Elements, Compounds, Symbols, and Chemical Formulae

Fundamentals of Atoms and Molecules

Definition of Matter Construction: Matter is composed of particles called molecules, which possess an independent existence. Molecules are further composed of even smaller particles known as atoms.
Definition of an Atom: An atom is defined as the smallest indivisible unit of an element that exhibits all the properties of that element. It may or may not have an independent existence but is the unit that participates in chemical reactions.
Definition of a Molecule: A molecule is defined as the smallest unit of an element or a compound which exhibits all the properties of that element or compound and has an independent existence. Molecules are divisible into atoms and can take part in chemical reactions.

Classification of Molecules

Molecules of Elements: These are formed when two or more atoms of the same element combine.
- Hydrogen ( $H_2$ ): Formed by the combination of two atoms of hydrogen.
- Ozone ( $O_3$ ): Formed by the combination of three atoms of oxygen.
Molecules of Compounds: These are formed when atoms of two or more different elements combine chemically.
- Water ( $H_2O$ ): Formed by two atoms of hydrogen and one atom of oxygen.
- Carbon Dioxide ( $CO_2$ ): Formed by one atom of carbon and two atoms of oxygen.
- Hydrogen Chloride ( $HCl$ ): Formed by the combination of hydrogen and chlorine.
- Zinc Oxide ( $ZnO$ ): Formed by the combination of zinc and oxygen.

Molecular Formulae and Atomicity

Definition of a Formula: A formula (plural: formulae) is a short way of representing the molecule of an element or a compound. It is designed to save time, space, and effort and is accepted universally.
Definition of Atomicity: The number of atoms present in a single molecule of an element is called its atomicity.
Types Based on Atomicity:
- Monoatomic: Molecules containing only one atom. The formula is simply the symbol. Examples: Helium ( $He$ ), Potassium ( $K$ ), Sodium ( $Na$ ), Calcium ( $Ca$ ), Zinc ( $Zn$ ), Gold ( $Au$ ), Copper ( $Cu$ ).
- Diatomic: Molecules containing two atoms of an element. The subscript $2$ is used. Examples: Hydrogen ( $H_2$ ), Oxygen ( $O_2$ ), Nitrogen ( $N_2$ ), Chlorine ( $Cl_2$ ), Fluorine ( $F_2$ ), Bromine ( $Br_2$ ), Iodine ( $I_2$ ).
- Triatomic: Molecules containing three atoms of an element. The subscript $3$ is used. Example: Ozone ( $O_3$ ).
- Polyatomic: Molecules containing more than three atoms of an element. The number of constituent atoms is written as a subscript. Examples: Phosphorus ( $P_4$ ) contains 4 atoms; Sulphur ( $S_8$ ) contains 8 atoms.

Table 4.8: Formulae of Common Molecules

Metals (Monoatomic):
- Iron: $Fe$
- Zinc: $Zn$
- Copper: $Cu$
- Magnesium: $Mg$
- Lead: $Pb$
- Calcium: $Ca$
- Sodium: $Na$
- Cobalt: $Co$
- Chromium: $Cr$
- Gold: $Au$
- Manganese: $Mn$
- Silver: $Ag$
- Mercury: $Hg$
Non-metals:
- Carbon: $C$ (Monoatomic)
- Hydrogen: $H_2$ (Diatomic)
- Nitrogen: $N_2$ (Diatomic)
- Oxygen: $O_2$ (Diatomic)
- Fluorine: $F_2$ (Diatomic)
- Chlorine: $Cl_2$ (Diatomic)
- Bromine: $Br_2$ (Diatomic)
- Iodine: $I_2$ (Diatomic)
- Ozone: $O_3$ (Triatomic)
- Phosphorus: $P_4$ (Polyatomic)
- Sulphur: $S_8$ (Polyatomic)
Metalloids (Monoatomic):
- Antimony: $Sb$
- Arsenic: $As$
Inert Gases (Monoatomic):
- Helium: $He$
- Neon: $Ne$

Detailed Rules for Writing Formulae

Subscript Usage: Atomicity is mentioned as a subscript to the right of the symbol. Subscript $1$ is normally not written.
Distinguishing Atoms and Molecules:
- $2H$ represents two separate atoms of hydrogen.
- $H_2$ represents one molecule of hydrogen.
- $2O_2$ represents two molecules of oxygen.
- $2O$ represents two separate atoms of oxygen.
Combining Power (Valency): Atoms combine in definite whole numbers ( $1, 2, 3$ ) representing the combining capacity of elements. In water ( $H_2O$ ), hydrogen has a combining power of $1$ and oxygen has a combining power of $2$ . The ratio of atoms is $2:1$ .

Information Derived from a Chemical Formula

Types of Elements: Identifies the specific elements that constitute the compound.
Atom Count: Specifies the exact number of each kind of atom in one molecule.
Atomic Ratios: Shows the numerical ratio in which different types of atoms are present.
Example: Sulphuric Acid ( $H_2SO_4$ ): One molecule is formed by the combination of two atoms of hydrogen, one atom of sulphur, and four atoms of oxygen.
Example: Carbon Dioxide ( $2CO_2$ ): The coefficient on the left indicates two separate molecules of carbon dioxide. Each molecule contains one carbon atom and two oxygen atoms, totaling two carbon atoms and four oxygen atoms for the entire expression.

Table 4.11: Metallic Oxides and Sulphides

Metallic Oxides:
- Magnesium oxide: $MgO$
- Calcium oxide: $CaO$
- Iron (II) oxide: $FeO$
- Copper oxide: $CuO$
- Zinc oxide: $ZnO$
- Lead oxide: $PbO$
Metallic Sulphides:
- Magnesium sulphide: $MgS$
- Calcium sulphide: $CaS$
- Iron (II) sulphide: $FeS$
- Copper sulphide: $CuS$
- Zinc sulphide: $ZnS$
- Lead sulphide: $PbS$

Table 4.12: Common Compounds, States, and Composition

Water: Liquid; $H_2O$ ; Elements: Hydrogen and oxygen.
Common salt (Sodium chloride): Solid; $NaCl$ ; Elements: Sodium and chlorine.
Sand (Silicon dioxide): Solid; $SiO_2$ ; Elements: Silicon and oxygen.
Hydrochloric acid: Liquid; $HCl$ ; Elements: Hydrogen and chlorine.
Sodium hydroxide: Solid; $NaOH$ ; Elements: Sodium, oxygen, and hydrogen.
Marble and chalk (Calcium carbonate): Solid; $CaCO_3$ ; Elements: Calcium, carbon, and oxygen.
Sulphuric acid: Liquid; $H_2SO_4$ ; Elements: Hydrogen, sulphur, and oxygen.
Nitric acid: Liquid; $HNO_3$ ; Elements: Hydrogen, nitrogen, and oxygen.
Calcium hydroxide: Solid; $Ca(OH)_2$ ; Elements: Calcium, oxygen, and hydrogen.
Glucose: Solid; $C_6H_{12}O_6$ ; Elements: Carbon, hydrogen, and oxygen.
Cane sugar (Sucrose): Solid; $C_{12}H_{22}O_{11}$ ; Elements: Carbon, hydrogen, and oxygen.
Vinegar (Acetic acid): Liquid; $CH_3COOH$ ; Elements: Carbon, hydrogen, and oxygen.
Baking soda (Sodium bicarbonate): Solid; $NaHCO_3$ ; Elements: Sodium, hydrogen, carbon, and oxygen.

Questions and Discussion

Identify Symbols:
- (a) B: Boron
- (b) He: Helium
- (c) Si: Silicon
- (d) Be: Beryllium
- (e) Li: Lithium
- (f) P: Phosphorus
- (g) K: Potassium
- (h) F: Fluorine
Short Answer Definitions:
- Pure Substance: A substance consisting of only one type of atom or molecule.
- Types of Pure Substances: Elements and Compounds.
- Elements: Pure substances that cannot be broken down into simpler substances by chemical means.
- Compounds: Substances formed when two or more elements combine chemically in a fixed proportion.
Categorized Examples:
- Non-metals which are solids: Carbon and Sulphur.
- Metals which are soft: Sodium and Potassium.
- Non-metals which are lustrous: Iodine and Graphite (Carbon).
- Elements which are liquids: Mercury (metal) and Bromine (non-metal).
- Inert gases: Helium and Neon.
Compounds Analysis:
- Elements in Sugar: Carbon, Hydrogen, and Oxygen.
- Elements in Ammonia: Nitrogen and Hydrogen.
- Elements in Marble: Calcium, Carbon, and Oxygen.
- Elements in Washing Soda: Sodium, Carbon, and Oxygen.
Extended Comparison Tasks:
- Metals vs Non-metals: Differences based on luster, conductivity, and state.
- Elements vs Compounds: Differences based on chemical breakdown and composition.
- Sodium Chloride Justification: Sodium (highly reactive metal) and Chlorine (toxic gas) combine to form Sodium Chloride (edible table salt), showing properties entirely different from constituent elements.
Student Activity - Combining Powers:
- Powers provided: $Na: 1$ , $Ca: 2$ , $Fe: 2$ , $Zn: 2$ , $Al: 3$ , $O: 2$ , $S: 2$ .
- Expected Formulae to write: Iron oxide ( $FeO$ ), Iron sulphide ( $FeS$ ), Sodium oxide ( $Na_2O$ ), Calcium oxide ( $CaO$ ), Calcium sulphide ( $CaS$ ), Zinc chloride ( $ZnCl_2$ ), Zinc sulphide ( $ZnS$ ), Aluminium chloride ( $AlCl_3$ ).