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Ionic compounds are formed when metals transfer electrons to non-metals, resulting in the formation of positive and negative ions. These ions attract each other due to opposite charges and create a stable compound through ionic bonding. A classic example is sodium chloride (table salt), consisting of sodium ions

• In ionic compounds, metals typically lose electrons becoming cations with a positive charge.
• Non-metals gain electrons to become anions, carrying a negative charge.

To determine the formula of an ionic compound:

• Identify the metal (cation) and its charge based on its group in the periodic table. Elements in Group 1 have a +1 charge, Group 2 have a +2 charge, etc.


• Identify the non-metal (anion) and its charge using standard oxidation states, often found in reference tables.


• Balance the overall charge by adjusting the number of cations and anions until the total positive charge equals the total negative charge.

Polyatomic ions are ions that consist of more than one atom, acting as a single charged entity. Unlike monatomic ions, polyatomic ions comprise a group of atoms bonded together that collectively carry a charge. Examples of polyatomic ions include:

• Hydroxide (OH鈦�): A negative ion with oxygen and hydrogen, often seen in bases like sodium hydroxide (NaOH).
• Carbonate (CO鈧兟测伝): A carbonate group bonded to metals in compounds like calcium carbonate (CaCO鈧�), prevalent in rocks and shells.
• Ammonium (NH鈧勨伜): A positive ion formed by nitrogen and hydrogen, common in fertilizers.

When writing the chemical formula for compounds containing polyatomic ions, treat these ions as discrete units. If multiple polyatomic ions are needed to balance the charge, they should be enclosed in parentheses with a subscript indicating the number needed, such as in magnesium cyanide, Mg(CN)鈧�.

Charge balancing is a crucial concept in chemistry to ensure that a compound is electrically neutral. The sum of positive and negative charges in a compound must equal zero. Steps to balance charges in formulas:

• Calculate the total positive charge from all cations.


• Calculate the total negative charge from all anions.


• Adjust the number of ions to balance these charges.

For instance, in lead(IV) carbonate, the lead has a +4 charge, and the carbonate has a -2 charge. To balance these, two carbonate ions are needed for each lead ion, resulting in the formula Pb(CO鈧�)鈧�. It's also important to consider the use of parentheses in formulas when multiple units of a polyatomic ion are present. This ensures clarity in indicating how many of each ion is part of the compound, as seen in compounds like Cr(OH)鈧�, where three hydroxide ions are balanced with one chromium ion.