AP Chemistry · Lesson 1

Types of bonds and what they explain

Know Know it

In an ionic solid, metal cations and nonmetal anions are held in a lattice by their opposite charges. In a covalent bond, two atoms share electron pairs. In a metal, cations sit in a sea of valence electrons that can move, which is why metals conduct and can be bent without shattering.

Bond type is a range, not three boxes. The bigger the electronegativity difference, the more unevenly the electrons are shared: a small difference gives a nonpolar covalent bond, a larger one a polar covalent bond, and a large one between a metal and a nonmetal an ionic bond.

Two atoms bond because their potential energy is lowest at one distance, the bond length. Closer than that, the nuclei repel; farther, the attraction weakens. The depth of that energy well is the bond energy. More shared pairs (a higher bond order) give a shorter, stronger bond.

Lattice energy follows Coulomb's law: the energy of attraction between two ions is proportional to q1q2r\frac{q_1 q_2}{r}, where r is the distance between ion centers (about the sum of the two radii). Higher charges and smaller ions give a larger lattice energy and usually a higher melting point.

An alloy mixes a metal with another element. Atoms of similar size replace some of the metal atoms. Much smaller atoms fit into the gaps between metal atoms, which makes the layers harder to slide, so the alloy is harder and less malleable.

Calculator On the exam

A scientific or graphing calculator is allowed on both sections of the exam, and a four-function one is permitted but not recommended. A sheet of equations and constants is provided. Most bonding questions need no calculator: compare charges and radii with q1q2r\frac{q_1 q_2}{r} by hand, and show the comparison in words.

Worked example Easy

Which of these bonds is the most polar?

  1. H–FAnswer
  2. H–Cl
  3. H–Br
  4. H–I

How to solve it

  1. Bond polarity grows with the electronegativity difference between the two atoms.
  2. Every bond here has hydrogen, so compare the halogens: electronegativity falls down group 17.
  3. Fluorine is the most electronegative, so H–F is the most polar.

Why each choice is right or wrong

  • A. Correct. Fluorine is the most electronegative element, so the H–F bond has the largest electronegativity difference and the most uneven sharing.
  • B. Incorrect. H–Cl is polar, but chlorine is less electronegative than fluorine, so the difference with hydrogen is smaller.
  • C. Incorrect. H–Br is less polar than H–F, because bromine is lower in group 17 and less electronegative than fluorine.
  • D. Incorrect. H–I is the least polar of these: iodine is the least electronegative halogen listed, close to hydrogen.

Trap Breaking a bond never releases energy

Two bonded atoms sit at the bottom of an energy well. Pulling them apart always takes energy. When a reaction releases energy, it is because the new bonds formed release more than the old bonds took to break.

Explain Explain it

Task verbs: explain, compare

To compare two ionic solids, first compare the charges, because they multiply: 2+ with 2− is four times 1+ with 1−. If the charges are the same, compare the distance: smaller ions sit closer, so they attract more strongly.

A full answer names both factors, says which way each one points for the two solids, and ends with what it means: a stronger attraction takes more energy to overcome, so the solid melts at a higher temperature.

Worked example Medium

Ionic radii (pm)

IonNa⁺K⁺Mg²⁺F⁻Cl⁻O²⁻
Radius10213872133181140

Based on the radii and the ion charges, which compound has the largest lattice energy?

  1. NaF
  2. MgOAnswer
  3. NaCl
  4. KCl

How to solve it

  1. Charges first: MgO is 2+ and 2−, so q1q2=4q_1 q_2 = 4; the other three are 1+ and 1−, so q1q2=1q_1 q_2 = 1.
  2. Distance: MgO's ions are 72 + 140 = 212 pm apart, the smallest of the four.
  3. Both factors favor MgO, so it has the largest lattice energy.

Why each choice is right or wrong

  • A. Incorrect. NaF has small ions (102 + 133 = 235 pm apart), but its charges are only 1+ and 1−, so its attraction is about a quarter of MgO's.
  • B. Correct. MgO has charges of 2+ and 2−, so q1q2=4q_1 q_2 = 4, and its ions are the closest (72 + 140 = 212 pm), so it has the largest attraction.
  • C. Incorrect. NaCl has 1+ and 1− charges and a larger distance (102 + 181 = 283 pm) than NaF, so its lattice energy is smaller still.
  • D. Incorrect. KCl has 1+ and 1− charges and the largest ions here (138 + 181 = 319 pm), so it has the smallest lattice energy of the four.

Explain Your turn

Calcium oxide, CaO, melts at about 2600 °C, while potassium chloride, KCl, melts at about 770 °C. The ionic radii are Ca²⁺ 100 pm, O²⁻ 140 pm, K⁺ 138 pm and Cl⁻ 181 pm. Explain the difference in melting points using Coulomb's law.

Write it, then compare. Nothing you type here is saved or graded.

Compare with a model answer

A full answer does this

  • Says the ions in CaO have larger charges (2+ and 2−) than those in KCl (1+ and 1−), so they attract more strongly
  • Says the ions in CaO are smaller, so they are closer together (about 240 pm against 319 pm) and attract more strongly
  • Links the stronger attraction in CaO to more energy needed to separate its ions, so a higher melting point

Model answer

By Coulomb's law, the attraction between two ions grows with the product of their charges and falls as the distance between them grows. The ions in CaO have charges of 2+ and 2−, against 1+ and 1− in KCl, so their attraction is about four times as strong. The CaO ions are also smaller, so their centers are closer together: about 100 + 140 = 240 pm, against 138 + 181 = 319 pm in KCl. Both factors make the attraction in CaO much stronger, so much more energy is needed to separate its ions, and CaO melts at a far higher temperature.

Apply Apply it

Practice this topic. AP Practice questions on exactly what this lesson covers, with an explanation for every answer.