Chemical Bonding: How Atoms Join Together

Atoms rarely sit alone — they bond together into compounds and molecules because doing so makes them more stable. The driving force is the outer ring of electrons: most atoms "want" a full outer shell, and bonding is how they get it. They have two strategies. They can hand electrons off (an ionic bond) or share them (a covalent bond), and which one happens shapes the properties of the resulting substance.

This guide explains valence electrons, ionic and covalent bonds, and polarity, with worked and practice questions matched to the level seen in ACT Science and chemistry at Northside Tutoring.

Why Bonding Matters

Bonding explains why substances have the properties they do — why salt dissolves, why water is liquid, why diamond is hard. ACT Science chemistry passages often rely on bonding concepts. (The SAT has no science section.) It builds directly on atomic structure.

Valence Electrons & the Octet Rule

Valence electrons are the electrons in an atom's outermost shell — the ones involved in bonding. Most atoms are most stable with a full outer shell of 8 electrons (the octet rule). Bonding is simply how atoms reach that stable arrangement, by gaining, losing, or sharing electrons.

Ionic Bonds: Trading Electrons

An ionic bond forms when one atom transfers electrons to another — typically a metal giving electrons to a nonmetal. The metal becomes a positive ion, the nonmetal a negative ion, and their opposite charges attract. Table salt (NaCl) is the classic example: sodium gives its one valence electron to chlorine.

Covalent Bonds: Sharing Electrons

A covalent bond forms when two atoms share electrons rather than transfer them — typically between two nonmetals. The shared pair counts toward both atoms' octets. Water (H₂O) and carbon dioxide (CO₂) are held together by covalent bonds, forming distinct molecules.

Ionic vs. Covalent

	Ionic	Covalent
Electrons	transferred	shared
Typical atoms	metal + nonmetal	nonmetal + nonmetal
Result	charged ions attract	neutral molecules

Polarity

In a covalent bond, atoms don't always share electrons equally. When one atom pulls the shared electrons more strongly, the bond is polar — one end is slightly negative, the other slightly positive. Water is a polar molecule, which is why it dissolves so many substances. Equal sharing gives a nonpolar bond.

Where You'll See This — Test by Test

Bonding supports ACT Science chemistry passages; the SAT has no science section and the SSAT doesn't test it. It's core high-school and AP Chemistry.

Watch the Lesson

Sometimes a diagram needs a voice. In the short video below, one of our Northside tutors walks through the core idea and works through test-style problems in real time.

Worked Example Problems

These problems are calibrated to the difficulty you'll actually see on test day. Try each one before opening the solution.

Common Mistakes to Avoid

Practice Problems — You Try

Three problems below. Work each before checking the solution.

The Northside Method — How We Teach This 1-on-1

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