Unit 2 · Topic 2.1 Beta

Types of Chemical Bonds

Atoms bond because nuclei attract the valence electrons of neighboring atoms.

Practice 1: Models and RepresentationsPractice 4: Model AnalysisPractice 6: Argumentation

Question set for this topic

Part 1 · Hook

Why this matters

Table salt, a copper wire and the chlorine in a swimming pool are all made of atoms stuck together, yet salt shatters, copper bends and chlorine is a gas. The difference starts with how the electrons are held between the atoms: transferred, shared, or pooled.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. Going across period 3 from Na to Cl, electronegativity

  1. increases, because the nuclear charge grows while the shell stays the same
  2. decreases, because the atoms get larger
  3. stays the same, because the atoms are in the same period
  4. increases, because the atoms gain more shells
Show the answer

More protons pull on electrons in the same shell, so the attraction for shared electrons grows from left to right.

  • Correct: increases, because the nuclear charge grows while the shell stays the same:
  • decreases, because the atoms get larger:
  • stays the same, because the atoms are in the same period:
  • increases, because the atoms gain more shells:

2. A magnesium atom forms an ion by

  1. losing its two valence electrons to become Mg²⁺
  2. gaining six electrons to become Mg⁶⁻
  3. losing one electron to become Mg⁺
  4. sharing its electrons to stay neutral
Show the answer

Mg has two valence electrons in the 3s subshell; losing them leaves the noble gas configuration of Ne.

  • Correct: losing its two valence electrons to become Mg²⁺:
  • gaining six electrons to become Mg⁶⁻:
  • losing one electron to become Mg⁺:
  • sharing its electrons to stay neutral:

Part 4 · See it

See it first

An arrow labeled electronegativity difference runs from small to large over three panels. Nonpolar covalent: two Cl atoms with a shared electron pair midway between them. Polar covalent: H and Cl share a pair that sits closer to Cl; H is marked delta plus and Cl delta minus, with a dipole arrow pointing toward Cl. Ionic: an electron moves from Na to Cl, giving a small Na+ cation and a larger Cl- anion.
As the electronegativity difference between two atoms grows, the shared electrons are pulled further toward one atom, until an electron is effectively transferred and two ions form. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Two atoms come together and both nuclei attract the valence electronsthe electrons end up between the nuclei, holding the atoms together: a chemical bond
  2. Both atoms are nonmetals with the same electronegativitythe shared pair is attracted equally: a nonpolar covalent bond
  3. One nonmetal is more electronegative than the otherthe shared pair spends more time near it, giving partial charges δ− and δ+: a polar covalent bond
  4. A metal with a low ionization energy meets a nonmetal that attracts electrons stronglyan electron moves from the metal to the nonmetal, and the cation and anion attract: an ionic bond
  5. Many metal atoms each release their valence electronsthe cations sit in a sea of delocalized electrons that holds them together: metallic bonding

Part 6 · Key ideas

Key ideas

  • A covalent bond is a pair of electrons shared between two atoms; both nuclei attract it.
  • Unequal sharing makes a polar covalent bond: the more electronegative atom gets a partial negative charge (δ−), the other δ+.
  • An ionic bond is the Coulombic attraction between a cation and an anion, usually a metal with a nonmetal.
  • In metallic bonding, cations sit in a sea of delocalized valence electrons.
  • Bond character is a continuum: the larger the electronegativity difference, the more polar and the more ionic the bond.

Part 7 · Misconception

A common mistake

The wrong idea: A bond is either covalent or ionic, with nothing in between.

What actually happens: Bond character changes smoothly with the electronegativity difference. Cl–Cl shares equally, H–Cl shares unequally, and Na–Cl is so unequal that the electron is effectively transferred. Use the difference, and whether a metal is involved, to place a bond along that range.

Part 8 · Check yourself

Check yourself

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

Data table

Electronegativity values

Electronegativity values on the Pauling scale, rounded to one decimal place.

Electronegativity of selected elements (Pauling scale, no unit)
ElementElectronegativity
H2.2
C2.6
N3.0
O3.4
F4.0
Na0.9
Mg1.3
S2.6
Cl3.2
K0.8
Br3.0

1. Which of these bonds is the most polar?

  1. C–Cl
  2. O–H
  3. N–H
  4. C–S
Show the answer

The electronegativity differences are O–H 3.4 − 2.2 = 1.2, N–H 0.8, C–Cl 0.6 and C–S 0.0. The largest difference gives the most unequal sharing, so O–H is the most polar.

  • C–Cl: Cl is very electronegative, but so is C compared with H: the C–Cl difference is only 3.2 − 2.6 = 0.6. Polarity depends on the difference, not on one atom.
  • Correct: O–H: Right: a difference of 1.2 is the largest of the four.
  • N–H: The N–H difference is 3.0 − 2.2 = 0.8, smaller than the O–H difference of 1.2.
  • C–S: C and S both have an electronegativity of 2.6, so this bond is the least polar of the four. Larger atoms do not make a bond more polar.

2. A bond between which two atoms has its electrons shared most equally?

  1. H and Br
  2. N and O
  3. H and C
  4. C and S
Show the answer

C and S have the same electronegativity, 2.6, so neither nucleus pulls the shared electrons harder: the bond is nonpolar covalent.

  • H and Br: H and Br differ by 0.8, the largest difference of these four pairs.
  • N and O: N and O differ by 3.4 − 3.0 = 0.4, so O pulls the shared electrons a little harder. Two nonmetals next to each other in a period still differ.
  • H and C: H and C differ by 0.4, so the sharing is slightly unequal.
  • Correct: C and S: Right: a difference of 0.0 means equal sharing.

3. In a bond between sulfur and oxygen, which atom carries the partial negative charge, and why?

  1. O, because its larger electronegativity pulls the shared pair toward it
  2. S, because it is the larger atom and holds more electrons in total
  3. Neither, because a bond between two nonmetals has no polarity
  4. S, because its lower electronegativity lets it keep more of the pair
Show the answer

O (3.4) is more electronegative than S (2.6). The shared electrons spend more time near O, so O is δ− and S is δ+.

  • Correct: O, because its larger electronegativity pulls the shared pair toward it: Right: the electron density shifts toward the more electronegative atom.
  • S, because it is the larger atom and holds more electrons in total: Total electron count does not decide where the shared pair sits; the pull of each nucleus on the bonding electrons does, and O pulls harder.
  • Neither, because a bond between two nonmetals has no polarity: Two nonmetals share electrons, but they share them unequally unless their electronegativities match. S and O differ by 0.8.
  • S, because its lower electronegativity lets it keep more of the pair: Lower electronegativity means a weaker pull on shared electrons, so S ends up with less of the electron density, not more.

Particle view

Three samples at the particle level

Box 1Box 2Box 3+−+−+−+−+−+−+−+−+−+−++++++++++++++++++++Key: blue + = cation, yellow − = anion, gray + = metal cation, blue dots = electrons, green = nonmetal atom

4. Which box best represents solid potassium chloride, KCl?

  1. Box 3
  2. Box 2
  3. Box 1
  4. Box 2 or Box 3
Show the answer

KCl is a metal with a nonmetal, so it is ionic: K⁺ cations and Cl⁻ anions held in a regular pattern, each ion next to ions of opposite charge. That is Box 1.

  • Box 3: Box 3 shows separate units of two identical atoms sharing electrons, which is a covalent element such as Cl₂, not an ionic compound.
  • Box 2: Box 2 shows identical metal cations among free electrons, which is a metal. KCl has no free electrons.
  • Correct: Box 1: Right: alternating cations and anions.
  • Box 2 or Box 3: Neither picture has both cations and anions; an ionic compound must show both kinds of ion.

5. Which box best represents a sample of solid sodium metal?

  1. Box 3
  2. Box 1
  3. Box 2
  4. Box 1 or Box 2
Show the answer

In a metal, the atoms give up their valence electrons to a shared sea, leaving cations in a regular pattern with the electrons moving among them. Box 2 shows exactly that.

  • Box 3: Box 3 shows separate two-atom units of a nonmetal. Sodium atoms do not pair up into molecules.
  • Box 1: Box 1 has anions as well as cations. Pure sodium has no anions, because every atom is the same element.
  • Correct: Box 2: Right: metal cations in a sea of electrons.
  • Box 1 or Box 2: Box 1 shows an ionic compound; only Box 2 fits a pure metal.

6. Which compound is held together by ionic bonds?

  1. NF₃
  2. CaF₂
  3. CS₂
  4. SiCl₄
Show the answer

Calcium is a metal and fluorine a nonmetal with a very different electronegativity, so Ca gives up electrons and the compound is made of Ca²⁺ and F⁻ ions.

  • NF₃: N and F are both nonmetals, so they share electrons in polar covalent bonds.
  • Correct: CaF₂: Right: a metal with a nonmetal.
  • CS₂: C and S are both nonmetals with almost the same electronegativity; the bonds are covalent.
  • SiCl₄: Si is a metalloid, not a metal, and Cl is a nonmetal; they share electrons, so SiCl₄ has polar covalent bonds.

7. Which description of the dipole arrow for an H–F bond is correct?

  1. The arrow points from F to H, because H gives up electron density
  2. The arrow points from F to H, with the crossed tail at the F end
  3. No arrow is drawn, because H and F are both nonmetals
  4. The arrow points from H to F, with the crossed tail at the H end
Show the answer

A dipole arrow points toward the more electronegative atom, where the electrons spend more time (δ−). The crossed tail, like a plus sign, sits at the δ+ end. F is far more electronegative than H.

  • The arrow points from F to H, because H gives up electron density: H does give up electron density, which is exactly why the arrow points away from it, toward F.
  • The arrow points from F to H, with the crossed tail at the F end: That drawing points toward H, which would mean H is δ−. F is the more electronegative atom.
  • No arrow is drawn, because H and F are both nonmetals: Two different nonmetals still share unequally; H–F differs by 1.8, one of the most polar covalent bonds.
  • Correct: The arrow points from H to F, with the crossed tail at the H end: Right: toward F, plus-shaped tail at H.

Part 9 · Summary

Summary

Atoms bond because nuclei attract the valence electrons of neighboring atoms. Equal sharing gives a nonpolar covalent bond, unequal sharing a polar covalent bond with partial charges, and transfer from a metal to a nonmetal an ionic bond between oppositely charged ions. Metal atoms pool their valence electrons into a sea of delocalized electrons around the cations. The electronegativity difference sets where a bond falls on this continuum.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections