Fundamental Chemistry Laws, Atomic Models, and Periodic Properties
Fundamental Chemical Laws
Law of Conservation of Mass
In any chemical reaction that takes place in a closed system, mass is conserved: The mass of starting substances (reactants) is the same as that of the final products.
Law of Definite Proportions
When two or more elements combine to yield a compound, they do so in a fixed proportion.
Law of Multiple Proportions
When two or more elements combine to form different compounds, they do so in a ratio of small whole numbers.
Dalton’s Atomic Theory
- Chemical elements are made of particles called atoms, which are indivisible.
- All atoms of the same element are equal (have the same mass and properties). Different atoms have different masses and properties.
- Chemical compounds are formed by the union of different atoms, which combine in a ratio of simple whole numbers.
- Atoms are neither created nor destroyed, only rearranged.
Gas Laws
Boyle’s Law
At constant temperature, the volume occupied by a mass of gas is inversely proportional to the pressure: P * V = K; P1 * V1 = P2 * V2.
Justification of Boyle’s Law
If temperature is constant, compressing a gas by half, according to kinetic theory, means the gas molecules continue with the same average speed, but collide twice as often because the distance has been halved. In the same time, they travel half the distance, thus colliding twice as often, and pressure increases twice.
Charles-Gay Lussac’s Law
At constant pressure, volume is directly proportional to temperature: V1/T1 = V2/T2.
Justification of Charles-Gay Lussac’s Law
If we increase the temperature of a gas twice, according to kinetic theory, the speed of the particles doubles, increasing pressure. Since the external pressure is constant, the gas will double in volume to equalize pressure.
Combined Gas Law
Ideal gases perfectly fulfill Boyle’s and Gay-Lussac’s laws: P1 * V1/T1 = P2 * V2/T2
Ideal Gas Law
P * V = n * R * T, where R = 0.082 atm * L / mol * K
Kinetic-Molecular Theory
- Gases consist of particles (atoms or molecules) separated by distances.
- Gas molecules continuously move randomly, colliding with each other and the container walls.
- Collisions are perfectly elastic, so there’s no kinetic energy change.
- Average kinetic energy is directly proportional to temperature.
Rationale for Gas Properties
Since gas molecules are in constant motion and collide with container walls, gases take the container’s shape, can be compressed, and exert pressure.
Atomic Models
Thompson’s Atomic Model
Thompson suggested a model of a solid sphere with embedded electrons neutralized by a positive charge. Adding electrons would give a negative charge, and losing electrons would give a positive charge.
Discovery of the Proton (Rutherford)
Rutherford observed cathode rays in a cathode ray tube, traveling in straight lines opposite the cathode. Canal rays originated from collisions of cathode rays with atoms, tearing off electrons and creating ions.
Discovery of the Electron
Electrons travel in straight lines, have mass, have a negative charge, and are fundamental particles of the atom.
Bohr Model
Electrons orbit the nucleus in circular orbits without losing energy. Allowed orbits have specific energy levels (n). Energy increases with distance from the nucleus. Electrons can jump between energy levels by emitting or absorbing radiation.
Corrections to the Bohr Model
Bohr’s model was incorrect because it couldn’t explain fine structure. Orbits could be elliptical and have different spatial orientations, and electrons behave like waves.
Atomic Properties and Periodic Trends
Atomic Number (Z)
The number of protons, a characteristic property of an element.
Mass Number (A)
A = Z + N (sum of protons and neutrons)
Isotopes
Atoms of the same element with the same atomic number but different mass numbers.
Chemical Element
A pure substance formed by atoms with the same atomic number.
Heisenberg Uncertainty Principle
It is impossible to know both the position and velocity of an electron simultaneously.
Atomic Orbital
The area of space where there is the highest probability of finding an electron with a specific energy.
Pauli Exclusion Principle
No two electrons in an atom can have the same four quantum numbers. Each orbital holds a maximum of two electrons.
Electronic Configurations
- Aufbau Principle: Electrons fill orbitals in order of increasing energy.
- Pauli Exclusion Principle: Two electrons in the same orbital have opposite spins.
- Hund’s Rule: Electrons fill degenerate orbitals singly before pairing up, with parallel spins.
Periodic Properties
- Atomic Radius: Decreases across a period and increases down a group. Cations are smaller than their neutral atoms, and anions are larger.
- Ionization Energy: Energy required to remove an electron. Increases across a period and decreases down a group.
- Electron Affinity: Energy change when an electron is added. Increases across a period and decreases down a group.
- Electronegativity: Tendency of an atom to attract electrons in a bond. Increases across a period and decreases down a group.
Metallic and Non-Metallic Character
- Metals: Low ionization energy, low electron affinity, tend to form cations.
- Non-metals: High ionization energy, high electron affinity, tend to form anions.
- Metalloids: Properties intermediate between metals and non-metals.