General Chemistry Exam Study Guide: Atomic Structure, Chemical Quantities, and Periodic Trends

Atomic Structure and Subatomic Particles

Subatomic Particles

Atoms are composed of three main subatomic particles: protons, neutrons, and electrons. Protons and neutrons have approximately equal masses, while electrons are much lighter.

• Protons: Positively charged particles found in the nucleus.

• Neutrons: Neutral particles also located in the nucleus.

• Electrons: Negatively charged particles found in orbitals around the nucleus.

Example: A 10+ ion has lost 10 electrons compared to its neutral atom.

Chemical Quantities and Moles

Mole Calculations

The mole is a fundamental unit in chemistry used to express amounts of a chemical substance. The number of moles is calculated using the formula:

$\text{Moles} = \frac{\text{Mass (g)}}{\text{Molar Mass (g/mol)}}$

• Molar Mass: The mass of one mole of a substance, usually in g/mol.

• Avogadro's Number: $6.022 \times 10^{23}$ particles per mole.

Example: To find the number of moles of H2CO7 in 450.0 g, divide the mass by the molar mass of H2CO7.

Atoms Calculation: To find the number of atoms, multiply the number of moles by Avogadro's number.

Scientific Notation and Significant Figures

Reporting Measurements

Measurements in chemistry should be reported using the correct number of significant figures and, when appropriate, scientific notation.

• Significant Figures: Digits that carry meaning contributing to a measurement's precision.

• Scientific Notation: Expresses numbers as a product of a coefficient and a power of ten, e.g., $1.75 \times 10^{-8}$ m.

Example: The atomic radius of uranium is approximately $1.75 \times 10^{-8}$ cm.

Atomic Orbitals and Electron Configuration

Valence Orbital Diagrams

Valence orbital diagrams show the arrangement of electrons in the outermost shell of an atom. The valence electrons are those involved in chemical bonding.

• P (Phosphorus): 5 valence electrons in the 3p orbital.

• S (Sulfur): 6 valence electrons in the 3p orbital.

• Na (Sodium): 1 valence electron in the 3s orbital.

Orbital Shapes: s orbitals are spherical, p orbitals are dumbbell-shaped.

Quantum Numbers

Quantum numbers describe the properties of atomic orbitals and the electrons in them:

• Principal quantum number (n): Indicates the energy level.

• Angular momentum quantum number (l): Indicates the shape of the orbital.

• Magnetic quantum number (ml): Indicates the orientation of the orbital.

Example: For n = 7 and l = 3, ml can be -3, -2, -1, 0, 1, 2, 3.

Periodic Trends

First Ionization Energy

Ionization energy is the energy required to remove an electron from a gaseous atom. Trends in the periodic table:

• Increases across a period (left to right).

• Decreases down a group (top to bottom).

Order Example: Na < P < S (Lowest to Highest IE)

Atomic Radius

Atomic radius is the distance from the nucleus to the outermost electron shell. Trends:

• Decreases across a period.

• Increases down a group.

Order Example: S < P < Na (Smallest to Largest)

Electromagnetic Radiation and Spectroscopy

Emission and Absorption

Atoms emit and absorb electromagnetic radiation at characteristic wavelengths. The Fraunhofer lines are dark lines in the solar spectrum caused by absorption of specific wavelengths by elements in the sun's atmosphere.

• Emission: Atoms emit light when electrons fall to lower energy levels.

• Absorption: Atoms absorb light when electrons are excited to higher energy levels.

Example: The transition in hydrogen that absorbs a photon with the longest wavelength is from n = 2 to n = 3.

Electron Configuration and Unpaired Electrons

Unpaired Electrons

The number of unpaired electrons in an ion can be determined from its electron configuration.

• Co2+ (Cobalt(II)): Has 3 unpaired electrons.

Isotopes and Average Atomic Mass

Calculating Average Atomic Mass

The average atomic mass of an element is calculated using the masses and abundances of its isotopes:

$\text{Average Atomic Mass} = \sum (\text{Isotope Mass} \times \text{Fractional Abundance})$

Isotope	Mass (g/mol)	Abundance (%)
Cri-301	300.94	73.10%
Cri-303	302.95	26.90%

Example: Average atomic mass = (300.94 × 0.7310) + (302.95 × 0.2690)

Stoichiometry and Mass Calculations

Stoichiometric Calculations

Stoichiometry involves calculating the amounts of reactants and products in chemical reactions.

• Use molar mass to convert between grams and moles.

• Apply ratios from chemical equations to determine required quantities.

Example: To find how much chocolate is needed to consume a certain amount of caffeine, use the mass of caffeine per gram of chocolate and set up a proportion.

Summary Table: Periodic Trends

Element	First Ionization Energy	Atomic Radius
Na	Lowest	Largest
P	Middle	Middle
S	Highest	Smallest

Additional info: Some context and examples were inferred to provide complete academic explanations and self-contained study notes.