Anatomy and Physiology 2e · The Chemical Level of Organization
Chemical Bonds
On this page 9 sections
In 30 seconds
Atoms rarely exist alone in the body; they combine into molecules and compounds. The forces holding atoms together are chemical bonds, and they form because atoms are more stable with filled outer (valence) electron shells. Most atoms have incomplete shells, so they gain, lose, or share electrons with neighbors — a pattern commonly taught as the Octet rule Atoms tend to gain, lose, or share electrons to reach eight in the outer shell Full entry →: atoms tend to end with eight electrons in their outer shell (hydrogen is the exception, settling for two).
There are two major bond classes. Ionic bonds form when one atom transfers electrons to another, creating oppositely charged ions that attract. Covalent bonds form when atoms share electron pairs, and they can be nonpolar (equal sharing) or polar (unequal sharing). Two weaker interactions also shape life: hydrogen bonds, weak attractions between molecules (not true bonds), and van der Waals interactions, very weak attractions between atoms in close contact. Together these interactions build everything from table salt to DNA, and they explain why water — the body's most important molecule — behaves as it does.
Why this matters
Nearly every physiological process depends on bonds being formed, broken, or rearranged — the subject of the next topic, Chemical Reactions. Water's ability to dissolve salts and cushion tissues comes from its polar covalent bonds and hydrogen bonding. The DNA double helix is held together by hydrogen bonds between its strands. Proteins fold into the precise shapes that let enzymes, antibodies, and muscle proteins work because of weak interactions within the chain. The ions driving nerve signals — sodium, potassium, calcium — are created by the same electron transfer that makes ionic bonds.
In health and disease, bond chemistry is everywhere: electrolyte balance, oxygen transport (oxygen binds covalently to hemoglobin), and the reason fats and oils do not dissolve in water (their bonds are nonpolar). A student who understands bonds can predict which substances dissolve in water, why some molecules are "sticky," and how a protein's shape determines its function.
The college version
Core Concepts
Why atoms bond: the octet rule
An atom's chemical personality is set by its valence electrons — those in the outermost shell. Atoms with full shells (the noble gases) stay alone; everyone else tends to gain, lose, or share electrons to fill the shell, most commonly to eight. This octet rule is a simplified but very useful teaching model (some larger atoms exceed eight; hydrogen and helium follow a "duet" of two). It explains why sodium gives away an electron, why chlorine accepts one, and why two oxygen atoms share two pairs — each arrangement leaves both partners with a filled outer shell.
Ionic bonds: electron transfer
An Ionic bond Attraction between oppositely charged ions formed by electron transfer Full entry → forms when one atom transfers one or more electrons to another. The donor becomes a positively charged cation; the acceptor becomes a negatively charged anion. Oppositely charged ions attract strongly, and that electrostatic attraction is the ionic bond. In sodium chloride (NaCl), sodium transfers its single valence electron to chlorine, producing Na⁺ and Cl⁻, which pack into a crystal lattice.
In the body, ionic compounds dissociate in water into free ions — the electrolytes (sodium, potassium, calcium, chloride) that conduct electrical currents in body fluids and are essential for nerve, muscle, and fluid balance. Ionic bonds are strong in solid crystals but dissolve readily in water, exactly what the body needs: solid salt is stable on the shelf, but dissolved ions are what physiology uses.
Covalent bonds: electron sharing
A Covalent bond Bond formed by sharing electron pairs Full entry → forms when two atoms share one or more pairs of electrons so both can count them toward a full shell. One shared pair makes a single bond, two pairs a double bond, three a triple bond. Covalent bonds are strong and hold most of the body's molecules together — water (H₂O), carbon dioxide (CO₂), glucose, proteins, and DNA.
Covalent bonds come in two flavors depending on how evenly electrons are shared:
- Nonpolar covalent bonds — equal sharing, because the atoms attract electrons equally. Examples: bonds between identical atoms (O₂, N₂) and carbon–hydrogen bonds. Nonpolar molecules do not dissolve well in water — why oils and fats (rich in C–H bonds) separate from water.
- Polar covalent bonds — unequal sharing, because one atom attracts electrons more strongly. That attracting ability is Electronegativity An atom's ability to attract shared electrons Full entry →. In water, oxygen is more electronegative than hydrogen, so shared electrons spend more time near oxygen: that end gains a partial negative charge (δ−), the hydrogen ends a partial positive charge (δ+). The molecule stays neutral overall but has separated charge — a polar molecule.
Hydrogen bonds: the weak but mighty attraction
A Hydrogen bond Weak attraction between a δ+ hydrogen and an electronegative atom Full entry → is not a true Chemical bond The force holding atoms together in molecules and compounds Full entry → — no electrons are transferred or shared. It is a weak attraction between a hydrogen carrying a partial positive charge (because it is bonded to an electronegative atom like oxygen or nitrogen) and another electronegative atom nearby. Individually weak and easily broken, hydrogen bonds are collectively enormously important.
They give water its remarkable properties: high surface tension, cohesion between molecules, and the ability to dissolve many substances. They hold the two DNA strands together. They drive protein folding — weak attractions between parts of a chain determine its three-dimensional shape, and shape determines function. This is why heat denatures proteins: added energy breaks the hydrogen bonds and other weak interactions holding the folded shape.
Van der Waals interactions
Van der Waals interactions are very weak attractions that arise when atoms or molecules come extremely close, caused by transient, uneven electron distributions creating brief dipoles. Individually negligible, they matter collectively when many atoms pack together — such as the lipid tails in a cell membrane. They help explain why lipids pack into membranes and why large molecules fit together like puzzle pieces.
Bond types at a glance
| Interaction | Electron transfer or sharing? | Relative strength | Key biological roles |
|---|---|---|---|
| Ionic bond | Electron transfer | Strong in crystals; dissociates in water | Electrolytes: Na⁺, K⁺, Ca²⁺, Cl⁻ in body fluids |
| Covalent bond | Electron sharing | Strong | Holds water, glucose, proteins, DNA together |
| Hydrogen bond | Neither — weak attraction | Weak individually, powerful collectively | Water's properties, DNA double helix, protein folding |
| Van der Waals | Neither — transient attraction | Very weak | Lipid membranes, molecular packing |
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| A hydrogen bond | A covalent bond involving hydrogen | In a hydrogen bond, hydrogen is merely attracted to another atom — not shared or transferred. In a covalent bond (O–H), the hydrogen is actually bonded |
| A polar molecule | A charged (ionic) molecule | Polar molecules have partial charges (δ+/δ−) and are neutral overall; ions carry a full net charge |
| Nonpolar molecule | Hydrophobic | "Nonpolar" describes electron sharing; poor water solubility is a practical consequence, not the definition |
| Ionic bonds being "the strongest" | Covalent bonds | Ionic bonds are strong in crystal form but dissolve in water; covalent bonds are strong and hold the body's molecules together |
| "Sharing" in a polar covalent bond being equal | Equal sharing | Polar sharing is unequal — one atom keeps the electrons longer (oxygen in water), creating partial charges |
| All weak attractions being hydrogen bonds | Van der Waals interactions | Hydrogen bonds need a specific δ+ hydrogen and electronegative partner; van der Waals interactions are weaker, nonspecific transient attractions |
| Electronegativity | Atomic number | Electronegativity is the pull on shared electrons in a bond; atomic number is the proton count — related, but not the same |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Atoms are a bit like kids who want a full set of eight pocket-parts in their outermost pocket. Some kids have one extra part and give it away (that makes an ionic bond, like salt), while others hold hands and share parts so each one counts eight (that makes a covalent bond, like water). There is also a weaker "sticky handshake" between molecules called a hydrogen bond — like Velcro that is easy to pull apart one strip at a time but holds strongly when many strips are together. That Velcro sticks the two sides of DNA together and makes water bead up.
Worked example
Picture a salad dressing of oil and vinegar. Shake it and the droplets disperse; let it sit and the oil rises again. The chemistry is simple once you know the bonds. Vinegar is mostly water, a polar molecule: oxygen's electronegativity pulls shared electrons toward itself, leaving the oxygen end δ− and the hydrogen ends δ+. The oil is made of long molecules full of nonpolar carbon–hydrogen bonds, where electrons are shared evenly and no end carries a lasting partial charge.
Water molecules would rather hydrogen-bond with each other than interact with oil, and oil molecules prefer their own kind through weak van der Waals attractions. Nothing "repels" — there is simply no energetic payoff for mixing, so they separate.
Now scale this to the human body. The cell membrane is a lipid bilayer: fatty (nonpolar) tails face each other in the middle, and polar head groups face the watery inside and outside of the cell. That arrangement — built from nonpolar covalent bonds, van der Waals interactions, and the water-loving behavior of polar heads — is what separates the cell's contents from its surroundings. It is also why fat-soluble molecules slip through membranes easily while ions and polar molecules need channels and pumps. One everyday observation about salad dressing explains a foundational feature of every cell in your body.
Key takeaways
- Bonds exist because atoms seek filled outer shells — the octet rule (eight electrons; hydrogen needs two) is the standard teaching model.
- Ionic bonds = electron transfer, producing cations (+) and anions (−) that attract; in water they dissociate into electrolytes.
- Covalent bonds = electron sharing; single/double/triple; strong; the backbone of the body's molecules.
- Polar vs. nonpolar covalent depends on electronegativity: O–H and N–H bonds are polar; C–H and identical-atom bonds are nonpolar. "Like dissolves like."
- Hydrogen bonds are weak attractions, not true bonds — they explain water's properties, hold DNA strands together, and drive protein folding.
- Van der Waals interactions are very weak, short-range attractions that matter when many atoms pack together (lipid membranes).
- Bond type predicts water solubility: ionic and polar covalent substances dissolve in water; nonpolar substances (fats, oils) do not.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Why do atoms form chemical bonds in the first place?
Show answer
To fill their outermost electron shells and reach a more stable configuration (the octet rule; hydrogen seeks two). Atoms gain, lose, or share electrons to achieve this.
What happens to the electrons when sodium and chlorine form an ionic bond, and what are the resulting ions?
Show answer
Sodium transfers its single valence electron to chlorine. Sodium becomes Na⁺ (cation), chlorine becomes Cl⁻ (anion), and the opposite charges attract, forming sodium chloride.
What makes a covalent bond polar rather than nonpolar? Give an example of each found in the body.
Show answer
Unequal sharing caused by a difference in electronegativity. Water's O–H bonds are polar (oxygen pulls electrons). Nonpolar examples: C–H bonds in fats and the O=O bond in oxygen gas.
Why is a hydrogen bond not a true chemical bond, and why is it still essential for life?
Show answer
It is only a weak electrostatic attraction between a δ+ hydrogen and an electronegative atom — no electron is transferred or shared. But collectively, hydrogen bonds give water its properties, hold DNA strands together, and stabilize protein shapes.
Using bond chemistry, explain why fats and oils do not dissolve in water.
Show answer
Fats are full of nonpolar C–H bonds with evenly shared electrons, so they carry no partial charges and have no reason to interact with polar water molecules, which prefer hydrogen-bonding with each other.
Rank ionic, covalent, hydrogen, and van der Waals interactions by strength, and state where each matters in the body.
Show answer
Strongest to weakest: covalent (strongest; holds water, DNA, proteins), ionic (strong in crystals, dissociates in water; electrolytes), hydrogen (weak individually, powerful collectively; water, DNA, protein folding), van der Waals (weakest; lipid membranes and molecular packing).
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Chemical bond
- The force holding atoms together in molecules and compounds
- Octet rule
- Atoms tend to gain, lose, or share electrons to reach eight in the outer shell
- Electronegativity
- An atom's ability to attract shared electrons
- Ionic bond
- Attraction between oppositely charged ions formed by electron transfer
- Cation / anion
- Positive ion (lost electrons) / negative ion (gained electrons)
- Covalent bond
- Bond formed by sharing electron pairs
- Polar covalent bond
- Unequal sharing; one end slightly negative, one slightly positive
- Nonpolar covalent bond
- Equal sharing; no separated charge
- Hydrogen bond
- Weak attraction between a δ+ hydrogen and an electronegative atom
- Van der Waals interaction
- Very weak attraction between atoms in close contact
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.