5.2 Chemical Bonding & Reactions

Atoms rarely exist alone — most matter is held together by chemical bonds, and most of the practical chemistry tested on the NAPT (and used aboard ship in batteries, corrosion control, fuel combustion, and water treatment) comes down to two questions: how do atoms bond, and what happens when compounds react? This section covers ionic vs. covalent bonding, how to predict bond type from electronegativity and periodic-table position, the five common reaction types, and how to balance chemical equations with fully worked examples.

Ionic vs. Covalent Bonding

A chemical bond is the force that holds atoms together in a compound, arising from interactions between valence electrons (see Section 5.1). There are two main types:

• Ionic bond — one or more electrons transfer completely from a metal atom (which gives electrons up easily) to a nonmetal atom (which readily accepts them). The result is a positively charged cation and a negatively charged anion that attract each other electrostatically. In table salt, sodium chloride (NaCl), sodium's lone valence electron transfers to chlorine: Na becomes Na⁺, Cl becomes Cl⁻, and the two ions lock into a rigid crystal lattice — which is why ionic compounds tend to be hard, brittle solids with high melting points.
• Covalent bond — two nonmetal atoms share one or more pairs of electrons instead of transferring them outright, with each atom counting the shared pair toward its own stable octet. In water (H2O), oxygen shares one electron pair with each hydrogen atom, forming two O-H covalent bonds.

Covalent bonds are further split by how evenly the electrons are shared:

Predicting Bond Type from Electronegativity

Electronegativity measures how strongly an atom pulls shared electrons toward itself in a bond. On the common Pauling scale, values range from about 0.7 (cesium and francium, the least electronegative) to 3.98 (fluorine, the most electronegative element on the table). A handful of values worth knowing: H = 2.20, C = 2.55, N = 3.04, O = 3.44, F = 3.98, Na = 0.93, Cl = 3.16, K = 0.82, Ca = 1.00.

To predict bond type, calculate the electronegativity difference (ΔEN) between the two bonded atoms and compare it to this rule of thumb:

Worked examples:

• NaCl: ΔEN = 3.16 - 0.93 = 2.23 → well above 1.7 → ionic.
• HCl: ΔEN = 3.16 - 2.20 = 0.96 → between 0.4 and 1.7 → polar covalent.
• Cl2: ΔEN = 3.16 - 3.16 = 0 → nonpolar covalent (identical atoms always share equally).

If you don't have exact electronegativity numbers memorized, a reliable shortcut is periodic-table position: a metal bonded to a nonmetal is (almost always) ionic; two nonmetals bonded together are covalent — polar if they're different elements, nonpolar if they're identical.

Common Reaction Types

Most chemical reactions you'll see on the NAPT fall into five recognizable patterns:

A few things worth noticing in these examples:

• The synthesis reaction (making ammonia from nitrogen and hydrogen gas) is the basis of the industrial Haber process.
• The decomposition of hydrogen peroxide (H2O2) into water and oxygen gas is why peroxide bubbles when it contacts a wound — it's decomposing, catalyzed by an enzyme in blood.
• The single replacement reaction between zinc and hydrochloric acid produces visible hydrogen gas bubbles — a classic lab demonstration of one element displacing another from a compound.
• The double replacement reaction between lead nitrate and potassium iodide produces lead iodide, a bright yellow solid that precipitates out of solution — visible proof that a reaction happened, since two soluble compounds swapped partners to form one insoluble one.
• Combustion reactions always produce carbon dioxide and water when a hydrocarbon burns completely in enough oxygen, releasing energy as heat and light.

Balancing Chemical Equations

Balancing enforces the law of conservation of mass: atoms are neither created nor destroyed in an ordinary chemical reaction, so the same number of atoms of each element must appear on both sides of the equation. The rules:

1. Write correct chemical formulas first — never change a subscript to balance an equation (that changes what substance you're describing).
2. Count atoms of each element on both sides.
3. Add whole-number coefficients in front of formulas to balance — usually metals first, then other nonmetals, then hydrogen, then oxygen last.
4. Reduce coefficients to the simplest whole-number ratio.
5. Recheck every element.

Worked Example 1 — Single Replacement: Balance Al + CuSO4 → Al2(SO4)3 + Cu.

• Start: Al + CuSO4 → Al2(SO4)3 + Cu (unbalanced — only 1 Al on the left, 2 on the right).
• Balance aluminum: 2Al + CuSO4 → Al2(SO4)3 + Cu.
• Balance the sulfate group (SO4): the right side needs 3 (inside Al2(SO4)3), so use 3 CuSO4 on the left: 2Al + 3CuSO4 → Al2(SO4)3 + Cu.
• Balance copper: 3 Cu now appear on the left (from 3CuSO4), so put 3 Cu on the right: 2Al + 3CuSO4 → Al2(SO4)3 + 3Cu.
• Check: Al (2 = 2), Cu (3 = 3), S (3 = 3), O (12 = 12). Balanced.

Worked Example 2 — Combustion: Balance C3H8 + O2 → CO2 + H2O.

• Balance carbon first: propane has 3 C atoms, so use 3 CO2: C3H8 + O2 → 3CO2 + H2O.
• Balance hydrogen next: propane has 8 H atoms, so use 4 H2O (4 × 2 = 8 H): C3H8 + O2 → 3CO2 + 4H2O.
• Balance oxygen last: the right side now has (3 × 2) + (4 × 1) = 6 + 4 = 10 oxygen atoms, so use 5 O2 (5 × 2 = 10): C3H8 + 5O2 → 3CO2 + 4H2O.
• Check: C (3 = 3), H (8 = 8), O (10 = 10). Balanced.

Key Takeaways

• Ionic bonds transfer electrons (metal → nonmetal); covalent bonds share electrons (nonmetal + nonmetal), and can be polar or nonpolar depending on electronegativity difference.
• An electronegativity difference (ΔEN) greater than about 1.7 predicts an ionic bond; below that, the bond is covalent (polar above ΔEN ≈ 0.4, nonpolar below it).
• The five core reaction types are synthesis, decomposition, single replacement, double replacement, and combustion — each with a distinct general pattern.
• Balancing an equation means adjusting coefficients only, never subscripts, until every element's atom count matches on both sides.
• The balanced combustion equation C3H8 + 5O2 → 3CO2 + 4H2O demonstrates the standard balancing order: carbon, then hydrogen, then oxygen.