Basics

Question 1

Atomic Number (Z) and Mass Number (A)

Answer 1

• Atomic Number (Z): Number of protons.
• Mass Number (A): Protons + Neutrons.
• Number of protons = Number of electrons in ground state.
• Atomic Mass (in amu) = Mass Number.
• Atomic Weight = Weighted average of the mass numbers of different isotopes.

Question 2

Energy of Emitted/Absorbed Photon

Answer 2

• E = h f, where Planck’s constant h = 6.626 × 10^(-34) J•s.
• c = f λ.
• E = h c / λ.

Question 3

Quantum Mechanical Model

Answer 3

• Heisenberg Uncertainty Principle: The momentum and position of electron cannot be simultaneously determined with perfect accuracy.
• Pauli Exclusion Principle: No two electrons in a given atom can possess the same set of four quantum numbers.
• Principle Quantum Number (n): Represents energy level and shell number. Maximum number of electrons within a shell = 2(n)².
• Azimuthal Quantum Number (l): Represents shape of subshell. Range of possible l values is 0 to (n - 1). Converted to “s” subshell (l=0), “p” subshell (l=1), “d” subshell (l=2), “f” subshell (l=3).
• Magnetic Quantum Number (mₗ): Represents orbital within a subshell. Range of possible mₗ values is -l to +l, including 0. “s” orbital is spherical and each “p” orbital is dumbbell-shaped.
• Spin Quantum Number (mₛ): Represents spin orientations +1/2 and -1/2.

• Electrons in Second Energy Level:
n = 2
l = {0,1} (s,p)
mₗ = {0} (1 orbital); {-1,0,+1} (3 orbitals)
mₛ = {+1/2,-1/2}; {+1/2,-1/2}; {+1/2,-1/2}; {+1/2,-1/2}

Question 4

Electron Configurations

Answer 4

• Aufbau Principle: Electrons fill from lower-energy subshells to higher-energy subshells. Use n+l rule to figure out that lowest energy fills first. 6s subshell (6+0=6) has lower energy than and will fill before 5d subshell (5+2=7).
• Hund’s Rule: Orbitals in a given subshell are filled such that there are a maximum number of half-filled orbitals with parallel spins. Group of Chromium moves electron from “s” subshell to “d” subshell to have half-filled “d” subshell ([Ar]4s²3d⁴ -> [Ar]4s¹3d⁵). Group of Copper moves electron from highest “s” subshell to highest “d” subshell to have fully-filled “d” subshell ([Ar]4s²3d⁹ -> [Ar]4s¹3d¹⁰). Paramagnetic materials have unpaired electrons that can be induced to have parallel spins by magnetic field and are attracted to the magnetic field. Diamagnetic materials have only paired electrons that are opposed to being reoriented by magnetic field and are repelled by the magnetic field.

Question 5

Periodic Trends

Answer 5

• Effective Nuclear Charge (Zeff): Electrostatic attraction between the valence shell electrons and the nucleus; increases from left to right across a period, with little change from top to bottom of group. Valence electrons increasingly separated from nucleus as principle quantum number (n) increases from top to bottom in group.
• Atomic Radius decreases from left to right across a period and increases from top to bottom in a group.
• Ionic Radius of nonmetals is largest at metalloid boundary and Ionic Radius of metals is smallest at metalloid boundary. Cations are generally smaller than their corresponding neutral atom, while anions are generally larger than their corresponding neutral atom.
• Ionization Energy: Amount of energy necessary to remove an electron from the valence shell (endothermic process); increases from left to right across a period and decreases from top to bottom in a group.
• Electron Affinity: Amount of energy released when an atom gains an electron in its valence shell (exothermic process); increases from left to right across a period and decreases from top to bottom in a group.
• Electronegativity: Measure of the attractive force between nucleus and electrons within a bond; increases from left to right across a period and decreases from top to bottom in a group.

Question 6

Groups in Periodic Table

Answer 6

• Alkali Metals (Group IA): Take on oxidation state of +1. Most reactive metals.
• Alkaline Earth Metals (Group IIA): Take on oxidation state of +2. Most reactive metals.
• Chalcogens (Group VIA): Take on oxidation state of -2. Biologically important elements.
• Halogens (Group VIIA): Take on oxidation state of -1. Nonmetals with highest electronegativities.
• Noble Gases (Group VIIIA): Have fully filled balance shell, very high ionization energies, and nonexistent electronegativities and electron affinities. Inert gases.
• Transition Metals (B Groups): Take on multiple oxidation states, and have low electronegativities, low ionization energies, and low electron affinities.

Question 7

Formal Charge

Answer 7

• FC = Valence Electrons - Nonbonding Lone Electrons - 1/2(Electrons in Bonds).
• FC = VEs - Dots - Sticks.
• Charge of an ion or compound is equal to the sum of the formal charges of the individual atoms comprising the ion or compound.

• Formal Charge on central N atom of [NH₄]+ is FC = 5 - 0 - 4 = +1.

Question 8

Mole and Molar Mass

Answer 8

• Avogadro’s Number = 6.022 × 10²³ per mole.

* Molar Mass: Mass of one mole of a compound; expressed in g/mol.

Question 9

Normality and Gram Equivalent Weight

Answer 9

• N = Molarity × n.
• Units are equivalents/L.
• 1N H₂CO₃ solution, Molarity of H₂CO₃ is 0.5M.
• N = 0.5M × 2 mol protons/mol H₂CO₃ = 1N protons.
• Gram Equivalent Weight = Molar Mass / n.
• Amount of a compound in grams needed to produce one equivalent of particle of interest.
• Gram Equivalent Weight of H₂CO₃ is (62 g/mol) / 2 = 31g to produce 1 equivalent of hydrogen ions.

Question 10

Percent Composition

Answer 10

• Percent Composition = (Mass of element in formula / Molar Mass) × 100%.
• Percent Composition of Chromium in K₂Cr₂O₇ is (2(52.0)/294.2) × 100% = 35.4%.

Question 11

Stoichiometric Conversions

Answer 11

• 1 mole of ideal gas at STP = 22.4 L.
• Avogadro’s Number = 6.022 × 10²³ per mole.
• Molar Mass obtained from periodic table.

Question 12

Percent Yield

Answer 12

• Percent Yield: (Actual Yield/Theoretical Yield) × 100%.

Question 13

Common Polyatomic Anions

Answer 13

• Nitrate: NO₃-
• Carbonate: CO₃²-
• Sulfate: SO₄²-
• Phosphate: PO₄³-