In the balanced chemical equation for the reaction of magnesium with oxygen gas to form magnesium oxide, how should the equation Mg + O2 → MgO be correctly balanced?

Introduction / Context:
Balancing chemical equations is a fundamental skill in chemistry. A balanced equation ensures that the law of conservation of mass is satisfied, meaning that the number of atoms of each element is the same on both sides of the equation. The reaction between magnesium metal and oxygen gas to form magnesium oxide is a common example used in introductory courses. This question asks you to choose the correctly balanced form of the basic equation Mg + O2 → MgO.

Given Data / Assumptions:

• The unbalanced equation is magnesium plus oxygen gas gives magnesium oxide.
• Oxygen exists as a diatomic molecule O2 under standard conditions.
• Magnesium oxide has the empirical formula MgO, with a one to one ratio of Mg and O.
• We must add coefficients to balance the number of Mg and O atoms on each side.

Concept / Approach:
To balance the equation, count atoms of each element on both sides. In the unbalanced equation Mg + O2 → MgO, there is one magnesium atom on each side, but two oxygen atoms on the left and only one on the right. We cannot change subscripts in formulas; we can only add coefficients. To balance oxygen, we need two oxygen atoms on the product side, which requires two formula units of MgO. That introduces two magnesium atoms on the product side, so we must also have two magnesium atoms on the reactant side. The balanced equation is therefore 2Mg + O2 → 2MgO.

Step-by-Step Solution:
Step 1: Write the unbalanced equation: Mg + O2 → MgO. Step 2: Count atoms on the left side: 1 magnesium atom and 2 oxygen atoms. Step 3: Count atoms on the right side: 1 magnesium atom and 1 oxygen atom in MgO. Step 4: To balance oxygen, place a coefficient of 2 in front of MgO, giving 2MgO on the right side. Now there are 2 oxygen atoms on each side but 2 magnesium atoms on the right and only 1 on the left. Step 5: To balance magnesium, place a coefficient of 2 in front of Mg on the left side, giving 2Mg + O2 → 2MgO. Now magnesium and oxygen are balanced.

Verification / Alternative check:
Check the final balanced equation 2Mg + O2 → 2MgO carefully. On the left, there are 2 magnesium atoms and 2 oxygen atoms. On the right, there are 2 magnesium atoms and 2 oxygen atoms in the two units of MgO. The numbers match for each element, so mass is conserved. Also, the coefficients are the smallest whole numbers that achieve balance, which is the usual convention. Any alternative equation that changes subscripts, such as MgO2, would represent a different compound and would not be correct for magnesium oxide formation.

Why Other Options Are Wrong:
The option 2Mg + O → 2MgO is wrong because oxygen does not exist as monatomic O under normal conditions; it is diatomic O2. The equation 2Mg + O → MgO2 is incorrect because it changes the product formula to MgO2, which is not the usual magnesium oxide and also leaves atoms unbalanced. The equation 2Mg + O2 → MgO2 is also wrong because the product again is not MgO and the numbers of atoms do not match. These forms violate either the actual formulas of the substances or the conservation of atoms.

Common Pitfalls:
Beginners sometimes try to balance equations by changing subscripts inside chemical formulas, which changes the identity of the compounds and is not allowed. Others may forget that many gases, including oxygen, exist as diatomic molecules and incorrectly write O instead of O2. To avoid such mistakes, always treat formulas as fixed and balance equations by placing integer coefficients in front of them. For this reaction, remembering the simple pattern metal plus oxygen gives metal oxide, balanced as 2Mg + O2 → 2MgO, can be a helpful memory aid.

Final Answer:
The correctly balanced equation for the reaction of magnesium with oxygen is 2Mg + O2 → 2MgO.