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1.8Atomic Structure and Properties

Valence Electrons and Ionic Compounds — AP CHEMISTRY Revision Notes & Topical Questions

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1.8 Valence Electrons and Ionic Compounds — Notes — AP CHEMISTRY notes: Identify valence electrons, predict ion charges and write formulae of ionic compounds.Valence electrons = electrons in the outermost shell (highest n), which take part in bonding.The number can be read from t…

  • Ionic bond
  • Covalent bond
  • Formula unit
  • Polyatomic ion
  • The number can be read from the electron configuration
  • For main-group elements the number equals the last digit of the group number

Section 1 of 10Valence electrons

Valence electrons
Valence electrons electrons in the outermost shell (highest n), which take part in bonding.
The number can be read from the electron configuration.
For main-group elements the number equals the last digit of the group number.
Elements in the same group have the same number of valence electrons, so they form analogous compounds, such as NaCl and KBr, or MgO and CaS.
The interactions between valence electrons and nuclei determine whether two elements bond.

Section 2 of 10Bond types

Bond types
Ionic bond
  • Ionic bond between a metal and a nonmetal.
Covalent bond
  • Covalent bond between two nonmetals.

Section 3 of 10Forming ions

1
Atoms form ions with the electron configuration of the nearest noble gas, whose full outer shell is stable.
2
Metals lose valence electrons to become cations; nonmetals gain electrons to become anions.
3
Removing more electrons than the valence count would take core electrons, which needs much more energy, so it does not occur in ordinary compounds.
4
The electrostatic attraction between cation and anion is the ionic bond.

Section 4 of 10Ion charges by group

GroupValence electronsIon chargeExample
11+1Na+
22+2Mg2+
133+3Al3+
155−3N3−
166−2O2−
177−1F−
Ion charges by group
Group 14 elements tend not to form ions.
For Groups 1 to 3 the charge equals the group number and for Groups 15 to 17 it equals the group number − 10.

Section 5 of 10Transition metals and polyatomic ions

Transition metals and polyatomic ions
Transition metals often form several cations, such as Fe2+ and Fe3+, and lose 4s electrons first.
Common polyatomic ions are NO3−, SO42−, CO32−, NH4+, PO43− and OH−.
They combine using the same charge-balance rule.

Section 6 of 10Writing an ionic formula

1
Find the group of each element.
2
Predict each ion's charge from the number of valence electrons.
3
Find the lowest common multiple of the charges.
4
Choose subscripts so total positive charge equals total negative charge.
5
Write the cation first and reduce to the simplest ratio.
  • Example
    • Al3+ with O2− gives Al2O3.

Section 7 of 10Reactivity trends

Reactivity trends
Group 1 metal reactivity increases down the group.
Halogen reactivity decreases down the group.
Higher ion charges and smaller ions give stronger ionic lattices, so MgO is stronger than NaCl.

Section 8 of 10More from the specification

More from the specification
Electronegativity periodic trends across groups
  • Electronegativity periodic trends across groups systematic periodic trends dictate ionization energies and ionic bond strength across representative main group elements.
Periodic trend
  • A predictable change in a property across a period or down a group.
Atomic radius
  • Half the distance between the nuclei of two bonded atoms of an element.
Ionic radius
  • Distance from the nucleus to the outermost electrons of an ion.
Shielding
  • The reduction in attraction to the nucleus caused by inner electron shells.
Trend
  • General pattern down a group or across a period.
Period
  • A row of the Periodic Table, equal to the number of occupied shells.
Property
  • A feature that describes what a substance is like and what it does.

Section 9 of 10Key terms

TermMeaning
Valence electronsOuter-shell electrons involved in bonding.
CationPositive ion formed by losing electrons.
AnionNegative ion formed by gaining electrons.
Ionic bondElectrostatic attraction between cations and anions.
Formula unitSimplest ratio of ions in an ionic compound.
Polyatomic ionGroup of covalently bonded atoms carrying a charge.

Section 10 of 10Exam technique

1
Predicting a Group 2 ion as 1+ — it loses two electrons
2
Using the group number for anion charge — use group number minus 10 (or minus 8 in the older numbering)
3
Forgetting that ionic compounds are electrically neutral
4
Removing 3d electrons before 4s when forming transition-metal cations

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