Valence Electrons and Ionic Compounds
Connect repeating valence patterns to familiar ion charges and electrically neutral compound formulas.
AP Chemistry Topic 1.8 · CED printed p. 38 / PDF p. 45. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Valence configurations
- Charge arithmetic
Similar outer electrons give analogous chemistry
In covalent bonding, shared valence-electron density is attracted to both positive nuclei, not just to the nucleus of the atom from which an electron was counted. Nuclei repel one another and electrons repel one another. The balance of these interactions helps explain a stable separation; pushing nuclei arbitrarily close does not keep lowering the energy. A more stable bonded arrangement has lower total energy than separated partners under the stated conditions; atoms do not “want” an octet in a literal sense. Outer electrons are accessible for transfer or sharing. Elements with similar outer-shell patterns often react in analogous ways: Li, Na and K commonly form +1 ions; Mg and Ca commonly form +2 ions; F and Cl commonly form −1 ions.
For representative monatomic ions in familiar binary compounds, group 1 commonly gives +1, group 2 +2, group 13 often +3, group 15 often −3, group 16 −2 and group 17 −1. These are useful predictions, not universal charges for every compound. Transition metals can have multiple common charges; use a stated charge or other evidence. Noble gases have filled valence shells and generally low reactivity, though “never react” would be too strong.
The compound must balance charge
An ionic formula records the smallest ratio of ions that makes the compound electrically neutral. Mg2+ requires two Cl− ions: MgCl2. Al3+ and O2− balance at six positive and six negative charge units: Al2O3. Reduce ratios such as Ca2O2 to CaO. Keep polyatomic ions intact when they repeat, as in Ca(NO3)2.
Periodicity predicts analogous formulas: replacing Mg with Ca in MgCl2 gives CaCl2. It does not imply identical reactivity rates or physical properties. Down group 1, outer electrons are generally farther away and easier to remove, which helps explain greater ease of electron loss. Bond formation and reaction outcomes also depend on the reacting partners and conditions, so use periodicity as a model rather than an unconditional recipe.
Let's work through it
Work it through
Predict the formula formed by Ca and N, then the analogous formula formed by Mg and N.
- For the familiar monatomic-ion model, Ca is group 2 → Ca2+; nitrogen as nitride is group 15 → N3−.
- Find the least common charge total: 6. Three Ca2+ contribute +6 and two N3− contribute −6, so the neutral formula is Ca3N2.
- Mg has the analogous two-valence-electron group pattern and commonly forms Mg2+, giving Mg3N2.
- The 3:2 subscripts give numbers of ions, while 2+ and 3− are charges on each ion.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Predict a formula for Al3+ and S2−.
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Predict a formula for Al3+ and S2−.
Worked answer and reason
Al2S3; two Al ions give +6 and three sulfide ions give −6.
Neutrality determines the integer ratio.
Retrieval 2
Why are NaCl and KCl analogous formulas?
Worked answer and reason
Na and K share an outer ns1 pattern and commonly form +1 ions, balancing Cl− in a 1:1 ratio.
Group patterns support analogous ionic charges and compounds.
Catch the tempting mistake
- An ion’s charge becomes its own subscript.
- Subscripts balance the total charges of all ions, so each subscript depends on both ions.
- Every element has one universal ionic charge.
- Typical main-group charges are a useful subset; transition-metal and other chemistry require more evidence.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 1 problem 17 · ap application · symbolic
- Unit 1 problem 18 · ap synthesis · verbal
Scientific calculator workspace
Use this workspace for arithmetic, logarithms and elementary functions. Keep intermediate precision and report final values with appropriate units and significant figures. Graphing and general equation solving need a suitable approved tool.
Scientific calculator
This calculator requires JavaScript. Use the physics formula sheets and show your working manually.
+ − * / ^ ! · pi · e · sqrt · sin, cos, tan · asin, acos, atan · atan2(y,x) · ln, log · abs · exp · hypot(x,y). Use * between factors. Keep units in your written working.
For the teacher
- Require the charge sum to equal zero rather than accepting a criss-cross mnemonic.
- Use an analogous-compound task to assess periodicity without a hazardous reaction demonstration.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 1.8.A
- Explain the relationship between trends in the reactivity of elements and periodicity.
- 1.8.A.1
- The likelihood that two elements will form a chemical bond is determined by the interactions between the valence electrons and nuclei of elements.
- 1.8.A.2
- Elements in the same column of the periodic table tend to form analogous compounds.
- 1.8.A.3
- Typical charges of atoms in ionic compounds are governed by the number of valence electrons and predicted by their location on the periodic table.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 (supplied source) — Exact topic-numbered learning objectives, essential knowledge, science practices and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
- OpenStax Chemistry 2e §8.1, Valence Bond Theory — Qualitative shared-electron attraction to both nuclei, competing repulsions and stable bonded separation. New questions and feedback are independently authored.
Glossary and deeper reading · 2 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
-
Covalent bond
A covalent bond involves shared electron density between atoms. The sharing can be approximately equal or unequal depending on the atoms and their environment. For example, the bond in a hydrogen molecule shares electrons between two identical atoms.
-
Ion
An ion is an atom or group of atoms with a net electric charge. Ordinary ion formation changes electron count without changing the element's proton count. For example, a sodium atom losing one electron becomes a sodium ion with charge +1.