UPCAT Chemistry — Atomic Theory & StructureFlash Cards

State Dalton's Atomic Theory (5 main points)

1. All matter consists of tiny particles called atoms 2. Atoms of one element cannot be subdivided or transformed into atoms of another element 3. Atoms cannot be created or destroyed 4. All atoms of the same element are identical in mass, shape, and properties 5. Atoms of different elements combine in simple, whole-number ratios to form compounds

Calculate the number of protons, neutrons, and electrons in ³⁹K⁺ (potassium ion)

Step 1: Identify atomic number of K = 19 (from periodic table) Step 2: Mass number = 39 Step 3: Calculate particles: - Protons = 19 (always equals atomic number) - Neutrons = 39 - 19 = 20 - Electrons = 19 - 1 = 18 (lost 1 electron due to +1 charge) Answer: 19 protons, 20 neutrons, 18 electrons

Compare Thomson's Plum Pudding Model with Rutherford's Nuclear Model

Thomson's Model: - Atom is a sphere of positive charge with electrons embedded like 'plums in pudding' - No nucleus concept - Uniform distribution of positive charge Rutherford's Model: - Dense, positively charged nucleus at center - Electrons orbit around nucleus - Mostly empty space - Based on alpha scattering experiment

What are isotopes? Give an example with Carbon.

Isotopes are atoms of the same element with: - Same number of protons (same atomic number) - Different number of neutrons (different mass numbers) - Same chemical properties but different physical properties Carbon Isotopes: - Carbon-12: 6 protons, 6 neutrons - Carbon-13: 6 protons, 7 neutrons - Carbon-14: 6 protons, 8 neutrons (radioactive)

Write the electron configuration for Calcium (Ca, Z=20)

Step 1: Follow Aufbau principle (fill lowest energy orbitals first) Step 2: Order of filling: 1s, 2s, 2p, 3s, 3p, 4s, 3d... Step 3: Distribute 20 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² Verification: 2+2+6+2+6+2 = 20 ✓ Noble gas notation: [Ar] 4s²

What happens during alpha decay? Write the nuclear equation for Uranium-238.

Alpha Decay Process: - Nucleus emits an alpha particle (⁴He or ⁴α) - Alpha particle = 2 protons + 2 neutrons - Mass number decreases by 4 - Atomic number decreases by 2 Uranium-238 decay: ²³⁸U → ²³⁴Th + ⁴He Result: Uranium becomes Thorium + helium nucleus

List the four quantum numbers and their meanings

1. Principal Quantum Number (n): - Values: 1, 2, 3, ... - Describes: Energy level/shell size 2. Angular Momentum (ℓ): - Values: 0 to (n-1) - Describes: Orbital shape (s, p, d, f) 3. Magnetic (mₗ): - Values: -ℓ to +ℓ - Describes: Orbital orientation 4. Spin (mₛ): - Values: +½ or -½ - Describes: Electron spin direction

Calculate the atomic mass of Chlorine given: Cl-35 (75%) and Cl-37 (25%)

Formula: Atomic mass = (mass₁ × %₁) + (mass₂ × %₂) Step 1: Convert percentages to decimals 75% = 0.75, 25% = 0.25 Step 2: Calculate weighted average Atomic mass = (35 × 0.75) + (37 × 0.25) Atomic mass = 26.25 + 9.25 = 35.5 amu Answer: 35.5 amu

Why do atoms form ions? Explain cation and anion formation.

Atoms form ions to achieve stable electron configuration (noble gas configuration): Cations (positive ions): - Metals lose electrons - Example: Na → Na⁺ + e⁻ - Achieve stability by losing valence electrons Anions (negative ions): - Nonmetals gain electrons - Example: Cl + e⁻ → Cl⁻ - Achieve stability by filling outer shell Driving force: Lower energy state with complete outer shell

Explain the nuclear stability rules for even/odd protons and neutrons

Nuclear Stability Pattern: 1. Even protons + Even neutrons = MOST STABLE (168 nuclides) Example: ¹²C, ¹⁶O 2. Even protons + Odd neutrons = STABLE (57 nuclides) Example: ¹³C 3. Odd protons + Even neutrons = STABLE (50 nuclides) Example: ¹⁵N 4. Odd protons + Odd neutrons = LEAST STABLE (4 nuclides) Example: ²H, ⁶Li Reason: Pairing energy makes even numbers more stable

What is the maximum number of electrons in n=3 shell? Show orbital breakdown.

Formula: Maximum electrons = 2n² For n=3: Maximum = 2(3)² = 18 electrons Orbital breakdown: - 3s orbital: 2 electrons - 3p orbitals (3): 6 electrons - 3d orbitals (5): 10 electrons Total: 2 + 6 + 10 = 18 electrons Subshells: 3s² 3p⁶ 3d¹⁰

Compare beta decay and gamma decay processes

Beta Decay: - Neutron converts to proton + electron - Equation: n → p⁺ + e⁻ + ν̄ - Mass number stays same, atomic number +1 - Example: ²³⁴Th → ²³⁴Pa + e⁻ Gamma Decay: - Nucleus releases energy (no particles) - High-energy electromagnetic radiation - No change in mass or atomic number - Often follows alpha or beta decay - Example: ⁶⁰Co* → ⁶⁰Co + γ

Explain periodic trends: atomic radius and ionization energy across Period 3

Across Period 3 (Na to Ar): Atomic Radius DECREASES: - More protons = stronger nuclear charge - Same electron shielding - Electrons pulled closer to nucleus - Na > Mg > Al > Si > P > S > Cl > Ar Ionization Energy INCREASES: - Stronger attraction to nucleus - Harder to remove electrons - Na < Mg < Al < Si < P < S < Cl < Ar Both trends due to effective nuclear charge increase

What are the quantum number values for a 3d electron?

For 3d electron: 1. Principal (n) = 3 (third energy level) 2. Angular momentum (ℓ) = 2 (d subshell corresponds to ℓ = 2) 3. Magnetic (mₗ) = -2, -1, 0, +1, +2 (5 possible values for d orbitals) 4. Spin (mₛ) = +½ or -½ (electron can spin up or down) Total possible combinations: 5 orbitals × 2 spins = 10 electrons maximum

Calculate the number of neutrons in an atom with mass number 127 and 53 protons

Given: - Mass number (A) = 127 - Number of protons (Z) = 53 Formula: Mass number = Protons + Neutrons A = Z + N Solve for neutrons (N): N = A - Z N = 127 - 53 = 74 Answer: 74 neutrons Note: This is Iodine-127 (¹²⁷I)

Why was Bohr's atomic model an improvement over Rutherford's model?

Bohr's Improvements: 1. Fixed Energy Levels: - Electrons in specific orbits (not spiraling) - Solved stability problem 2. Quantized Energy: - Energy levels: E₁, E₂, E₃... - Electrons can only exist in these levels 3. Light Emission/Absorption: - Explained spectral lines - Energy difference = hν 4. Stability: - Electrons don't radiate energy in stationary orbits - Solved classical physics contradiction Limitation: Only worked for hydrogen

What is the electron configuration of Fe³⁺ (iron with +3 charge)?

Step 1: Find neutral Fe configuration Fe (Z=26): [Ar] 4s² 3d⁶ Step 2: Remove electrons for Fe³⁺ - Remove 3 electrons total - Remove 4s electrons first (2 electrons) - Then remove 1 electron from 3d Step 3: Final configuration Fe³⁺: [Ar] 3d⁵ Or expanded: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ Note: 4s electrons are removed before 3d when forming cations

Define half-life and calculate remaining atoms after 2 half-lives

Half-life (t₁/₂): Time needed for half of radioactive nuclei to decay Calculation for 2 half-lives: Starting amount = N₀ After 1 half-life: N₁ = N₀/2 = 0.5N₀ After 2 half-lives: N₂ = N₁/2 = N₀/4 = 0.25N₀ General formula: N = N₀(1/2)ⁿ where n = number of half-lives For n=2: N = N₀(1/2)² = N₀/4 Answer: 25% of original atoms remain

Explain Hund's Rule with an example using nitrogen (N)

Hund's Rule: Single electrons with same spin occupy each equal-energy orbital before pairing occurs Nitrogen (Z=7): 1s² 2s² 2p³ 2p orbital filling: 2p: ↑ ↑ ↑ (correct - follows Hund's rule) □ □ □ NOT: ↑↓ ↑ □ (incorrect - violates Hund's rule) □ □ □ Reason: - Lower energy when electrons are unpaired - Minimizes electron-electron repulsion - Maximum number of unpaired electrons = maximum spin

What is the wavelength-frequency relationship for electromagnetic radiation? Calculate frequency if λ = 500 nm.

Relationship: c = λν where: c = speed of light = 3.00 × 10⁸ m/s λ = wavelength (m) ν = frequency (Hz) Given: λ = 500 nm = 500 × 10⁻⁹ m Solve for frequency: ν = c/λ ν = (3.00 × 10⁸ m/s)/(500 × 10⁻⁹ m) ν = 6.00 × 10¹⁴ Hz Answer: 6.00 × 10¹⁴ Hz (green light)