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1. Understanding Compounds

• Definition: A compound is a substance formed when two or more elements chemically combine in a fixed ratio.
• Example: Water (H2OH_2OH2​O) is formed by combining two hydrogen atoms with one oxygen atom.

2. The Role of the Periodic TableThe Periodic Table helps predict how elements combine based on their valency.
Valency:

• Valency is the combining power of an element, determined by the number of electrons in its outer shell.
• Elements tend to:
  • Lose, gain, or share electrons to achieve a full outer shell (following the octet rule).

Group Numbers and Valency:

1. Group 1 (Alkali Metals): Valency = 1 (e.g., Sodium NaNaNa).
2. Group 2 (Alkaline Earth Metals): Valency = 2 (e.g., Magnesium MgMgMg).
3. Group 13: Valency = 3 (e.g., Aluminium AlAlAl).
4. Group 14: Valency = 4 (e.g., Carbon CCC).
5. Group 15: Valency = 3 (e.g., Nitrogen NNN).
6. Group 16: Valency = 2 (e.g., Oxygen OOO).
7. Group 17 (Halogens): Valency = 1 (e.g., Chlorine ClClCl).
8. Group 18 (Noble Gases): Valency = 0 (generally non-reactive).

3. Predicting the Ratio of Atoms in CompoundsStep 1: Identify the Elements and Their Valency:

• Example: Sodium (NaNaNa) has a valency of 1, and Chlorine (ClClCl) also has a valency of 1.

Step 2: Balance the Charges:

• Combine elements in a ratio that balances their valency.
  • Example: Sodium chloride (NaClNaClNaCl) combines 1 sodium atom and 1 chlorine atom because both have valency 1.

Step 3: Write the Formula:

• The formula shows the ratio of atoms in the compound.
  • Example: In magnesium chloride (MgCl2MgCl_2MgCl2​), 1 magnesium atom (valency 2) combines with 2 chlorine atoms (each with valency 1).