Chemical Formulae
Definition of chemical formula: A chemical formula is a short and easy way to show what elements are in a substance and how many atoms of each element it contains. Instead of writing a full sentence like “water is made of two hydrogen atoms and one oxygen atom,” we just write H₂O. The chemical formula uses element symbols and little numbers called subscripts to do this job quickly and clearly.
Element symbols: Every element has its own unique symbol. These symbols are usually one or two letters long. The first letter is always a capital letter, and if there’s a second letter, it’s always lowercase. For example, “H” is for hydrogen, “O” is for oxygen, and “Na” is for sodium. These symbols come from the element’s name, often in English or Latin.
Atomic quantities: The small number written at the lower right of an element symbol is called a subscript. This subscript tells us how many atoms of that element are in the compound. For example, in H₂O, the subscript 2 after H tells us there are two hydrogen atoms. If no number is shown after the symbol, like the O in H₂O, it means there is just one atom of that element.
Water example: Let’s take the example of water. Its chemical formula is H₂O. This means that every molecule of water contains 2 hydrogen atoms (H) and 1 oxygen atom (O). These atoms are joined together in a specific way to form the water that we drink, cook with, and see in rivers and rain.
Types of Chemical Formulae
Empirical Formula
Empirical formula definition: The empirical formula of a compound tells us the simplest whole-number ratio of atoms of each element in the compound. It doesn’t show how many atoms are actually in a molecule, just the simplest version of the ratio between the elements.
Lowest ratio only: For example, a compound might actually have 6 carbon atoms and 12 hydrogen atoms, like glucose. But the empirical formula for glucose is CH₂, because both 6 and 12 can be divided by 6, giving us the lowest whole-number ratio of 1 carbon to 2 hydrogen atoms.
Conversion to empirical: To write an empirical formula, we usually start with the masses of the elements in a compound. We then convert the masses to moles, divide the moles to find the smallest whole-number ratio, and finally use that ratio to write the formula.
Glucose example: Glucose is a sugar with the molecular formula C₆H₁₂O₆. That means each molecule has 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. But its empirical formula is CH₂O, which shows the simplest ratio of 1 carbon, 2 hydrogen, and 1 oxygen atom.
Same empirical formula: Different compounds can have the same empirical formula even though their molecular formulas are different. For instance, ethene (C₂H₄) and butene (C₄H₈) both simplify to CH₂ when reduced to their lowest ratio.
Sulphur dioxide: The chemical formula SO₂ is both the empirical and the molecular formula of sulphur dioxide. That’s because the actual number of atoms (1 sulfur and 2 oxygen) is already the simplest possible ratio.
Molecular Formula
Molecular formula definition: The molecular formula tells us the actual number of each type of atom in a single molecule of a compound. It gives more detailed information than the empirical formula.
Actual composition: Instead of just showing the ratio, the molecular formula shows exactly how many atoms are in the molecule. This helps us understand the full structure of the substance.
Multiple of empirical: The molecular formula is always a whole-number multiple of the empirical formula. That means we can multiply the subscripts in the empirical formula by a number (like 2, 3, etc.) to get the molecular formula.
Formula relation: We can write it like this: Molecular formula = (Empirical formula)ⁿ. Here, ‘n’ is a whole number that tells us how many times bigger the molecular formula is compared to the empirical one.
Finding molecular formula: To find the molecular formula, we first determine the empirical formula and calculate its mass. Then we divide the actual molecular mass (often given in the question) by the empirical mass to find ‘n’. Finally, we multiply each subscript in the empirical formula by ‘n’.
Glucose example: Glucose has the molecular formula C₆H₁₂O₆. This means each glucose molecule contains 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. The empirical formula CH₂O multiplied by 6 gives this full formula.
Ethene example: Ethene has the molecular formula C₂H₄. It means 2 carbon atoms and 4 hydrogen atoms are in each molecule. The empirical formula is CH₂, and the molecular formula is twice that.
Butane example: Butane’s molecular formula is C₄H₁₀, which tells us each molecule has 4 carbon atoms and 10 hydrogen atoms. The empirical formula for butane is C₂H₅, so the molecular formula is 2 times that.
Formula of a Substance in Ionic Form
Ionic compounds: Ionic compounds are made of ions, which are atoms or groups of atoms with electric charges. A positively charged ion is called a cation, and a negatively charged ion is called an anion. These oppositely charged ions stick together to form a compound.
Neutral formula: When ions come together to form an ionic compound, the total positive charge must equal the total negative charge. This is called charge balance. The result is a compound that has no overall charge.
Writing ionic formulas: To write a formula for an ionic compound, we first write the symbol for the cation, then the anion. Then we look at their charges and adjust how many of each ion we need so the total charges cancel out. One easy way to do this is the cross-over method, where you swap the charge numbers to become subscripts.
Simplify ratio: After using the cross-over method, we make sure the formula shows the lowest possible whole-number ratio of the ions.
Sodium chloride example: Sodium chloride (NaCl) is made of one sodium ion (Na⁺) and one chloride ion (Cl⁻). Each has a charge of +1 and -1, so they balance perfectly and form a 1:1 ratio.
Magnesium chloride example: Magnesium has a charge of +2 (Mg²⁺), and chloride has a charge of -1 (Cl⁻). To balance this, we need two Cl⁻ ions for every Mg²⁺ ion. That’s why the formula is MgCl₂.
Calcium oxide example: Calcium (Ca²⁺) and oxide (O²⁻) both have charges of 2, but in opposite directions. One of each balances out the charges, so the formula is CaO.
Writing Chemical Formulae
General Rules
Element symbols: Each element is shown by a symbol. These symbols are taken from the periodic table. They help keep formulas short and easy to understand. For example, Na is for sodium, Cl is for chlorine.
Subscripts usage: The little numbers after symbols tell us how many atoms are present. If no number is written, it means there is only one atom. For example, CO₂ means one carbon atom and two oxygen atoms.
For Covalent Compounds
Molecular atom count: In covalent compounds (where atoms share electrons), the subscripts tell us how many atoms of each kind are in the molecule.
Water example revisited: Water has the formula H₂O. This tells us that every molecule has 2 hydrogen atoms and 1 oxygen atom bonded together.
For Ionic Compounds
Cation–anion order: In ionic formulas, the positive ion (cation) is always written first, followed by the negative ion (anion).
Net zero charge: When writing formulas for ionic compounds, we must balance the charges so that the final formula has no net charge. This may involve using more than one of each ion.
Magnesium chloride example again: In MgCl₂, we use one magnesium ion (Mg²⁺) and two chloride ions (Cl⁻) to cancel out the charges and make a neutral compound.
Using Oxidation Numbers
Polyatomic ions: Some compounds include polyatomic ions. These are groups of atoms that behave like a single charged unit, such as sulfate (SO₄²⁻) or nitrate (NO₃⁻).
Charge balance: Just like with simple ions, we need to make sure the total positive and negative charges from the polyatomic ions cancel each other out.
Ammonium nitrate example: Ammonium nitrate (NH₄NO₃) is made by combining one ammonium ion (NH₄⁺, with a +1 charge) and one nitrate ion (NO₃⁻, with a -1 charge). The charges cancel out, so the compound is neutral.