Atomic and Molecular Weights, Subatomic Particles and Solutions

Atomic and Molecular Weights

O₂ 1:7.93

The relationship between oxygen and hydrogen is 1:7.93. Dalton stated that the atomic weight of oxygen (with valence 2) was 7.92; the modern relative atomic mass of oxygen is about 16.

Avogadro's Principle and Molar Number

Avogadro

Avogadro's principle states that equal volumes of different gases, at the same temperature and pressure, contain the same number of molecules. This allows comparison of molecular weights: the relative density of one gas to another reflects the ratio of their molecular weights.

In a gas, there are approximately 6.023 × 10²³ molecules per mole (Avogadro's number).

Equivalent Weights and Valence

Equivalent weight of a compound is the molecular weight divided by its valence (the combining capacity). Valence is the combining power of an element — how many hydrogen atoms it can replace or combine with.

Subatomic Particles: Electrons, Protons, Neutrons

Electrons: Electrons are negatively charged particles. When an atom loses electrons it becomes a positive ion (a cation); when it gains electrons it becomes a negative ion (an anion).

Protons: Protons are positively charged particles located in the atomic nucleus. An atom becomes positively charged (a cation) if it loses electrons, but the proton itself is a stable positive particle in the nucleus.

Neutrons: Neutrons are electrically neutral particles in the nucleus. They can also be produced in certain nuclear reactions — for example, by bombarding beryllium (Be) with alpha particles, which yields neutrons. Neutrons, like protons and electrons, are fundamental components of atoms.

Oxidation and Reduction

Oxidation: A chemical change in which an atom or group of atoms loses electrons. Oxidation often corresponds to an increase in oxidation state (or valence).

Reduction: A chemical change in which an atom or group of atoms gains electrons. Reduction corresponds to a decrease in oxidation state.

Acid-Base Reactions and Electrolytes

Acids and bases ionize in solution and can produce positive or negative ions. Electrolytes are substances (including many salts, acids, and bases) that, when dissolved in water, dissociate into ions and conduct an electric current.

Homogeneous and Heterogeneous Materials

Homogeneous material: A material that is uniform in properties and composition throughout; it may consist of a single element, compound, or a single phase.

Heterogeneous material: A material composed of two or more distinct parts or phases, separated by definite boundaries where properties change abruptly. A mixture with more than one phase is called a heterogeneous mixture, and each distinct portion is called a phase.

Solutions: Solvent, Solute and Saturation

If a homogeneous phase has a variable composition, it is called a solution. The components of a solution can be solid, liquid, or gas.

• The component present in the larger amount is the solvent.
• The component present in the smaller amount is the solute.
• When no more solute can dissolve at the given conditions, the solution is saturated.