Dot-and-Cross Diagram for Magnesium Chloride

Introduction

Magnesium supplies two electrons when forming its usual ion, but one chlorine atom needs only one electron to form chloride. The solution is a different particle ratio: two chlorine atoms accept one electron each. This example shows how electron accounting determines both the number of ions in a drawing and the subscripts in a formula.

Core explanation

Begin with magnesium's outer-shell count of two and chlorine's count of seven. One magnesium atom can lose two electrons to form Mg²⁺. Each chlorine atom gains one electron to form Cl⁻. Therefore, the two electrons must go to two different chlorine atoms if these are the specified final ions.

Draw one magnesium atom and two chlorine atoms before transfer. Mark the magnesium electrons as crosses and the chlorine electrons as dots. After transfer, each chloride has seven dots and one cross. Put each ion in its own bracket: one Mg²⁺ bracket and two separate Cl⁻ brackets. Neither chloride should receive both crosses.

The combined charges are +2 + (−1) + (−1) = 0. The ion ratio is one magnesium to two chlorides, so the formula is MgCl₂. The subscript two applies to chlorine's count in the ratio. It does not mean one chloride ion has charge −2, and it does not indicate a neutral chlorine molecule inside the crystal.

Total valence-electron accounting provides a second check. The starting atoms supply 2 + 7 + 7 = 16 valence electrons. The final two chloride outer shells contain eight each, giving sixteen. Magnesium's original two valence electrons now occur among those chloride symbols; its ten core electrons have not disappeared simply because the valence-only drawing omits them.

As with other ionic formulas, MgCl₂ represents a repeating ratio in the solid. A three-ion drawing is a convenient accounting unit, not proof that the crystal is built from separate MgCl₂ molecules. The extended lattice and the ions' mobility in different states explain its bulk properties.

Step-by-step reasoning

1. Write the common ion charges Mg²⁺ and Cl⁻. 2. Match magnesium's two lost electrons with two chlorine atoms gaining one each. 3. Place one transferred cross in each chloride outer shell and label all three ion brackets. 4. Check sixteen tracked valence electrons and net charge zero, then write MgCl₂.

Visual explanation

Place magnesium between two chlorine atoms, with one transfer arrow pointing left and one right. In the final drawing, separate the three charged brackets. Each chloride's eight symbols should contain exactly one cross, showing the electrons were distributed.

Real-world analogy

Two seats are available, but each ticket admits only one person. Filling both seats therefore requires two tickets rather than making one ticket count twice. Similarly, each chlorine atom accepts one electron in this model, so magnesium's donation needs two acceptors.

Real-world example

Magnesium chloride is one of the salts associated with seawater and brines. In aqueous solution, its dissolved ions are represented as one Mg²⁺ for every two Cl⁻. Water surrounds the ions, but dissolving the salt does not change the charge-neutral composition ratio supplied by the formula.

Why?

Why cannot a single Cl⁻ balance Mg²⁺? Their combined charge would be +1, leaving the proposed bulk formula electrically unbalanced. A second chloride supplies another negative charge while maintaining the identity of each chloride as a 1− ion.

Common misconception

“MgCl₂ contains one Mg ion and one Cl₂ ion.” The formula expresses two chloride ions per magnesium ion in the ionic model. A subscript counting chlorine atoms is not permission to invent a combined Cl₂²⁻ particle.

Worked example

A drawing shows one Mg²⁺ and one bracket containing a chlorine symbol with seven dots and two crosses, labelled 2−. It fails the intended chemistry and gives chlorine nine outer electrons. Correct it by drawing two separate chlorides, each with seven dots, one cross and charge −1. The corrected charges balance magnesium's +2.

Quick check

1. How many chloride ions are required to balance five magnesium ions in this ratio? Answer: Ten chloride ions, because each Mg²⁺ requires two Cl⁻ ions.

Exam focus

Draw the complete formula ratio when asked for the compound's dot-and-cross diagram. A correct single chloride ion beside magnesium is insufficient if the second required chloride is missing.

Advanced insight

The same electron-and-charge matching predicts CaCl₂ and MgBr₂ from their common ion charges. Reusing the reasoning is more reliable than memorising pictures, but ion identities must still be established before transferring the pattern to a new compound.

Summary

One magnesium atom loses two electrons, and two chlorine atoms gain one each. The resulting Mg²⁺ and two Cl⁻ ions balance electrically. Separate charged brackets and one transferred symbol per chloride lead directly to the ratio and formula MgCl₂.

Practice questions

1. How many original chlorine valence electrons occur across the two chlorides before counting transferred electrons? Answer: Fourteen, seven from each of the two chlorine atoms. 2. What does the two in MgCl₂ count? Answer: Two chlorides per magnesium in the simplest composition ratio, not charge on one chloride. 3. Write the total charge of one Mg²⁺ with two Cl⁻ ions. Answer: +2 − 1 − 1 = 0, confirming a neutral combination.