Difficulty: Introductory | Prerequisites: General chemistry basics, periodic table familiarity
This chapter sits right at the start of organic chemistry and covers the two fundamental ways atoms bond: by transferring electrons (ionic) or by sharing them (covalent). If you are coming in cold, you need to know what valence electrons are and have a rough sense of where metals and nonmetals sit on the periodic table. Everything in organic chemistry builds on understanding how and why atoms form bonds, so this is foundational.
Atoms bond in two main ways: ionic bonding transfers electrons entirely from one atom to another, creating oppositely charged ions that attract; covalent bonding shares electrons between atoms, usually nonmetals with similar electronegativities. Polarity describes how unevenly those shared electrons are distributed, and it depends on electronegativity differences and molecular symmetry. The octet rule drives both types of bonding, as atoms seek a full set of eight valence electrons (a noble gas configuration).
Octet rule
The principle that atoms tend to gain, lose or share electrons until they have eight valence electrons, matching the electron configuration of the nearest noble gas. Think of it as: atoms "want" a full outer shell, and bonding is how they get there.
Ionic bond
A bond formed by the complete transfer of one or more valence electrons from one atom (typically a metal) to another (typically a nonmetal), producing oppositely charged ions held together by electrostatic attraction. In simple terms, one atom hands over its electrons and both end up charged.
Covalent bond
A bond formed when two atoms share one or more pairs of electrons, usually between nonmetals with similar electronegativities. Think of it as: instead of giving electrons away, atoms hold on to them together.
Cation
A positively charged ion formed when an atom loses one or more electrons. In simple terms, it is the atom that gave electrons away and now has more protons than electrons.
Anion
A negatively charged ion formed when an atom gains one or more electrons. In simple terms, it is the atom that accepted extra electrons and now carries a negative charge.
Valence electrons
The electrons in the outermost shell of an atom, which participate in bonding. These are the electrons that matter for chemical reactions.
Electronegativity
A measure of an atom's ability to attract shared electrons towards itself in a covalent bond. Think of it as: how greedy an atom is for electrons. Higher electronegativity means a stronger pull.
Polarity
A measure of the separation of charge within a compound, arising when electrons in a covalent bond are shared unequally. In simple terms, a polar bond has a slight positive end and a slight negative end because one atom hogs the electrons more than the other.
Tags: ionic bond, electron transfer, cation, anion, electrostatic attraction, NaCl, noble gas configuration
Ionic bonding involves the complete transfer of valence electrons from one atom to another
This creates two oppositely charged ions that are held together by electrostatic force
Electron donor and acceptor roles:
The metal loses electrons (electron donor) and becomes a cation (positive charge)
The nonmetal accepts electrons (electron acceptor) and becomes an anion (negative charge)
Driving force: atoms transfer electrons to achieve a noble gas configuration (full octet)
Energy considerations:
The predicted overall energy of electron transfer alone is usually positive (endothermic, unfavourable)
The actual reaction is favourable because the electrostatic attraction between the resulting ions releases enough energy to more than compensate
Example: NaCl formation
Sodium (Na) loses one electron to become Na+
Chlorine (Cl) gains that electron to become Cl-
The electrostatic attraction between Na+ and Cl- holds the compound together
Tags: covalent bond, electron sharing, electronegativity, single bond, double bond, triple bond, quadruple bond, PCl3
Covalent bonding involves the sharing of electron pairs between atoms
Sharing is rarely equal; the more electronegative atom pulls the shared electrons closer to itself
Where it occurs:
Normally between nonmetals
Can also occur between metals and nonmetals in some cases
Occurs between atoms of similar electronegativities
Bond multiplicity:
Single bond: one shared pair of electrons
Double bond: two shared pairs
Triple bond: three shared pairs
Quadruple bond: four shared pairs (rare, mainly in transition metal chemistry)
Example: PCl3 (phosphorus trichloride)
Phosphorus shares one electron with each of three chlorine atoms
Each P-Cl bond is a single covalent bond
Phosphorus ends up with a lone pair and three bonding pairs
Tags: polarity, polar bond, nonpolar bond, electronegativity difference, charge separation, molecular symmetry
Polarity is a measure of the separation of charge within a compound
Polarity depends on:
The difference in electronegativity between the bonded atoms
The symmetry of the molecule (a symmetric arrangement can cancel out individual bond polarities)
Polarity arises when electrons in a covalent bond are shared unequally
The bonding spectrum:
One extreme: ionic bonds (complete electron transfer, maximum charge separation)
Middle: polar covalent bonds (unequal sharing)
Other extreme: nonpolar covalent bonds (equal or near-equal sharing, as in diatomic elements like I2)
Bonding is not a strict either/or; it sits on a continuum from fully ionic to fully nonpolar covalent
Identifying bond type in compounds:
CH4: covalent (carbon and hydrogen are both nonmetals with similar electronegativities)
Fe2O3: ionic (metal + nonmetal with large electronegativity difference)
KNO3: ionic (K+ paired with the covalently bonded nitrate ion NO3-)
I2: nonpolar covalent (identical atoms, equal sharing)
C4: covalent (same element)
H2O: polar covalent (oxygen is more electronegative than hydrogen)
BeCl2: covalent with ionic character (beryllium is a metal, but the electronegativity difference is moderate)
NaCl formation (ionic bond)
Na + Cl → Na+ + Cl-. Sodium donates its single valence electron to chlorine. Both atoms achieve a noble gas configuration. The resulting ions are held together by electrostatic attraction.
PCl3 formation (covalent bond)
Phosphorus shares one electron with each of three chlorine atoms, forming three single covalent bonds. Each chlorine retains three lone pairs; phosphorus retains one lone pair.
The bonding spectrum
Bonding type exists on a continuum. At one extreme sit ionic bonds (complete electron transfer). At the other sit nonpolar covalent bonds (equal sharing between identical atoms). Polar covalent bonds fall in the middle, with unequal sharing driven by electronegativity differences.
Ionic compounds like NaCl (table salt) dissolve in water and conduct electricity because their ions dissociate and move freely, which is the basis of electrolyte chemistry in batteries and biological systems. Covalent bonding is what holds together virtually every organic molecule in your body, from the sugars you metabolise to the DNA that carries your genetic code. Understanding polarity is what lets chemists predict whether substances will dissolve in water (polar solvents dissolve polar solutes), which underpins everything from drug design to paint formulation.
Students often think ionic bonds are "weaker" than covalent bonds. In fact, ionic compounds typically have very high melting points because of the strong electrostatic forces in their crystal lattices.
Students assume that if a compound contains a metal, the bonding must be entirely ionic. Some metal-nonmetal bonds (like Be-Cl in BeCl2) have significant covalent character due to a moderate electronegativity difference.
Students frequently forget that polyatomic ions (like NO3- in KNO3) contain covalent bonds within the ion, even though the compound as a whole is ionic. A compound can be ionic overall while having covalent bonding inside its ions.
Students think electron sharing in covalent bonds is always equal. It rarely is. Unless the two atoms are identical (as in I2 or O2), one atom will pull the shared electrons more strongly, creating polarity.
⚠️ Being able to classify a compound as ionic or covalent by inspecting its formula is a bread-and-butter exam skill. Know the metal + nonmetal = ionic shortcut, but also know its exceptions.
⚠️ Expect questions that give you a list of compounds and ask you to sort them by bond type. The examples from the source notes (CH4, Fe2O3, KNO3, I2, H2O, BeCl2) are representative of what appears on exams.
⚠️ The energy paradox in ionic bonding (endothermic electron transfer, but exothermic overall due to lattice energy / electrostatic attraction) is a common conceptual exam question.
⚠️ Identifying bond types within a reaction (distinguishing ionic from covalent reactants and products) is tested directly. The source notes include worked examples of this with Na2CO3 + CaCl2 and sodium amide reactions.
True or False: Ionic bonding involves the sharing of electrons between atoms. (False, it involves complete transfer.)
Fill in the blank: An atom that loses electrons becomes a ______ and carries a ______ charge. (Cation, positive.)
True or False: Covalent bonds can only be single bonds. (False, they can be single, double, triple, or quadruple.)
Fill in the blank: Polarity arises when electrons are shared ______ between two atoms. (Unequally.)
True or False: The predicted energy of ionic electron transfer alone is favourable. (False, it is endothermic and unfavourable on its own; electrostatic attraction makes the overall reaction favourable.)
Q: Classify each of the following as ionic or covalent: CH4, Fe2O3, KNO3, I2, H2O, BeCl2.
A: CH4 is covalent (two nonmetals). Fe2O3 is ionic (metal + nonmetal). KNO3 is ionic overall (K+ ion paired with the covalent polyatomic ion NO3-). I2 is nonpolar covalent (identical atoms). H2O is polar covalent (nonmetals with different electronegativities). BeCl2 is covalent with some ionic character.
Q: In the reaction Na2CO3 + CaCl2 → CaCO3 + 2NaCl, which compounds are ionic and which contain covalent bonds?
A: All four compounds are ionic (metal cations paired with anions). However, the carbonate ion (CO3 2-) within Na2CO3 and CaCO3 contains covalent bonds between carbon and oxygen.
Q: Why is the overall formation of NaCl energetically favourable, even though electron transfer alone is endothermic?
A: The electrostatic attraction between the Na+ and Cl- ions releases a large amount of energy (lattice energy), which more than compensates for the endothermic cost of the electron transfer itself.
Q: What determines whether a covalent bond is polar or nonpolar?
A: The difference in electronegativity between the two bonded atoms. If the atoms have similar electronegativities, the bond is nonpolar. If one atom is significantly more electronegative, the bond is polar. Molecular symmetry also matters at the whole-molecule level.
Q: What is the nature of the bond between sodium and amide (NH2-)?
A: The bond between sodium (Na+) and the amide ion (NH2-) is ionic. Sodium has donated an electron to become Na+, and the amide ion carries a negative charge. Within the amide ion itself, the N-H bonds are covalent.
This connects to Lewis structures (Chapter 1.3+), where you will draw out the exact arrangement of bonding and lone pair electrons for covalent molecules. Understanding polarity here feeds directly into molecular geometry and VSEPR theory, since the shape of a molecule determines whether individual bond dipoles cancel or add up. The ionic vs covalent distinction also matters for understanding reaction mechanisms later in organic chemistry: nucleophiles and electrophiles behave the way they do because of how electrons are distributed in bonds.
Ionic bond, covalent bond, octet rule, electron transfer, electron sharing, cation, anion, electronegativity, polarity, polar covalent, nonpolar covalent, noble gas configuration, valence electrons, electrostatic attraction, lattice energy, bond multiplicity, single bond, double bond, triple bond, polyatomic ion, Lewis dot structure, NaCl, PCl3, bonding in organic chemistry, Purdue organic chemistry, Chapter 1.2 bonding notes