Chemical Reactions, Mass Conservation and Atomic Models

Chemical Reactions and Mass Conservation

A chemical reaction is a process whereby one or more substances are transformed into one or more different substances.

The Law of Conservation of Mass

Lavoisier's Law: The mass in a chemical reaction remains constant. This law states that the mass of a system remains constant, regardless of the transformation that occurs within it. In any chemical transformation that takes place in a closed system, the total mass of existing substances is conserved.

Proust's Law of Constant Proportions

When two or more elements combine to form a single compound, they do so in mass proportions that are always defined and constant. While a homogeneous mixture may be formed of the same components as a compound but supports different proportions, a compound has single and unchanging proportions.

The Electrical Nature of Matter

Methods of Electrification

• Electrification by friction
• Contact electrification
• Electrification by induction or influence

The Electric Charge

Electrical phenomena are a manifestation of electric charge.

Coulomb's Law and Electric Forces

Forces exist between electric charges: like charges repel each other, creating a repulsive force, while unlike charges attract each other, creating a force of attraction.

The force of attraction or repulsion exerted between two point electric charges is directly proportional to the product of these charges and inversely proportional to the square of the distance that separates them.

Evolution of Atomic Models

Thomson Atomic Model and Ions

An ion is an atom that has gained or lost one or more electrons. If an atom loses one or more electrons, it acquires a net positive charge and becomes a cation. If it gains one or more electrons, it acquires a net negative charge and is called an anion.

Rutherford Atomic Model

The atom has a central core in which almost all its mass is concentrated, provided by the protons and neutrons. The positive charge of protons is compensated by the negatively charged electrons, which are outside the nucleus. Thus, the atom is electrically neutral. The core contains, therefore, protons in a number equal to the electrons of the atom. The electrons spin at high speed around the nucleus and are separated from it by a great distance.

Bohr Model and Electron Shells

The Bohr model introduced key amendments to the Rutherford model:

1. The single electron moves in circular orbits (permitted or stable) that do not emit energy. The electron in each orbit has a certain energy that is greater the further away the orbit is from the nucleus.
2. The emission of energy occurs when an electron jumps from an initial state of higher energy to a lower energy state.

Distribution of Electrons

• First shell (internal): 2 electrons
• Second shell: 8 electrons
• Third shell: 18 electrons
• Fourth shell: 32 electrons