Why do atoms bond, and what decides whether a bond is ionic, covalent or metallic?
Types of chemical bonds: explain ionic, covalent and metallic bonding in terms of valence electrons and electronegativity, and predict bond type from the periodic table.
A focused Virginia SOL Chemistry answer on bonding under CH.3: why atoms bond to reach a stable octet, how ionic, covalent and metallic bonds form, and how to predict the bond type from electronegativity difference and position on the periodic table.
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What this topic is asking
Standard CH.3 begins with chemical bonding. Virginia expects you to explain why atoms bond, to describe ionic, covalent and metallic bonds in terms of valence electrons, and to predict the bond type from the electronegativity difference and the elements' positions on the periodic table. Bonding is the bridge from atomic structure to the properties of compounds.
Why atoms bond
Valence electrons are the currency of bonding. Metals have few valence electrons and lose them easily; nonmetals are short of a full octet and gain or share to complete it. The electronegativity difference between two atoms decides which mechanism operates.
Ionic bonding
Sodium chloride is the classic example: sodium loses its one valence electron to become , chlorine gains it to become , and the ions attract in a lattice. Ionic compounds have high melting points, are brittle, and conduct electricity when molten or dissolved (the ions are then free to move) but not as solids.
Covalent bonding
A covalent bond forms when two nonmetals share a pair of electrons so that each atom counts the shared pair toward its octet. If the two atoms have equal electronegativity (the same element, as in ), the sharing is equal and the bond is nonpolar covalent. If one atom pulls harder (as in ), the sharing is unequal and the bond is polar covalent, with a partial negative charge on the more electronegative atom and a partial positive charge on the other. Molecular (covalent) substances tend to have lower melting points than ionic compounds and usually do not conduct electricity.
Metallic bonding
Predicting bond type
The electronegativity difference is the quickest predictor: roughly to is nonpolar covalent, about to is polar covalent, and above about is ionic. A faster shortcut from the periodic table: metal plus nonmetal is usually ionic, nonmetal plus nonmetal is covalent, and metal plus metal is metallic.
Try this
Q1. What type of bond holds the atoms together in solid copper? [1 point]
- Cue. A metallic bond, with copper cations in a sea of delocalized electrons.
Q2. Explain why sodium chloride conducts electricity when molten but not when solid. [1 point]
- Cue. When molten the ions are free to move and carry charge; in the solid the ions are locked in the lattice and cannot move.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SOL (multiple choice)1 marksWhich type of bond forms between a metal and a nonmetal when electrons are transferred? (A) nonpolar covalent (B) polar covalent (C) ionic (D) metallicShow worked answer →
The answer is (C) ionic.
An ionic bond forms when electrons are transferred from a metal (low electronegativity, loses electrons to form a cation) to a nonmetal (high electronegativity, gains electrons to form an anion). The oppositely charged ions then attract. A metal with a nonmetal and a large electronegativity difference signals an ionic bond. Covalent bonds (A and B) share electrons between nonmetals, and a metallic bond (D) is between metal atoms.
The trap is confusing transfer with sharing; transfer of electrons gives an ionic bond.
SOL (tech-enhanced, drag and drop)3 marksClassify each pair as forming an ionic, polar covalent, or nonpolar covalent bond: (a) Na and Cl (electronegativity difference ); (b) H and Cl (difference ); (c) Cl and Cl (difference ).Show worked answer →
A 3-point classification item using electronegativity difference.
(a) Na and Cl: difference is large, so the bond is ionic (1 point).
(b) H and Cl: difference is moderate, so the bond is polar covalent (1 point).
(c) Cl and Cl: difference means electrons are shared equally, so the bond is nonpolar covalent (1 point).
Markers reward using the electronegativity difference: roughly to is nonpolar covalent, about to is polar covalent, and above about is ionic. The boundary values are guidelines, not hard cutoffs.
Related dot points
- Lewis structures and molecular geometry: draw electron-dot (Lewis) structures for simple molecules and use VSEPR to predict molecular shapes.
A focused Virginia SOL Chemistry answer on structure under CH.3: drawing electron-dot (Lewis) structures for simple molecules, counting bonding and lone pairs, and using VSEPR to predict shapes such as linear, bent, trigonal planar and tetrahedral.
- Polarity and intermolecular forces: determine molecular polarity from shape and bond polarity, and compare dispersion, dipole-dipole and hydrogen-bonding forces and their effect on properties.
A focused Virginia SOL Chemistry answer on polarity under CH.3: how bond polarity and molecular shape combine to make a molecule polar or nonpolar, the three intermolecular forces (dispersion, dipole-dipole, hydrogen bonding), and how they set boiling and melting points and solubility.
- Naming compounds and writing formulas: name and write formulas for ionic compounds (including polyatomic ions), binary molecular compounds and simple acids.
A focused Virginia SOL Chemistry answer on nomenclature under CH.3: writing formulas for ionic compounds by balancing charges (the crossover method), using polyatomic ions and roman numerals, and naming binary molecular compounds with prefixes and simple acids.
- The periodic table and periodic trends: describe the organization of the periodic table and the trends in atomic radius, ionization energy, electronegativity and reactivity across periods and down groups.
A focused Virginia SOL Chemistry answer on the periodic table under CH.2: how it is organized into groups, periods, metals, nonmetals and metalloids, and the trends in atomic radius, ionization energy, electronegativity and reactivity and why each runs the way it does.
- Electron configuration and energy levels: describe how electrons occupy energy levels, write electron configurations, identify valence electrons, and relate ground and excited states to spectra.
A focused Virginia SOL Chemistry answer on electron arrangement under CH.2: energy levels and sublevels, writing electron configurations, counting valence electrons, and the difference between ground state and excited state and how it produces line spectra.
Sources & how we know this
- 2018 Science Standards of Learning - Chemistry — Virginia Department of Education (2018)
- Chemistry Curriculum Framework — Virginia Department of Education (2018)