English:Chemical Bonds
Chemical Bonds
Introduction
Chemical bonds are the attractions that hold atoms or ions together in molecules, crystals, metals, and many other structures. Bonding helps explain why table salt forms crystals, why copper conducts electricity, why water behaves differently from oxygen gas, and why diamond and graphite can have very different properties even though both are made of carbon.
At this level, you can understand much of bonding by focusing on valence electrons, the electrons in the outermost occupied energy level of an atom. You will use simple particle models to compare ionic, covalent, and metallic bonding. These models are useful, but real chemical bonding is explained more fully by quantum mechanics.
Learning Goals
By the end of this aiMOOC, you should be able to:
- Valence electrons: Explain why outer electrons are especially important in bonding.
- Ions: Distinguish between cations and anions and connect their charges to electron loss or gain.
- Ionic bonding: Explain electron transfer and the attraction between oppositely charged ions.
- Covalent bonding: Explain how atoms share electron pairs.
- Metallic bonding: Describe a lattice of positive metal ions attracted to delocalized electrons.
- Properties: Use bonding and structure to predict conductivity, melting behavior, hardness, and flexibility.
- Chemical reactions: Explain that breaking bonds requires energy and forming bonds releases energy.
Atoms, Electrons, and Stability
An atom has a tiny positively charged nucleus surrounded by negatively charged electrons. Electrons occupy energy levels. The electrons furthest from the nucleus are called valence electrons. They are especially important because they are the electrons most involved when atoms bond.
Simple shell diagrams can help you count valence electrons. Sodium has one valence electron in its third shell, while chlorine has seven. In an introductory model, this makes transfer of one electron from sodium to chlorine a useful way to explain the ions found in sodium chloride.
The Octet Rule as a Model
For many main-group atoms, a filled outer shell is a useful clue to stable bonding patterns. The octet rule says that many atoms tend to form bonds so that eight electrons are associated with their valence shell. Hydrogen is a common exception because its first shell is full with two electrons. The octet rule is a helpful school-level model, not a universal law: many real compounds do not follow it exactly.
A deeper idea is energy. A bond can form when the bonded arrangement is lower in energy than the separated atoms. Breaking a chemical bond requires energy; forming a chemical bond releases energy. Whether a whole reaction releases or absorbs energy depends on the balance between all the bonds broken and all the bonds formed.
Ionic Bonding
Ionic bonding is the electrostatic attraction between oppositely charged ions. In many simple examples, a metal atom loses one or more electrons and a nonmetal atom gains them. Losing electrons produces a positive ion called a cation; gaining electrons produces a negative ion called an anion.
In sodium chloride, sodium atoms form Na+ ions and chlorine atoms form Cl− ions. The electron transfer creates charged particles, but the ionic bond itself is the attraction between those opposite charges.
In a solid ionic compound, the ions usually form a large repeating three-dimensional arrangement called a crystal lattice. Sodium chloride therefore is not best pictured as separate little NaCl molecules in the solid. Each ion is surrounded by ions of opposite charge in an extended structure.
Typical Properties of Ionic Substances
Many ionic substances have high melting points because strong electrostatic attractions act throughout the lattice. They are often hard but brittle: if layers of ions shift so that like charges line up, repulsion can help split the crystal.
A solid ionic substance usually does not conduct electricity because its ions cannot move freely. When an ionic substance is molten, or when it dissolves in water and forms mobile ions, it can conduct electricity. These are patterns, not perfect rules for every substance.
Covalent Bonding
Covalent bonding forms when atoms share pairs of electrons. It is especially common between nonmetal atoms. A shared electron pair is attracted to both nuclei and can help hold the atoms together.
One shared pair is called a single bond. Two shared pairs form a double bond, and three shared pairs form a triple bond. Lewis structures use dots or lines to show valence electrons and shared pairs.
Molecules and Giant Covalent Structures
Many covalent substances consist of separate molecules. Examples include H2, O2, H2O, and CO2. The covalent bonds inside each molecule can be strong, while the attractions between separate molecules may be much weaker.
Other covalent substances form giant networks rather than small molecules. Diamond and Graphite are both made only of carbon atoms, but their atoms are connected differently. In diamond, each carbon is strongly bonded in a three-dimensional network. In graphite, carbon atoms form strong layers, while the layers can slide past one another more easily. Graphite also has mobile delocalized electrons within its layers, so it can conduct electricity.
Polar Covalent Bonds
Sometimes a shared electron pair is not shared equally. An atom with a stronger attraction for bonding electrons pulls the shared electrons closer. This creates a polar covalent bond with small partial charges.
Water contains polar O–H bonds. Oxygen attracts the shared electrons more strongly than hydrogen. Because the water molecule is bent, these bond polarities do not cancel, so the whole molecule is polar.
Metallic Bonding
Metallic bonding occurs in metals. A useful model describes a regular lattice of positive metal ions surrounded by delocalized electrons. These electrons are not tied to one single atom or one single bond; they can move through the structure.
The attraction between the positive ions and the delocalized electrons holds the metal together. The mobile electrons help explain why metals conduct electricity and thermal energy. Metals can also be malleable and ductile because layers of ions can shift while the attraction to the electron cloud remains.
Comparing the Main Bonding Types
| Bonding type | Main particle picture | What happens to valence electrons | Typical structure | Typical electrical behavior |
|---|---|---|---|---|
| Ionic | Positive and negative ions | Electrons are transferred in the formation of ions | Giant ionic lattice | Conducts when ions can move, such as when molten or dissolved |
| Covalent | Atoms joined by shared electron pairs | Electrons are shared | Small molecules or giant covalent networks | Simple molecular substances usually do not conduct; some giant structures such as graphite do |
| Metallic | Positive metal ions and delocalized electrons | Outer electrons are delocalized through the structure | Giant metallic lattice | Usually conducts well as a solid and as a liquid |
The bonding type is only part of the story. A material's properties also depend on its full structure, the particles present, and how those particles are arranged.
From Bonding to Observable Properties
Bonding models are valuable because they connect tiny particles to properties you can observe.
Table salt: Sodium chloride has an ionic lattice. It forms crystals, has a relatively high melting point, and can conduct when molten or dissolved because charged ions can move.
Water: Each water molecule contains covalent bonds. Its bent shape and polar bonds make the molecule polar, which strongly influences how water interacts with ions and other polar molecules.
Copper: Metallic bonding gives copper mobile electrons and a structure that can be drawn into wires. This helps explain its high electrical conductivity and ductility.
Diamond and graphite: Both are forms of carbon with covalent bonding, but their different structures lead to very different hardness and electrical behavior. This is strong evidence that structure and bonding together determine material properties.
Models, Evidence, and Common Misconceptions
Lewis and Particle Models
Lewis structures, electron-shell diagrams, and ball-and-stick models are representations. They highlight useful ideas but leave out details. Electrons are not tiny planets traveling on fixed circular tracks, and a line in a Lewis structure is not a physical stick. Scientific models are tools for thinking and predicting.
Bonding Is Not the Same as Intermolecular Attraction
A chemical bond holds atoms together within a molecule or extended structure. Intermolecular forces act between separate molecules. For example, the O–H covalent bonds inside a water molecule are different from the hydrogen-bond attractions that can occur between water molecules.
Breaking Bonds Does Not Release Energy
A common misconception is that breaking a bond releases energy. In fact, energy must be supplied to break a bond. Energy is released when new bonds form. An exothermic reaction releases energy overall when forming new bonds releases more energy than is required to break the original bonds.
Key Vocabulary
| Term | Meaning |
|---|---|
| Valence electron | An electron in the outermost occupied energy level of an atom that can participate in bonding |
| Ion | An atom or group of atoms with an overall electric charge |
| Cation | A positively charged ion |
| Anion | A negatively charged ion |
| Ionic bond | Electrostatic attraction between oppositely charged ions |
| Covalent bond | A bond involving shared electron pairs between atoms |
| Metallic bond | Attraction between positive metal ions and delocalized electrons |
| Lattice | A regular repeating arrangement of particles |
| Delocalized electron | An electron spread through a structure rather than assigned to one atom or one bond |
| Polarity | Uneven distribution of electric charge in a bond or molecule |
Interactive Tasks
Quiz: Test Your Knowledge
Which electrons are most directly involved in ordinary chemical bonding? (Valence electrons) (!Core electrons) (!Neutrons) (!Protons)
What happens when a neutral atom loses an electron? (It becomes a positive ion) (!It becomes a negative ion) (!It becomes a neutron) (!It loses its nucleus)
What is an ionic bond? (Electrostatic attraction between oppositely charged ions) (!Equal sharing of neutrons) (!A force that joins only identical atoms) (!Attraction between two neutral metal nuclei)
What happens to electrons in a covalent bond? (They are shared between atoms) (!They are changed into protons) (!They disappear from the atoms) (!They remain only in the nucleus)
Why can molten sodium chloride conduct electricity? (Its ions can move) (!Its neutrons can move) (!Its crystal lattice becomes metallic) (!Its electrons turn into ions)
Which model best describes metallic bonding? (Positive metal ions attracted to delocalized electrons) (!Negative metal ions sharing protons) (!Separate molecules held only by gravity) (!Neutral atoms with no electrons)
What must happen when a chemical bond is broken? (Energy must be supplied) (!Energy must always be released) (!A proton must leave the nucleus) (!The atom must become radioactive)
Why are diamond and graphite different even though both contain only carbon? (Their atoms are arranged and bonded differently) (!They contain different kinds of protons) (!Graphite contains no electrons) (!Diamond is an ionic compound)
What does a single covalent bond represent in a Lewis structure? (One shared pair of electrons) (!One transferred proton) (!One pair of neutrons) (!One positive ion)
Why is water a polar molecule? (Its polar bonds and bent shape create an uneven charge distribution) (!Its oxygen atom has no electrons) (!Its hydrogen atoms are negative ions) (!Its covalent bonds contain no shared electrons)
Memory Game
| Valence electron | An electron in the outermost occupied energy level that can take part in bonding |
| Cation | A positively charged ion |
| Anion | A negatively charged ion |
| Ionic bond | Attraction between oppositely charged ions |
| Covalent bond | Attraction associated with a shared pair of electrons between atoms |
| Delocalized electron | An electron that can move through a larger structure instead of belonging to one atom |
Drag and Drop
| Match the correct terms. | Topic |
|---|---|
| Electron transfer | Ionic bonding |
| Electron pair sharing | Covalent bonding |
| Delocalized electrons | Metallic bonding |
| Oppositely charged ions | Ionic lattice |
| Unequal sharing | Polar covalent bond |
Match each process or particle description to the bonding idea that best fits it.
Crossword Puzzle
| Valence | What word describes electrons in the outermost occupied shell that can take part in bonding? |
| Cation | What is a positively charged ion called? |
| Anion | What is a negatively charged ion called? |
| Covalent | What kind of bond forms when atoms share electron pairs? |
| Lattice | What one-word term describes a regular repeating arrangement of particles in a crystal? |
| Metallic | What kind of bonding includes delocalized electrons moving through a metal structure? |
LearningApps
Cloze Text
Open-Ended Tasks
Easy
- Bond Spotter: Make a photo collage or drawing of at least six everyday materials and predict whether ionic, covalent, or metallic bonding is most important in each; add one sentence of evidence for every prediction.
- Lewis Structure Card: Create a clear study card showing simple electron-dot or line models for H2, Cl2, and H2O, and explain what each shared pair means.
- Property Sort: Sort a teacher-provided set of substance cards by conductivity, brittleness, melting behavior, or flexibility, then connect each group to a possible bonding model.
- Teach Chemical Bonds in Sixty Seconds: Produce a one-minute audio or video explanation that correctly compares electron transfer, electron sharing, and delocalized electrons.
Standard
- Crystal Lattice Model: Build a two-color model of a sodium chloride lattice, photograph it, and explain why each ion is attracted to ions of the opposite charge in several directions.
- Conductivity Investigation: Under teacher supervision, use only a teacher-approved low-voltage conductivity tester to compare safe samples such as a metal, solid salt, salt solution, sugar solution, and distilled water; record results and explain them with particle mobility.
- Materials Expert Interview: Interview a science teacher, technician, engineer, electrician, jeweler, or materials worker about how conductivity, hardness, brittleness, or flexibility matters in their work, then connect one answer to chemical bonding.
- Diamond and Graphite Visual Explanation: Create a poster, infographic, or narrated slide showing how different carbon structures can lead to different hardness and electrical behavior.
Advanced
- Unknown Solid Challenge: Use a set of property data for several unknown solids to infer the most likely bonding type for each and write a claim-evidence-reasoning explanation.
- Design a Material: Invent a material for a specific job such as an electrical wire, heat-resistant cutting surface, soluble tablet, or flexible metal sheet, and justify which bonding and structure would best fit the required properties.
- Model Limitations Investigation: Compare electron-shell diagrams, Lewis structures, and ball-and-stick models; create a short report explaining what each model shows well and what it leaves out.
- Bond Energy Storyboard: Create a storyboard for a chemical reaction showing that breaking old bonds requires energy while forming new bonds releases energy, then explain how the balance can make a reaction exothermic or endothermic.
Learning Assessment
- Bonding from Evidence: Given melting point, conductivity, solubility, and brittleness data for an unknown substance, identify the most likely bonding type and justify your conclusion using at least three pieces of evidence.
- Salt and Sugar Comparison: Explain why solid salt, salt solution, and sugar solution can show different electrical behavior even when both salt and sugar may dissolve in water.
- Copper Wire Reasoning: Use the metallic bonding model to explain why copper can conduct electricity and be drawn into wire without immediately breaking apart.
- Carbon Structure Transfer: Compare diamond and graphite and explain how two substances made of the same element can have different properties because of structure and electron behavior.
- Water Polarity Reasoning: Explain how unequal electron sharing and molecular shape work together to make water polar, then predict how this can affect attraction to ions.
- Reaction Energy Explanation: Correct the statement “breaking bonds releases energy” and use bond breaking and bond formation to explain why a reaction may release energy overall.
Evidence of Learning
Knowledge: You can define valence electrons, ions, ionic bonding, covalent bonding, metallic bonding, lattices, delocalized electrons, and polarity in your own words.
Skills: You can read simple bonding diagrams, compare models, interpret property data, build evidence-based explanations, and connect particle behavior to macroscopic properties.
Products: Useful evidence can include a Lewis-structure study card, a lattice model, an experiment record, a poster or infographic, an interview summary, a short explanatory video, and a claim-evidence-reasoning response.
Transfer: You can apply bonding ideas to unfamiliar materials, predict likely properties from a structural model, identify limitations of simplified diagrams, and reason about energy changes when bonds break and form.
OERs on the Topic
IUPAC Gold Book: chemical bond
OpenStax Chemistry 2e: Ionic Bonding
OpenStax Chemistry 2e: Covalent Bonding
Wikimedia Commons: Chemical bonding media
Linked Learning Areas