Chemical Bonds

Why atoms bond. An atom is most stable when its valence shell (outermost energy level) is full. The first shell holds 2 electrons; the next shells are stable with 8 (the octet rule). Atoms with unfilled valence shells gain, lose, or share electrons to reach stability — and that electron activity is chemical bonding.

Ionic bonds. When one atom transfers electrons to another, both become charged ions. A classic example is table salt: sodium (Na) donates one electron to chlorine (Cl), becoming Na⁺ (a cation); chlorine becomes Cl⁻ (an anion). The opposite charges attract, forming an ionic bond. In water, these bonds dissociate, releasing free ions called electrolytes (Na⁺, K⁺, Ca²⁺, Cl⁻) that carry electrical charge — essential for nerve impulses, muscle contraction, and fluid balance.

Covalent bonds. Instead of transferring electrons, atoms can share them, forming strong covalent bonds. This is the dominant bond in the body's organic molecules (carbohydrates, lipids, proteins, nucleic acids). Sharing can be equal or unequal:

• Nonpolar covalent: electrons shared equally, so there are no charged regions. Example: the bonds in O₂ or between carbons in a fat's tail. Nonpolar molecules do not mix well with water.
• Polar covalent: electrons shared unequally, because one atom pulls harder. The classic example is water (H₂O): oxygen pulls the shared electrons closer, giving the oxygen end a slight negative charge and the hydrogen ends a slight positive charge. This uneven charge makes water polar — a property that underlies nearly all of the body's chemistry.

Hydrogen bonds. These are weak attractions, not true bonds within a molecule, that form between (or within) molecules when a partially positive hydrogen is drawn to a partially negative atom nearby. Individually weak, collectively they are powerful: hydrogen bonds hold water molecules together (giving water its cohesion and high heat capacity), stabilize the 3-D shape of proteins, and zip the two strands of DNA together — weakly enough to be "unzipped" for copying.

How it works

Predicting bond type:

1. Big electron transfer (metal + nonmetal) → ionic bond, forms ions/electrolytes.
2. Electron sharing between nonmetals → covalent bond (equal = nonpolar; unequal = polar).
3. Between molecules, partial charges attract → hydrogen bonds (weak but numerous).

Comparisons

Common confusions

• Ionic vs covalent. Ionic = electrons transferred (charges form); covalent = electrons shared.
• Polar vs nonpolar. Both are covalent; the difference is equal vs unequal sharing.
• Hydrogen bond vs covalent bond in water. The O–H bonds within one water molecule are polar covalent; the attraction between separate water molecules is a hydrogen bond.
• Cation vs anion. Cation = positive (think "cat-ion paws-itive"); anion = negative.

Memory aids

• "Ionic = I owe you an electron" (transfer); "coVALENT = co-sharing."
• Cation is Positive ("t" looks like a plus sign).
• Polar bear in water: water is polar.

Quick review

• Atoms bond to complete their valence shell (octet rule).
• Ionic bonds transfer electrons, forming ions/electrolytes that dissociate in water.
• Covalent bonds share electrons — nonpolar (equal) or polar (unequal, as in water).
• Hydrogen bonds are weak attractions between partial charges; collectively they shape proteins, hold DNA together, and give water its special properties.