Chemical Formula of Compounds | Chemistry for Grade 11 (IGCSE) PDF Download

Molecular Formulae

Element symbols

• Each element is represented by its own unique symbol as seen on the Periodic Table. For example, H represents hydrogen.
• Symbols consisting of two letters have the first letter capitalized and the second in lowercase. For instance, sodium is represented as Na, not NA.
• Atoms combine in fixed ratios to attain stable electron configurations. A chemical formula indicates these ratios. For instance, H₂O shows two hydrogen atoms combining with one oxygen atom.
• By knowing the number of atoms in a molecule, you can deduce its chemical formula. For example, if a molecule has 3 hydrogen and 1 nitrogen atom, its formula is NH₃.
• Diagrams or models can also represent chemical formulas. For instance, an ammonia molecule comprises a central nitrogen atom bonded to three hydrogen atoms.

Chemical formulae

• The structural formula illustrates how atoms are bonded in a molecule, depicted through a diagram or written form. It provides a detailed arrangement of atoms within the compound.
• On the other hand, the molecular formula gives the actual count of each element's atoms present in one molecule of the compound. For instance, H₂ signifies two hydrogen atoms, while HCl denotes one hydrogen atom and one chlorine atom.

Example: Butane

• Structural Formula (Displayed):
  
• Structural Formula (Simplified): Offers a condensed version of the structural formula in written form. Example: CH₃CH₂CH₂CH₃
• Molecular Formula: Indicates the number of each element's atoms in one molecule. For Butane, the molecular formula is C₄H₁₀.
• Empirical Formula: Represents the simplest whole number ratio of different atoms in a compound. The empirical formula for Butane is C₂H₅.

Deducing Formulae by Combining Power (Valency)

• The concept of valency is crucial in determining the formulas of compounds, whether they are molecular or ionic in nature.
• Valency, also known as combining power, indicates the number of bonds an atom can form with another atom or the number of electrons it can lose, gain, or share to create a compound. For instance, carbon belongs to Group IV, allowing a single carbon atom to establish 4 single bonds or 2 double bonds.
• The valencies associated with elements in various groups are as follows: