2.1 Early Ideas in Atomic Theory

1)  All matter is composed of tiny, indivisible particles called atoms.

2) Atoms of the same element are identical in mass and properties.

3) Atoms of one element differ in properties from atoms of all other elements.

4) Compounds are formed when atoms of different elements combine in fixed, simple whole-number ratios.

5) Chemical reactions involve the rearrangement of atoms, but the atoms themselves remain unchanged.

The Law of Definite Proportions, also known as Proust's Law, states that:

• a chemical compound always contains the same elements in exactly the same proportions by mass, regardless of the sample size or source of the compound.

Can we use Dalton's postulates to describe the law of definite proportions?

Dalton's Postulate: All matter is composed of indivisible atoms. Atoms of a given element are identical in mass and properties.

• This postulate explains that a compound, like water, is made of atoms that combine in fixed ratios, so any molecule of water will always have two hydrogen atoms and one oxygen atom.

Dalton's Postulate: Atoms combine in simple whole-number ratios to form compounds.

• This reinforces the Law of Definite Proportions because it implies that compounds have fixed, definite ratios of atoms, leading to consistent mass proportions between elements.

The Law of Multiple Proportions, also proposed by John Dalton, states that:

• 'When two elements combine with each other to form two or more different compounds, the ratios of the masses of one element that combines with the fixed mass of the other are simple whole numbers'.

Can we use Dalton's postulates to describe the law of multiple proportions?

Dalton's Postulate: Atoms of different elements differ in mass and properties.

• This explains why elements like carbon and oxygen can combine in different ratios to form distinct compounds with different properties, such as CO (carbon monoxide) and CO₂ (carbon dioxide).

Dalton's Postulate: Atoms combine in simple whole-number ratios to form compounds.

• The Law of Multiple Proportions directly reflects this postulate, as it describes how the mass ratios of one element combining with a fixed mass of another element are small whole numbers, like in the carbon-oxygen example (1:2 ratio of oxygen atoms).

Dalton's Postulate: Chemical reactions involve the rearrangement of atoms.

• This reinforces the idea that different compounds are formed by rearranging atoms in different proportions, further supporting the concept of multiple proportions.

1)  The atom is the smallest unit of matter that can take part in a chemical reaction & are neither created nor destroyed during a chemical change

2)  Atoms are rearranged in chemical reactions to yield substances that are different from those present before the change

• Democritus (Ancient Greece) – Predict the presence of an atom

• Dalton (UK, 1800) – Applied atomic chemistry

• J. J. Thomson (UK, 1897) – Determination of the presence of electrons through a cathode ray tube experiment

• Rutherford (Great Britain, 1909) – Confirming the existence of atomic nuclei

• N. Bohr (Denmark, 1913) – Planetary hypothesis of the atom

• N. Bohr (Denmark, 1922) – Describing the shape of an atomic nucleus

• de Broglie (France, 1923) – Propose the oscillation of electron particles

• Schrödinger (Austria, 1926) – Mathematically identifying the electron orbits of an atom

• Heisenberg (Germany, 1927) – Describes the stability of the electron orbit through the uncertainty principle

• R. Feynman (USA, 1965) – Quantum Mechanics Research

• IBM (USA, 1990) – Making a microscope capable of viewing atoms, succeeding in moving atoms into particles