Unit 1 · Topic 1.8 Beta

Valence Electrons and Ionic Compounds

Main-group metals lose their valence electrons to form cations, and nonmetals gain electrons to form anions, usually reaching a noble gas configuration.

Practice 1: Models and RepresentationsPractice 6: Argumentation

Question set for this topic

Part 1 · Hook

Why this matters

Table salt is made of sodium, a soft metal that reacts violently with water, and chlorine, a poisonous green gas. Together they form a harmless white crystal you put on fries. The change comes from electrons: sodium gives one up and chlorine takes one, and the charges that result hold the solid together.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. How many valence electrons does magnesium, [Ne] 3s², have?

  1. 2
  2. 12
  3. 10
  4. 8
Show the answer

The outermost shell, n = 3, holds 3s²: 2 valence electrons.

  • Correct: 2:
  • 12:
  • 10:
  • 8:

2. Across period 3, how does first ionization energy change in general?

  1. It increases
  2. It decreases
  3. It stays the same
  4. It increases, then falls to zero
Show the answer

Effective nuclear charge rises across the period, so electrons are held more tightly.

  • Correct: It increases:
  • It decreases:
  • It stays the same:
  • It increases, then falls to zero:

Part 4 · See it

See it first

Na (1s² 2s² 2p⁶ 3s¹) loses one electron to become the smaller Na⁺, like neon; Cl ([Ne] 3s² 3p⁵) gains one to become the larger Cl⁻, like argon; one of each makes neutral NaCl.
Sodium loses its single 3s electron and chlorine gains one, each reaching a noble gas configuration. Na⁺ and Cl⁻ combine 1 : 1 so the charges cancel. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Metals hold their few valence electrons loosely (low ionization energy)they lose them to form cations
  2. Nonmetals have a high effective nuclear charge and attract added electronsthey gain electrons to form anions
  3. Removing electrons past the valence shell, or adding them past the next noble gas, costs far more energymain-group ions usually end up with a noble gas configuration
  4. Cations and anions attract, and the compound must be neutralthey combine in the ratio that makes the charges cancel

Part 6 · Key ideas

Key ideas

  • Group 1 → 1+, group 2 → 2+, group 13 → 3+ (Al); group 15 → 3−, group 16 → 2−, group 17 → 1−. Each ion has a noble gas configuration.
  • An ionic compound is made of cations and anions in the ratio that makes the total charge zero: Ca²⁺ + 2 F⁻ → CaF₂.
  • Write the cation first; subscripts give the simplest ratio (a formula unit), never 2 : 2.
  • Elements in a group have the same number of valence electrons, so they form the same ions and react alike.

Part 7 · Misconception

A common mistake

The wrong idea: Atoms form ions because they want a full outer shell.

What actually happens: Atoms do not want anything. A metal's valence electrons are loosely held, so little energy removes them; removing one more from the core would cost far more. A nonmetal attracts an added electron strongly. Energy and Coulombic attraction set the charge.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Particle view

Ions in three solid samples

2+−−2+−−2+−−2+−−Box 1+−+−−+−++−+−Box 22+2−2+2−2−2+2−2+2+2−2+2−Box 3

Key: small blue circle, cation (charge shown); larger green or orange circle, anion (charge shown). Each box is a small piece of a much larger solid.

1. Which box could represent magnesium chloride?

  1. Box 2
  2. Box 3
  3. Box 1
  4. Boxes 1 and 3 both could
Show the answer

Magnesium (group 2) forms Mg²⁺; chlorine (group 17) forms Cl⁻. Two Cl⁻ balance one Mg²⁺: MgCl₂, the ratio in Box 1.

  • Box 2: Box 2 has 1+ cations, but magnesium forms 2+ ions.
  • Box 3: Chloride is 1−, not 2−.
  • Correct: Box 1: Right: Mg²⁺ and Cl⁻ in a 1 : 2 ratio, so the charges cancel: MgCl₂.
  • Boxes 1 and 3 both could: Box 3 has 2− anions, and chloride is 1−. Box 1 matches: 2+ cations and 1− anions in a 1 : 2 ratio.

2. Which formula matches Box 3?

  1. MgO: a 2+ cation and a 2− anion, 1 : 1
  2. Mg₂O₂: two of each ion in each formula
  3. NaCl: a 1+ cation and a 1− anion, 1 : 1
  4. MgCl₂: twice as many anions as cations
Show the answer

Equal numbers of 2+ and 2− ions: a 1 : 1 ratio, like MgO.

  • Correct: MgO: a 2+ cation and a 2− anion, 1 : 1: Right: +2 and −2 cancel one for one.
  • Mg₂O₂: two of each ion in each formula: A formula gives the simplest ratio; 2 : 2 reduces to 1 : 1.
  • NaCl: a 1+ cation and a 1− anion, 1 : 1: Box 3's ions carry charges of 2+ and 2−.
  • MgCl₂: twice as many anions as cations: Box 3 has equal numbers of cations and anions.

3. Why does every box contain cations and anions in the ratio it does?

  1. Each cation can touch two anions at a time
  2. Larger ions are outnumbered by smaller ones
  3. The ions are counted by mass, so heavier ions appear less often
  4. The charges cancel, so each solid is neutral
Show the answer

An ionic compound is electrically neutral. Box 1: 4(+2) + 8(−1) = 0; Box 2: 6(+1) + 6(−1) = 0; Box 3: 6(+2) + 6(−2) = 0.

  • Each cation can touch two anions at a time: How ions touch is about packing, not about the formula ratio.
  • Larger ions are outnumbered by smaller ones: In Boxes 2 and 3 the numbers are equal; the charges decide the ratio.
  • The ions are counted by mass, so heavier ions appear less often: The ratio counts ions, not masses.
  • Correct: The charges cancel, so each solid is neutral: Right: the ratio is set by the charges, so the whole solid has no net charge.

4. A student draws calcium nitride (Ca²⁺ and N³⁻) as a box with equal numbers of each ion. What is wrong with the drawing?

  1. Nitrogen forms no ions, so it does not belong in the box
  2. Its charges do not cancel; Ca₃N₂ has 3 Ca²⁺ per 2 N³⁻
  3. The ratio should be 2 Ca²⁺ to 3 N³⁻, Ca₂N₃
  4. Calcium ions should carry a 1+ charge
Show the answer

Find the smallest numbers that make the charges cancel: 3 × (+2) = +6 and 2 × (−3) = −6. Ca₃N₂.

  • Nitrogen forms no ions, so it does not belong in the box: Nitrogen, in group 15, gains 3 electrons to form N³⁻.
  • Correct: Its charges do not cancel; Ca₃N₂ has 3 Ca²⁺ per 2 N³⁻: Right: 3(+2) + 2(−3) = 0.
  • The ratio should be 2 Ca²⁺ to 3 N³⁻, Ca₂N₃: That gives 2(+2) + 3(−3) = −5: not neutral.
  • Calcium ions should carry a 1+ charge: Calcium is in group 2 and forms Ca²⁺.

Data table

Five main-group elements

Electron configurations of five atoms.

Electron configurations
ElementGroupElectron configuration
Lithium11s² 2s¹
Calcium2[Ar] 4s²
Aluminum13[Ne] 3s² 3p¹
Sulfur16[Ne] 3s² 3p⁴
Bromine17[Ar] 4s² 3d¹⁰ 4p⁵

5. Why does aluminum form Al³⁺ and not Al⁴⁺?

  1. A 4th electron would come from the core, costing far more energy
  2. Aluminum has three electrons in total
  3. A 4+ charge is too large for any ion to carry
  4. Aluminum wants to have exactly three electrons fewer than before
Show the answer

Al: [Ne] 3s² 3p¹. Losing the 3 valence electrons is affordable; a 4th would come from the 2p core, close to the nucleus and poorly shielded.

  • Correct: A 4th electron would come from the core, costing far more energy: Right: after 3 electrons, Al³⁺ has neon's configuration; the next is a tightly held core electron.
  • Aluminum has three electrons in total: Aluminum has 13 electrons; 3 of them are valence electrons.
  • A 4+ charge is too large for any ion to carry: The limit comes from the energy to remove core electrons, not from a fixed maximum charge.
  • Aluminum wants to have exactly three electrons fewer than before: Atoms do not want things; the energy cost of removing each electron sets the charge.

6. What is the formula of the compound formed by aluminum and bromine?

  1. Al₃Br
  2. AlBr
  3. AlBr₃
  4. Al₂Br₃
Show the answer

Al³⁺ + 3 Br⁻: +3 − 3 = 0. AlBr₃.

  • Al₃Br: This puts the 3 on the wrong ion: 3(+3) + (−1) = +8.
  • AlBr: One Br⁻ would leave the compound with a 2+ charge.
  • Correct: AlBr₃: Right: Al³⁺ and Br⁻; three Br⁻ balance one Al³⁺.
  • Al₂Br₃: That would need Al with a charge of 3/2, which does not exist.

Part 9 · Summary

Summary

Main-group metals lose their valence electrons to form cations, and nonmetals gain electrons to form anions, usually reaching a noble gas configuration. Ionic compounds combine cations and anions in the smallest whole-number ratio that makes the total charge zero. Elements in the same group form ions of the same charge and behave alike.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections