Chemistry Practice Test: Chemical Bonding and Molecular Structure

Sodium, a metal with low ionization energy, loses its single valence electron, and chlorine, which is very electronegative, gains it. The oppositely charged ions attract in an ionic bond. Equal sharing of electrons would be a nonpolar covalent bond.

Carbon and oxygen are both nonmetals, so they share electrons in covalent bonds. KBr, MgO and CaF₂ each combine a metal with a nonmetal, which typically forms ionic bonds.

The total charge must be zero: 2 Al³⁺ give +6 and 3 O²⁻ give −6, so the formula is Al₂O₃. Al₃O₂ copies the charges as subscripts in the wrong places and has a net charge of +5.

Three oxide ions carry −6 in total, so the two iron ions carry +6, which is +3 each. Iron can form more than one ion, so the charge is shown in Roman numerals: iron(III) oxide. Prefixes like "di-" and "tri-" are used for molecular compounds of two nonmetals, not for ionic ones.

Nitrogen has 5 valence electrons. Three are used in the three N-H bonds, and the remaining 2 form one lone pair, which gives nitrogen an octet (3 bonding pairs + 1 lone pair).

Nitrogen has four electron groups (three bonds and one lone pair) arranged roughly tetrahedrally. The lone pair is not counted in the molecular shape, so the atoms form a pyramid with nitrogen at the top. Trigonal planar would require no lone pair, as in BF₃.

Oxygen is more electronegative than carbon, so each bond is polar. But CO₂ is linear (O=C=O), and the two equal dipoles point in opposite directions and cancel. Water, by contrast, is bent, so its dipoles do not cancel.

Bond polarity grows with the difference in electronegativity: H-F has 4.0 − 2.1 = 1.9, H-Cl has 0.9, C-H has 0.4 and H-H has 0. So H-F is the most polar bond.

Hydrogen bonded to a very electronegative atom such as oxygen forms strong hydrogen bonds between molecules, and these must be overcome to boil water. Boiling separates molecules; it does not break the covalent bonds inside them. H₂S is actually the larger molecule.

In metallic bonding, valence electrons move freely among the metal cations. These mobile electrons carry current, and because the bonding is not directional, layers of atoms can slide without breaking the metal. A rigid lattice of positive and negative ions describes ionic solids, which are brittle.

Strong attractions between ions give ionic compounds high melting points. In the solid the ions are locked in place, so it does not conduct; when melted or dissolved, the ions can move and carry current.

Each covalent bond is one shared pair of electrons, so a triple bond shares 3 pairs, which is 6 electrons. The answer 3 counts bonds instead of electrons; 10 is the total number of valence electrons in N₂, including the lone pairs.

Carbon has 4 valence electrons and needs 4 more to complete its octet, so it forms 4 covalent bonds, as in CH₄. A double bond counts as two of these four.

Methane is nonpolar, so its molecules are held together only by weak London dispersion forces. Ammonia, water and hydrogen fluoride all form hydrogen bonds, which are much stronger and raise the boiling point.

Four bonding pairs and no lone pairs spread out as far as possible in three dimensions, giving a tetrahedral shape with angles of 109.5°. The 90° angle comes from drawing the molecule flat on paper, which is not its real shape.