Early Ideas of Matter & Chemical Combination

• Ancient Indian and Greek philosophers pondered the nature of matter.

Early Ideas of Matter

• Around 500 BC, Maharishi Kanad suggested that dividing matter would eventually lead to fundamental particles called "Parmanu".
• Pakudha Katyayama stated these particles exist in combined forms.
• Democritus and Leucippus, Greek philosophers, proposed that matter division would lead to indivisible particles called "atoms."
• These ancient ideas were based on philosophy rather than experimentation.
• In the late 18th century, scientists distinguished between elements and compounds and explored how they combine.
• Antoine L. Lavoisier established laws of chemical combination, laying the foundation for chemical sciences.

Laws of Chemical Combination

• The laws of chemical combination were established through experiments by Lavoisier and Joseph L. Proust.
• The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction.
• The law of constant proportions, also known as the law of definite proportions, states that a chemical substance always has the same elements in the same proportions by mass, regardless of the source or preparation method.
• Proust stated that elements are always present in definite proportions by mass in a chemical substance.
• For example, in water, the hydrogen to oxygen mass ratio is always 1:8.
• Similarly, ammonia always has a nitrogen to hydrogen mass ratio of 14:3.

Dalton's Atomic Theory

• British chemist John Dalton proposed the basic theory about the nature of matter.
• Dalton's theory explained the laws of chemical combination.
• All matter is composed of atoms.
• Atoms of the same element are identical in mass and chemical properties.
• Atoms of different elements have different masses and chemical properties.
• Atoms combine in simple whole-number ratios to form compounds.
• The relative number and kinds of atoms are constant in a given compound.
• Atoms are indivisible and cannot be created or destroyed in a chemical reaction.

Atoms

• Atoms are the building blocks of all matter, being extremely small and affecting everything.
• Atomic radius is measured in nanometers (1 nm = 1/10^9 m = 10^-9 m).
• Dalton used symbols for elements to represent a definite quantity (one atom) of that element.
• Berzilius suggested using one or two letters from the element's name as its symbol.
• IUPAC (International Union of Pure and Applied Chemistry) approves names, symbols, and units for elements.
• Element symbols are one or two letters long, with the first letter capitalized and the second lowercase.
• Some symbols are derived from Latin, German, or Greek names (e.g., Fe for iron from "ferrum").

Atomic Mass

• Dalton stated that each element has a characteristic atomic mass.
• Atomic masses are measured relative to a standard.
• Originally, 1/16 of the mass of a naturally occurring oxygen atom.
• The method was then changed as the unit to carbon-12 isotope.
• One atomic mass unit (u) is defined as exactly one-twelfth (1/12th) the mass of one carbon-12 atom.
• The relative atomic mass of an element is the average mass of the atom compared to 1/12th the mass of a carbon-12 atom.
• Atoms of most elements combine to form molecules and ions.
• Molecules and ions aggregate to form matter.

Molecules

• A molecule is a group of two or more atoms chemically bonded together.
• Molecules exhibit all properties of the substance and can exist independently.
• Molecules of elements contain the same type of atoms, whereas molecules of compounds contain different types.

Molecules of Elements

• Molecules of elements are constituted by the same type of atoms
• Molecules of argon (Ar) and helium (He) consist of only one atom.
• A molecule of oxygen (O₂) is diatomic, while ozone (O3) consists of three oxygen atoms.
• Atomicity refers to the number of atoms in a molecule.
• Metals, like carbon, have complex structures with many atoms bonded together.

Molecules of Compounds

• Molecules of compounds are formed by atoms of different elements joining in definite proportions.

Ions

• Ions are charged species in compounds composed of metals and non-metals.
• Ions can be single atoms or groups of atoms with a net charge.
• Anions are negatively charged, and cations are positively charged.
• Polyatomic ions are groups of atoms carrying a charge.

Writing Chemical Formulae

• Chemical formulae symbolically represent a compound's composition.
• Valency is the combining power of an element.
• The valencies or charges on ions must balance in a chemical formula.
• In a compound of metal and non-metal, the metal's symbol is written first.
• The number of polyatomic ions is indicated by enclosing the ion's formula within brackets and writing the number of ions outside.

Formulae of Simple Compounds

• Simplest compounds of two different elements are called binary compounds
• Cross over the valencies of the combining atoms, to write the chemical formulae
• For magnesium chloride, write the symbol of cation (Mg2+) followed by the symbol of anion (Cl-).
• In magnesium chloride, there are two chloride ions (Cl-) for each magnesium ion (Mg2+).
• The positive and negative charges must balance each other and the overall structure must be neutral.
• Formula unit mass is the sum of the atomic masses of all atoms in a formula unit of a compound.