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Grade 10-11 (age 15-17)

high school Ionic Bond

When a metal that gives up electrons easily, such as sodium, meets a nonmetal that takes them readily, such as chlorine, an electron passes from one to the other and a cation and an anion appear. The two oppositely charged ions pull on each other by electrostatic attraction, and the bond held together by that pull is an ionic bond. The attraction falls off with the square of the distance, so it grows sharply as the ions approach, and larger charges on smaller ions make a tighter bond with a higher melting point. The top of the diagram shows the two atoms changing size as the electron moves; the bottom shows how quickly the attraction weakens as the distance grows.

When One Electron Moves Over

3 r₀
💡 Reading the Two Rows
①Na gives away its single outer 3s electron and becomes Na⁺. With a whole outer shell emptied, its radius shrinks from 186 pm to 102 pm
②Cl takes that electron and becomes Cl⁻. The nuclear charge stays the same while the extra electron adds repulsion, so the radius grows from 99 pm to 181 pm
③In the crystal the two ions touch at r₀, the sum of the radii, about 283 pm. Any closer and the electron clouds push back hard, so the ions settle at this distance
④Double the distance and the attraction drops to 1/4; triple it and it drops to 1/9. The arrows shrink so fast because the force falls off with the square of the distance

Coulomb's Law and Lattice Energy

Coulomb's Law
F = kq1 q2r2
q₁, q₂: charges of the two ions, r: distance between ion centers, k: Coulomb constant
Lattice Energy
U ∝ |q₁||q₂| / r
Energy needed to pull 1 mol of an ionic crystal apart into gaseous ions; the larger it is, the stronger the bonding
🔍 Charge and Size Set the Melting Point
①Larger charges mean larger lattice energy. The charge product of Mg²⁺ and O²⁻ is 4 times that of Na⁺ and Cl⁻, and the ions sit closer, so MgO (about 3800 kJ/mol) far exceeds NaCl (about 790 kJ/mol)
②With equal charges, smaller ions sit closer together and give larger lattice energy, which is why NaF melts at a higher temperature than NaCl
③The larger the lattice energy, the harder it is to pull the ions apart and the higher the melting point: NaCl melts at 801 °C, MgO at about 2800 °C

How an NaCl Crystal Is Built

💡 Properties of Ionic Crystals
①Na⁺ and Cl⁻ alternate along rows and columns, so every ion has only oppositely charged neighbors. In three dimensions each ion is surrounded by 6 ions of the opposite charge
②The formula NaCl does not name a single molecule; it means Na⁺ and Cl⁻ are present in a 1 : 1 ratio
③In the solid the ions are locked in place and no current flows, but melted or dissolved in water the ions can move and the substance conducts
④If a force shifts one layer by a single position, like charges end up face to face and repel, so ionic crystals shatter easily

Work It Out

Example 1
Find the chemical formula of the ionic compound made of sodium ions (Na⁺) and oxide ions (O²⁻).
1
Check the charge on each ion.
Na⁺ : +1, O²⁻ : −2
2
Choose the ratio of ions that makes the total charge zero.
2 × (+1) + 1 × (−2) = 0 → Na₂O
▸ Na₂O
The charges of two Na⁺ cancel the charge of one O²⁻. The subscript 2 in the formula records that ratio.
Example 2
When magnesium (Mg) and chlorine (Cl) form an ionic bond, find the resulting formula and each ion.
1
Work out which ion each element forms: Mg loses 2 electrons and Cl gains 1.
Mg → Mg²⁺ + 2e⁻, Cl + e⁻ → Cl⁻
2
Balance the number of ions so the total charge is zero.
1 × (+2) + 2 × (−1) = 0 → MgCl₂
▸ MgCl₂ (Mg²⁺, Cl⁻)
Two Cl atoms each take one of the 2 electrons Mg gives up, so one Mg²⁺ pairs with two Cl⁻.

Summary

When an Ionic Bond Forms
metal atom → cation, nonmetal atom → anion → electrostatic attraction
Forms readily between a metal and a nonmetal with a large difference in electronegativity
2022 KICE mock Chemistry I type, adapted
What is the formula of the ionic compound made of aluminum ions (Al³⁺) and oxide ions (O²⁻)?
①AlO
②Al₂O₃
③Al₃O₂
④AlO₂
⑤Al₂O
▸ ② Al₂O₃
1
Find the simplest whole-number ratio a : b that satisfies (+3) × a + (−2) × b = 0.
2
Matching the least common multiple of 3 and 2, which is 6, gives a = 2 and b = 3.
(+3) × 2 + (−2) × 3 = 0 → Al₂O₃
🎯 Exam Points
①An ionic bond is the electrostatic attraction between a metal cation and a nonmetal anion
②A metal atom shrinks when it becomes a cation; a nonmetal atom grows when it becomes an anion
③Lattice energy is proportional to the product of the charges and inversely proportional to the distance between ion centers
④The solid does not conduct electricity; the melted liquid and the aqueous solution do
⑤Write the cation first and use the simplest whole-number ratio that makes the total charge zero
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Covalent Bond

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