Atoms, Elements, Chemical Reactions, and Molecular Structure: General Chemistry Study Guide

Atoms & Elements

Atomic Structure

Atoms are the fundamental building blocks of matter, composed of subatomic particles: protons, neutrons, and electrons. Understanding atomic structure is essential for grasping chemical properties and reactions.

• Atoms: Smallest unit of matter, indivisible by chemical means.

• Subatomic Particles:

  • Protons: Positive charge, mass ≈ 1 amu.

  • Neutrons: Neutral charge, mass ≈ 1 amu.

  • Electrons: Negative charge, mass ≈ 0.0005 amu.

• Atomic Number (Z): Number of protons in the nucleus; defines the element.

• Mass Number (A): Sum of protons and neutrons.

• Isotopes: Atoms of the same element with different numbers of neutrons.

Example: Carbon-12 and Carbon-13 are isotopes of carbon.

Valence Electrons

Valence electrons are the outermost electrons involved in chemical bonding. Their configuration determines an atom's reactivity.

• Valence Electrons: Highest energy electrons in the outer shell.

• Core Electrons: Electrons in inner shells, not involved in bonding.

Symbols and Notation

Chemical symbols and notation are used to represent elements and their atomic structure.

• Element Symbol: One or two letters (e.g., C for carbon).

• Notation: $\text{A}^\text{Z}\text{X}$, where X = element symbol, Z = atomic number, A = mass number.

Atoms and Subatomic Particles

Changing Subatomic Particles

Changing the number of protons, neutrons, or electrons alters the identity or properties of an atom.

• Changing Protons: Results in a new element.

• Changing Neutrons: Creates isotopes.

• Changing Electrons: Forms ions.

Atomic Mass and Isotopes

Atomic mass is the weighted average of isotopes based on their natural abundance.

• Molecular Mass: Sum of masses of all atoms in a molecule.

• Atomic Weight: Weighted average of isotopic masses.

Chemical Reactions

Types of Chemical Reactions

Chemical reactions involve the rearrangement of atoms to form new substances. There are several main types:

1. Synthesis: Two or more atoms combine to form a single compound. $\text{A} + \text{B} \rightarrow \text{AB}$

2. Decomposition: Compound breaks down into two or more products. $\text{AB} \rightarrow \text{A} + \text{B}$

3. Single Displacement: One element replaces another. $\text{AB} + \text{C} \rightarrow \text{AC} + \text{B}$

4. Double Displacement: Exchange of ions between compounds. $\text{AB} + \text{CD} \rightarrow \text{AD} + \text{CB}$

5. Combustion: Substance reacts with O2 to produce energy. $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$

Physical vs. Chemical Changes

Physical changes alter the state or appearance of matter without changing its composition, while chemical changes result in new substances.

• Physical Change: Melting, boiling, dissolving.

• Chemical Change: Burning, rusting, reacting.

Quantum Theory and Electron Configuration

Quantum Numbers

Quantum numbers describe the properties of atomic orbitals and electrons.

• Principal Quantum Number (n): Energy level, n = 1, 2, 3, ...

• Azimuthal Quantum Number (l): Shape of orbital, l = 0 (s), 1 (p), 2 (d), 3 (f).

• Magnetic Quantum Number (ml): Orientation in space, ml = -l to +l.

• Spin Quantum Number (ms): Electron spin, ms = +1/2 or -1/2.

Electron Configuration

Electron configuration describes the arrangement of electrons in an atom's orbitals.

• Aufbau Principle: Electrons fill lowest energy orbitals first.

• Pauli Exclusion Principle: No two electrons in an atom can have the same set of quantum numbers.

• Hund's Rule: Electrons occupy orbitals singly before pairing.

Bonding & Molecular Structure

Ionic and Covalent Bonds

Atoms form bonds to achieve stable electron configurations. Ionic bonds involve electron transfer, while covalent bonds involve electron sharing.

• Ionic Bonds: Formed between metals and nonmetals; electrons are transferred.

• Covalent Bonds: Formed between nonmetals; electrons are shared.

Bond Types Theory

Bond strength, polarity, and distribution of charges affect molecular properties.

• Bond Energy: Energy required to break a bond.

• Polarity: Unequal sharing of electrons creates dipoles.

• Intermolecular Forces:

  • London Dispersion: Weak, temporary dipoles.

  • Dipole-Dipole: Attraction between polar molecules.

  • Hydrogen Bond: Strong dipole-dipole involving H bonded to N, O, or F.

  • Ion-Dipole: Attraction between ions and polar molecules.

Molecular Shapes & Valence Bond Theory

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) theory predicts molecular geometry based on electron domain repulsion.

• Linear: 2 electron domains, bond angle 180°.

• Trigonal Planar: 3 electron domains, bond angle 120°.

• Tetrahedral: 4 electron domains, bond angle 109.5°.

• Trigonal Bipyramidal: 5 electron domains, bond angles 90°, 120°.

• Octahedral: 6 electron domains, bond angle 90°.

Hybridization

Hybridization explains the mixing of atomic orbitals to form new, equivalent orbitals for bonding.

• sp: Linear geometry.

• sp2: Trigonal planar geometry.

• sp3: Tetrahedral geometry.

Lewis Dot Structures

Drawing Lewis Structures

Lewis dot structures represent valence electrons and help predict molecular bonding and shape.

• Octet Rule: Atoms tend to have eight electrons in their valence shell.

• Formal Charge: Used to determine the most stable structure. $\text{Formal Charge} = \text{Valence Electrons} - \text{Nonbonding Electrons} - \frac{1}{2} \times \text{Bonding Electrons}$

Periodic Properties

Periodic Trends

The periodic table organizes elements by increasing atomic number and reveals trends in properties.

• Atomic Radius: Increases down a group, decreases across a period.

• Ionization Energy: Increases across a period, decreases down a group.

• Electronegativity: Increases across a period, decreases down a group.

Additional info:

• Notes cover core general chemistry topics: atomic structure, chemical reactions, quantum theory, bonding, molecular shapes, and periodic properties.

• Images included are directly relevant to the explanation and reinforce key concepts.