The Magnesium Oxide Formula, represented as MgO, is one of the most fundamental ionic compound formulas studied in CBSE Chemistry from Class 8 through Class 12. Magnesium oxide forms when magnesium burns in the presence of oxygen, producing a white powdery solid with a high melting point. This compound is covered in NCERT Science (Class 8, Chapter 4) and NCERT Chemistry Part I (Class 11, Chapter 3 and Chapter 10). For JEE Main and NEET aspirants, MgO appears in topics covering ionic bonding, electrochemistry, and the reactivity series. This article covers the formula, structure, molar mass, properties, a complete formula sheet, three progressive solved examples, CBSE exam tips, and JEE/NEET applications.
Key Magnesium Oxide Formulas at a Glance
Quick reference for the most important formulas and values related to MgO.
- Chemical formula: MgO
- Molar mass: \( M = 24.31 + 16.00 = 40.31 \) g/mol
- Formation reaction: \( 2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \)
- Reaction with water: \( \text{MgO} + \text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2 \)
- Reaction with acid: \( \text{MgO} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\text{O} \)
- Lattice energy: approximately \( -3795 \) kJ/mol
- Percentage of Mg in MgO: \( \frac{24.31}{40.31} \times 100 \approx 60.3\% \)
What is Magnesium Oxide Formula?
The Magnesium Oxide Formula is MgO. It represents an ionic compound formed between magnesium (Mg) and oxygen (O). Magnesium belongs to Group 2 of the periodic table. It has an atomic number of 12 and loses two electrons to form the Mg²⁺ cation. Oxygen belongs to Group 16 and gains two electrons to form the O²⁻ anion. The two ions combine in a 1:1 ratio. This gives the neutral formula MgO.
In the NCERT curriculum, MgO is introduced as a basic oxide in Class 8 Science (Chapter 4 — Materials: Metals and Non-Metals). It reappears in Class 11 Chemistry (Chapter 3 — Classification of Elements and Periodicity in Properties) and Class 11 Chemistry (Chapter 10 — The s-Block Elements). The compound is also discussed in Class 12 Chemistry in the context of ionic bonding and Born-Haber cycles.
Magnesium oxide is a white, odourless solid at room temperature. It has a melting point of 2852°C, making it a refractory material. It is slightly soluble in water, forming magnesium hydroxide. It is widely used in medicine as an antacid, in construction as a fireproofing agent, and in industry as a refractory lining for furnaces.
Magnesium Oxide Formula — Expression and Variables
The chemical formula for magnesium oxide is written as:
\[ \text{MgO} \]
The molar mass of MgO is calculated by adding the atomic masses of magnesium and oxygen:
\[ M(\text{MgO}) = M(\text{Mg}) + M(\text{O}) = 24.31 + 16.00 = 40.31 \text{ g/mol} \]
The formation reaction of magnesium oxide from its elements is:
\[ 2\text{Mg}_{(s)} + \text{O}_{2(g)} \rightarrow 2\text{MgO}_{(s)} \]
| Symbol / Component | Quantity / Description | Value / SI Unit |
|---|---|---|
| MgO | Chemical formula of magnesium oxide | — |
| Mg²⁺ | Magnesium cation (loses 2 electrons) | Charge: +2 |
| O²⁻ | Oxide anion (gains 2 electrons) | Charge: −2 |
| M(Mg) | Atomic mass of magnesium | 24.31 g/mol |
| M(O) | Atomic mass of oxygen | 16.00 g/mol |
| M(MgO) | Molar mass of magnesium oxide | 40.31 g/mol |
| Melting point | Temperature at which MgO melts | 2852°C (3125 K) |
| Boiling point | Temperature at which MgO boils | 3600°C |
| Density | Mass per unit volume of MgO | 3.58 g/cm³ |
| Crystal structure | Rock salt (NaCl-type) cubic structure | — |
Derivation of the Magnesium Oxide Formula
The formula MgO is derived using the valency (combining capacity) of each element. Magnesium has a valency of 2 because it has 2 electrons in its outermost shell (electronic configuration: 2, 8, 2). It loses both electrons to achieve a stable noble gas configuration, forming Mg²⁺. Oxygen has a valency of 2 because it needs 2 electrons to complete its octet (electronic configuration: 2, 6). It gains 2 electrons to form O²⁻. Using the criss-cross rule of valencies, the formula becomes Mg²O² = MgO (simplified). The compound is electrically neutral because the +2 charge of Mg²⁺ exactly balances the −2 charge of O²⁻.
Structure of Magnesium Oxide
Magnesium oxide adopts the rock salt (NaCl-type) crystal structure. Each Mg²⁺ ion is surrounded by six O²⁻ ions in an octahedral arrangement. Similarly, each O²⁻ ion is surrounded by six Mg²⁺ ions. This 6:6 co-ordination gives MgO an exceptionally high lattice energy of approximately \( -3795 \) kJ/mol. The high lattice energy explains the very high melting point of MgO. The bonding is purely ionic, with complete electron transfer from magnesium to oxygen.
Complete Chemistry Formula Sheet — Magnesium Compounds and Related Ionic Formulas
| Formula Name | Expression | Variables / Description | Molar Mass (g/mol) | NCERT Chapter |
|---|---|---|---|---|
| Magnesium Oxide | \( \text{MgO} \) | Mg²⁺ + O²⁻; ionic compound | 40.31 | Class 11, Ch 10 |
| Magnesium Hydroxide | \( \text{Mg(OH)}_2 \) | MgO + H₂O; used as antacid (milk of magnesia) | 58.32 | Class 11, Ch 10 |
| Magnesium Chloride | \( \text{MgCl}_2 \) | Mg²⁺ + 2Cl⁻; soluble ionic salt | 95.21 | Class 11, Ch 10 |
| Magnesium Sulphate | \( \text{MgSO}_4 \) | Epsom salt; Mg²⁺ + SO₄²⁻ | 120.37 | Class 11, Ch 10 |
| Magnesium Carbonate | \( \text{MgCO}_3 \) | Decomposes on heating to give MgO + CO₂ | 84.31 | Class 11, Ch 10 |
| Magnesium Nitrate | \( \text{Mg(NO}_3)_2 \) | Mg²⁺ + 2NO₃⁻; soluble salt | 148.31 | Class 11, Ch 10 |
| Calcium Oxide (Quicklime) | \( \text{CaO} \) | Ca²⁺ + O²⁻; similar to MgO structure | 56.08 | Class 11, Ch 10 |
| Sodium Oxide | \( \text{Na}_2\text{O} \) | 2Na⁺ + O²⁻; basic oxide | 61.98 | Class 11, Ch 10 |
| Aluminium Oxide | \( \text{Al}_2\text{O}_3 \) | 2Al³⁺ + 3O²⁻; amphoteric oxide | 101.96 | Class 11, Ch 11 |
| Zinc Oxide | \( \text{ZnO} \) | Zn²⁺ + O²⁻; amphoteric oxide | 81.38 | Class 12, Ch 6 |
| Formation of MgO (reaction) | \( 2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \) | Combustion of magnesium in air | — | Class 8, Ch 4 |
| MgO + Water | \( \text{MgO} + \text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2 \) | Basic oxide reacts with water | — | Class 10, Ch 2 |
Magnesium Oxide Formula — Solved Examples
Example 1 (Class 8-10 Level): Calculating Molar Mass of MgO
Problem: Calculate the molar mass of magnesium oxide using the Magnesium Oxide Formula MgO. Given: atomic mass of Mg = 24 u, atomic mass of O = 16 u.
Given: Atomic mass of Mg = 24 u, Atomic mass of O = 16 u, Formula = MgO
Step 1: Write the formula: MgO contains 1 atom of Mg and 1 atom of O.
Step 2: Apply the molar mass formula: \( M(\text{MgO}) = M(\text{Mg}) + M(\text{O}) \)
Step 3: Substitute the values: \( M(\text{MgO}) = 24 + 16 = 40 \) g/mol
Answer
Molar mass of MgO = 40 g/mol
Example 2 (Class 11-12 Level): Finding the Mass of MgO Produced
Problem: 12 g of magnesium is burnt completely in excess oxygen. Calculate the mass of magnesium oxide produced. (Atomic mass: Mg = 24 g/mol, O = 16 g/mol)
Given: Mass of Mg = 12 g, Reaction: \( 2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \)
Step 1: Calculate moles of Mg used.
\( n(\text{Mg}) = \frac{\text{mass}}{\text{molar mass}} = \frac{12}{24} = 0.5 \) mol
Step 2: Use the mole ratio from the balanced equation. The ratio of Mg to MgO is 2:2, which simplifies to 1:1.
\( n(\text{MgO}) = n(\text{Mg}) = 0.5 \) mol
Step 3: Calculate the molar mass of MgO.
\( M(\text{MgO}) = 24 + 16 = 40 \) g/mol
Step 4: Calculate the mass of MgO produced.
\( m(\text{MgO}) = n \times M = 0.5 \times 40 = 20 \) g
Answer
Mass of MgO produced = 20 g
Example 3 (JEE/NEET Level): Percentage Composition and Empirical Formula Verification
Problem: An oxide of magnesium contains 60.3% magnesium by mass. Verify that this corresponds to the Magnesium Oxide Formula MgO and calculate the empirical formula. (Atomic mass: Mg = 24.3 g/mol, O = 16.0 g/mol)
Given: % Mg = 60.3%, % O = 100 − 60.3 = 39.7%
Step 1: Assume 100 g of the compound. Then mass of Mg = 60.3 g and mass of O = 39.7 g.
Step 2: Convert masses to moles.
\( n(\text{Mg}) = \frac{60.3}{24.3} \approx 2.481 \) mol
\( n(\text{O}) = \frac{39.7}{16.0} \approx 2.481 \) mol
Step 3: Find the simplest whole-number ratio by dividing both by the smaller value.
\( \text{Ratio} = \frac{2.481}{2.481} : \frac{2.481}{2.481} = 1 : 1 \)
Step 4: Write the empirical formula. The Mg:O ratio is 1:1, so the empirical formula is MgO.
Step 5: Verify the theoretical percentage of Mg in MgO.
\( \%\text{Mg} = \frac{24.3}{40.3} \times 100 \approx 60.3\% \) ✓
Answer
The empirical formula is MgO, which matches the Magnesium Oxide Formula. The theoretical percentage of Mg (60.3%) confirms the result.
CBSE Exam Tips 2025-26
- Memorise the formula and molar mass together: Always write MgO = 40 g/mol as a single unit. CBSE frequently asks molar mass calculations in 1-mark and 2-mark questions.
- Balance equations carefully: The formation reaction \( 2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \) must be balanced. Unbalanced equations lose marks in CBSE board exams.
- Know all reactions of MgO: The reactions with water, acids (HCl, H₂SO₄), and CO₂ are frequently asked in CBSE Class 10 and Class 11 exams. We recommend writing these reactions on flashcards.
- Understand basic vs. amphoteric oxides: MgO is a basic oxide, not amphoteric. Students often confuse it with Al₂O₃ or ZnO. This distinction appears regularly in CBSE objective questions.
- Use the criss-cross method for formula derivation: CBSE Class 8 and Class 9 exams often ask students to derive the formula of MgO from valencies. Practice writing Mg²⁺ and O²⁻ and applying the criss-cross rule.
- Practise percentage composition problems: Our experts suggest solving at least 5 percentage composition problems on MgO before the 2025-26 board exams. These problems carry 3 marks each.
Common Mistakes to Avoid
- Writing the formula as Mg₂O or MgO₂: This is the most common error. Magnesium has a valency of 2 and oxygen has a valency of 2. After criss-crossing and simplifying, the correct formula is MgO, not Mg₂O₂. Always simplify the ratio to its lowest terms.
- Using incorrect molar mass: Some students use 40 g/mol for both MgO and NaOH. The molar mass of MgO is 40.31 g/mol (Mg = 24.31, O = 16). For NaOH, it is 40 g/mol (Na = 23, O = 16, H = 1). Do not confuse these two compounds.
- Forgetting to balance the formation equation: Writing \( \text{Mg} + \text{O}_2 \rightarrow \text{MgO} \) without balancing it is incorrect. The correct balanced equation is \( 2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \).
- Treating MgO as amphoteric: MgO reacts with acids but does not react with bases (like NaOH). It is a basic oxide, not an amphoteric oxide. Amphoteric oxides (Al₂O₃, ZnO) react with both acids and bases.
- Ignoring the ionic nature of MgO: Students sometimes describe MgO as a covalent compound. MgO is purely ionic. The high electronegativity difference between Mg and O (approximately 2.13 on the Pauling scale) confirms complete electron transfer and ionic bonding.
JEE/NEET Application of Magnesium Oxide Formula
In our experience, JEE aspirants encounter the Magnesium Oxide Formula in at least three distinct topic areas. Understanding these applications can help you secure marks across multiple chapters.
1. Stoichiometry and Mole Concept (JEE Main, NEET)
MgO is a favourite compound for stoichiometry problems. JEE Main and NEET regularly ask questions involving the combustion of magnesium, limiting reagent calculations, and yield calculations. The key reaction is \( 2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \). You must be comfortable converting between moles, mass, and number of molecules using Avogadro’s number (\( 6.022 \times 10^{23} \) mol⁻¹).
2. Born-Haber Cycle and Lattice Energy (JEE Advanced)
MgO is the standard example used in Born-Haber cycle problems in JEE Advanced. The cycle involves the sublimation enthalpy of Mg, the first and second ionisation enthalpies of Mg, the bond dissociation enthalpy of O₂, the electron gain enthalpy of O, and the lattice energy of MgO. The lattice energy of MgO (approximately −3795 kJ/mol) is significantly higher than that of NaCl (approximately −788 kJ/mol). This is because MgO has doubly charged ions (Mg²⁺ and O²⁻), while NaCl has singly charged ions. JEE Advanced problems often ask students to compare lattice energies using the Born-Landé equation or Kapustinskii equation.
3. Periodic Trends and s-Block Chemistry (NEET, JEE Main)
NEET and JEE Main frequently test periodic trends using Group 2 oxides. MgO has a higher melting point and lattice energy than CaO, SrO, and BaO. This is because Mg²⁺ has a smaller ionic radius, leading to stronger electrostatic attraction. Questions may ask you to arrange Group 2 oxides in order of melting point or basic character. The basic character of Group 2 oxides increases down the group: MgO < CaO < SrO < BaO. In our experience, this trend appears in approximately 1-2 questions per NEET paper.
4. Qualitative Analysis (NEET Practical-Based Questions)
MgO reacts with water to form Mg(OH)₂, a slightly soluble base. This reaction is tested in NEET practical-based questions. The reaction \( \text{MgO} + \text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2 \) produces a milky white suspension. Mg(OH)₂ turns red litmus paper blue, confirming its basic nature. This is a standard observation in CBSE and NEET laboratory-based questions.
FAQs on Magnesium Oxide Formula
For more chemistry formulas and related topics, explore our comprehensive guides on the Chemistry Formulas hub. You may also find it useful to study the Barium Acetate Formula and the Zinc Bromide Formula for a deeper understanding of ionic compound formulas. For stoichiometry calculations involving MgO, our guide on the STP Formula will help you master mole-volume relationships. For the official NCERT syllabus reference, visit ncert.nic.in.