** Individually occupy each available degenerate orbital singly (with parallel/unpaired spins) before any orbital in that set receives a second, paired electron - this arrangement minimizes electron-electron repulsion by initially keeping electrons as spatially separated as possible within the available equal-energy orbitals
- **D)** Occupy orbitals in a completely random pattern with no predictable rule governing the order at all

Valence electrons, the electrons in an atom's outermost occupied principal energy level (shell), are particularly important in chemistry because they:

- **A)** Have no relationship whatsoever to an atom's chemical reactivity or bonding behavior
- **B)** Are always exactly identical in number for every element on the periodic table
- **C)** Are located deep within the atom's nucleus, not in the outer electron shells
- **D)** Are primarily responsible for an atom's chemical bonding behavior and reactivity - since valence electrons are the least tightly bound to the nucleus (being in the outermost shell) and thus the most accessible for interaction with other atoms during the formation of chemical bonds (ionic, covalent, or metallic)

Atomic radius, the general size of an atom, exhibits which trend across a period (row) of the periodic table, moving from left to right?

- **A)** Atomic radius generally increases moving left to right across a period
- **B)** Atomic radius generally DECREASES moving left to right across a period, primarily because the increasing nuclear charge (more protons) across a period pulls the valence electrons (which remain in the same principal energy level/shell across a period) progressively closer to the nucleus, without a proportional increase in electron shielding to offset this stronger nuclear pull
- **C)** Atomic radius remains completely constant and unchanging across an entire period
- **D)** Atomic radius trends are entirely random and unpredictable across the periodic table, with no consistent pattern

Atomic radius exhibits which trend moving DOWN a group (column) of the periodic table?

- **A)** Atomic radius generally INCREASES moving down a group, primarily because each successive element adds an entirely new outermost principal electron shell (a higher n value), and this new, more distant shell (along with increased inner-shell electron shielding of the nuclear charge) outweighs the also-increasing nuclear charge, resulting in a net increase in overall atomic size
- **B)** Atomic radius generally decreases moving down a group
- **C)** Atomic radius remains completely unchanged moving down any group
- **D)** Atomic radius trends are opposite for metals compared to nonmetals within the same group