Atoms and Molecules

• The study note discusses atoms and molecules.

Discovery of Atoms

• Maharishi Kanad (Indian) and Democritus (Greek) are credited with discovering atoms.

Atomic Theory of Maharishi Kanad

• Paramanus are the smallest particles.
• "Para" signifies ultimate.
• "Anu" means particle.
• Atoms do not exist in free state.
• Atoms combine to form molecules.

Democritus Atomic Theory

• Paramanu is the smallest particle.
• Paramamu is equivalent to an atom.
• Atoms are indivisible.

Learning Objectives

• Describe atoms, molecules, and subatomic particles.
• Hypothesize how the laws of chemical combination are applied to atoms.

Laws of Chemical Combination

• Law of conservation of mass: Mass is neither be created nor destroyed in a chemical reaction.
• During a chemical reaction, the sum of the masses of the reactants and products remains unchanged.
• Law of constant proportions: In a chemical compound, the elements are always present in a definite proportion by mass.
• Water (H2O) always contains hydrogen and oxygen combined in the ratio of 2:16 or 1:8 by mass.
• 9 g of water decomposes into 1 g of hydrogen and 8 g of oxygen.
• Ammonia (NH3) always contains nitrogen and hydrogen combined in the same ratio of 14:3 by mass.

Dalton's Atomic Theory

• Matter is made up of tiny particles called atoms.
• Atoms are indivisible and cannot be created or destroyed in a chemical reaction.
• Atoms of a given element are similar in mass and properties.
• Atoms of different elements have different masses and properties.
• Atoms combine in small whole number ratios to form compounds.
• In a given compound, the relative number and kind of atoms are constant.

Atoms

• An atom is the smallest particle of an element that may or may not exist independently and retains all its chemical properties.
• Atoms are very small in size.
• Atomic radius is measured in nanometers (nm).
• 1 nanometer = 10^-9 m or 1 meter = 10^9 nm.
• The atomic radius of a hydrogen atom is 10^-10 m.
• The radius of a water molecule is 10^-9 m.

Symbols of Atoms

• Elements are represented by letters.
• Symbols can be one or two letters.
• One-letter symbols are capitalized.
• Two-letter symbols have a capital first letter and a small second letter.

Atomic Mass

• Atomic mass is the mass of an atom compared to a standard atom, as atoms are very small in size its mass is very small and determining its mass is very difficult.
• Carbon-12 atom is the standard.
• Carbon-12 has an atomic mass of 12 atomic mass units (u).
• One atomic mass unit (u) is 1/12th the mass of a carbon-12 atom.
• Atomic mass of an element is the average mass of one atom of the element compared with 1/12th the mass of a carbon-12 atom.

Molecules

• A molecule is the smallest particle of an element or compound that exists independently and shows all the properties of that substance.
• A molecule is a group of two or more elements held together by attractive forces.
• Atoms of the same or different elements can join together to form molecules.

Molecules of Elements

• A molecule of an element contains atoms of the same element.
• Molecules of some elements contain only one atom.
• Molecules of some elements contain two or more atoms.
• Atomicity of an element is the number of atoms present in one molecule of the element.

Atomicity of Elements

• Argon (Ar): 1 (Monoatomic)
• Helium (He): 1 (Monoatomic)
• Oxygen (O2): 2 (Diatomic)
• Hydrogen (H2): 2 (Diatomic)
• Nitrogen (N2): 2 (Diatomic)
• Chlorine (Cl2): 2 (Diatomic)
• Phosphorus (P4): 4 (Tetratomic)
• Sulphur (S8): Polyatomic
• Sodium (Na): 1 (Monoatomic)
• Iron (Fe): 1 (Monoatomic)
• Aluminium (Al): 1 (Monoatomic)
• Copper (Cu): 1 (Monoatomic)

Molecules of Compounds

• Molecule of a compound contains atoms of two or more different types of elements.

Examples of Compound Molecules

• Water (H2O): Hydrogen, Oxygen (2 Hydrogen, 1 Oxygen)
• Ammonia (NH3): Nitrogen, Hydrogen (1 Nitrogen, 3 Hydrogen)
• Carbon dioxide (CO2): Carbon, Oxygen (1 Carbon, 2 Oxygen)
• Hydrochloric acid (HCl): Hydrogen, Chlorine (1 Hydrogen, 1 Chlorine)
• Nitric acid (HNO3): Hydrogen, Nitrogen, Oxygen (1 Hydrogen, 1 Nitrogen, 3 Oxygen)
• Sulphuric acid (H2SO4): Hydrogen, Sulphur, Oxygen (2 Hydrogen, 1 Sulphur, 4 Oxygen)

Ions

• Compounds containing metal and non-metal elements contain charged particles called ions.
• An ion is a charged particle having positive or negative charge.
• A positively charged ion is called 'cation', and a negatively charged ion is called 'anion'.
• Valency is the combining capacity of an element.

Common Ions and their Valencies

• Sodium (Na+): 1
• Potassium (K+): 1
• Silver (Ag+): 1
• Copper (I) (Cu+): 1
• Magnesium (Mg2+): 2
• Calcium (Ca2+): 2
• Zinc (Zn2+): 2
• Iron (II) (Fe2+): 2
• Copper (II) (Cu2+): 2
• Aluminium (Al3+): 3
• Iron (iii) (Fe3+): 3
• Hydrogen (H+): 1
• Hydride (H-): 1
• Chloride(CI-):1
• Bromide(Br-): 1
• Iodide (I-): 2
• Oxide (O2-): 2
• Nitride (N3-): 3
• Ammonium (NH4+): 1
• Hydroxide (OH-): 1
• Nitrate (NO3-): 1
• Hydrogen carbonate (HCO3-): 1
• Carbonate (CO32-): 2
• Sulphite (SO32- ): 2
• Sulphate (SO42-): 2
• Phosphate (PO43-): 3

Writing Chemical Formulae

• Write the symbols/formula of the elements or ions, placing the metal or positive ion on the left and the non-metal or negative ion on the right.
• Write the valencies of the elements or ions below their symbols.
• Cross over the valencies of the combining ions.
• Enclose polyatomic ions in brackets before writing the formula.

Examples of Chemical Formulae

• Hydrogen chloride: HCl
• Hydrogen sulphide: H2S
• Magnesium chloride: MgCl2
• Carbon tetrachloride: CCl4
• Calcium oxide: CaO
• Aluminium oxide: Al2O3
• Sodium nitrate: NaNO3
• Calcium hydroxide: Ca(OH)2
• Sodium carbonate: Na2CO3
• Ammonium sulphate: (NH4)2SO4

Molecular Mass/Formula Unit Mass

• The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance.
• Molecular mass is expressed in atomic mass units (u).
• Molecular mass of water (H2O): 18 u
• Molecular mass of nitric acid (HNO3): 63 u