Chemical Bonds, Formulas, and Molecules

Transfer, sharing, and attractions

Atoms bond through their valence electrons. In an ionic bond, one atom transfers electrons and charged ions attract. Sodium can lose an electron to become Na+, while chlorine gains one to become Cl−.

In a covalent bond, atoms share electrons. Water is a covalent molecule. A hydrogen bond is a weaker attraction between a partly positive hydrogen and a nearby partly negative region.

Electron transfer creates ions before ionic attraction holds them together. Electron sharing holds atoms together inside a covalent molecule. Hydrogen bonds act between partly charged regions and are weaker than the covalent bonds inside a water molecule.

Reading chemical formulas

A chemical formula shows the kinds and numbers of atoms. A subscript applies only to the symbol or group just before it. H2O has two hydrogen atoms and one oxygen atom.

A coefficient counts whole particles. In 3H2O, the 3 means three water molecules, for a total of six H atoms and three O atoms. Parentheses multiply the atoms inside them.

Worked example. Ca(NO3)2 contains one Ca atom. The outside 2 multiplies everything in the parentheses, so it contains two N atoms and six O atoms. A missing subscript means one.

Worked charge balance: Mg2+ and Cl− must combine with zero total charge. One Mg2+ contributes +2 and two Cl− contribute −2, so the formula is MgCl2. For unlike charge sizes, find the smallest whole-number counts with equal positive and negative totals; use supplied ion charges, not memorized oxidation-state lists.

Bonding and properties

Ionic solids often have high melting points because many opposite charges attract. They can conduct electricity when melted or dissolved because ions can move. Solid ionic compounds do not conduct well because the ions are fixed.

Covalent substances vary widely. Water’s polarity and hydrogen bonding help it dissolve many charged or polar substances. Bonding is one cause of a substance’s properties, not the only cause.

Polar covalent bonds share electrons unequally and create partial charges. Water is polar, so it interacts well with ions and other polar molecules. Oil is nonpolar, so it does not mix well with water.

Always distinguish a property of a pure substance from what happens after it dissolves or melts.

Worked evidence check: a solid does not conduct, but conducts when melted. This is consistent with fixed ions becoming mobile. It supports an ionic model; it does not alone identify which salt is present. A dissolved covalent sugar does not produce mobile ions just because the solution is liquid.

In a metal, some electrons can move through the solid and carry charge. These mobile electrons explain electrical conductivity; the term “delocalized” simply means they are not confined to one atom.

Summary

• Ionic bonding involves charged ions.
• Covalent bonding involves shared electrons.
• Hydrogen bonds are weaker attractions.
• Formula subscripts show atom counts.
• Bond type helps explain physical properties.

Transfer makes ions; sharing makes covalent bonds. Hydrogen bonds are attractions, not electron sharing.

Sources

• OpenStax Chemistry 2e
• IUPAC Gold Book