Fundamental Chemistry Principles

Atomic Structure and Properties

Atomic Models

The concept of matter, initially considered indivisible according to Dalton, is presented with a discontinuous structure based on cathode rays (Thomson) and canal rays (Goldstein).

Rutherford considered the atom to consist of a small, hard core containing protons and neutrons, with electrons rotating around this core.

Bohr proposed that electrons in the atom are located in specific energy levels and sublevels, representing the distribution of electrons around the nucleus.

Key Atomic Definitions

Atomic Number (Z)

Atomic number (Z): Z = Number of Protons = Number of Electrons (in a neutral atom).

Mass Number (A)

Mass number (A): A = Number of Protons + Number of Neutrons.

Isotopes

Isotopes: Atoms of the same element with the same atomic number but different mass numbers (due to a different number of neutrons).

Relative Atomic Mass (Ar)

Relative atomic mass (Ar): Ar is the mass of an element compared with the carbon-12 atom. The average atomic mass of an element with isotopes can be calculated using the formula: Average Atomic Mass = (Mass₁ × %abundance₁ / 100) + (Mass₂ × %abundance₂ / 100) + ...

Fundamental Chemical Laws

Laws of Mass Conservation and Definite Proportions

Law of Conservation of Mass (Lavoisier)

Law of conservation of mass (Lavoisier): The total mass of a closed system remains unchanged, irrespective of the transformation that takes place within it.

Law of Definite Proportions (Proust)

Law of definite proportions (Proust): When two or more elements combine to form the same compound, they always do so in definite and constant mass proportions.

Volumetric Laws for Gases

Gay-Lussac's Law of Combining Volumes

Gay-Lussac's Law for gases: When gases combine to form new gaseous compounds, their volumes bear a simple whole number ratio, provided they are under the same conditions of pressure and temperature.

Avogadro's Hypothesis

Avogadro's hypothesis: Equal volumes of different gases under the same conditions of temperature and pressure contain the same number of molecules.

Molar Volume

Molar volume: The volume occupied by one mole of any gas at standard temperature and pressure (STP). This volume is approximately 22.4 L and contains 6.022 × 10²³ particles (Avogadro's number).

Understanding pH and Indicators

The pH scale: It allows us to determine the degree of acidity or basicity of a substance. If pH is 7, it is neutral; if pH < 7, it is acidic; if pH > 7, it is basic.

To measure pH, phenolphthalein is used, which is colorless in acidic medium and turns bright pink in basic medium. Also, methyl orange indicator paper is used, which is colored red in acidic medium and yellow/orange in neutral/basic medium. pH is also measured digitally with a pH meter.

Energy in Chemical Reactions

Exothermic Reactions

Exothermic: Reactions that release energy.

Endothermic Reactions

Endothermic: Reactions that absorb energy.

Heat of Reaction

Heat of reaction: The amount of heat energy that is liberated or absorbed during a chemical reaction.

Bond Energy

Bond energy: The energy needed to break a chemical bond or the energy released when a bond is formed.