TEAS Ready⚗️ Science · Chemistry · Lesson 3 of 6 · 6 min
Chemical bonds and balancing equations
🔗 Why does salt dissolve, why does water bead up, and why must every equation balance? It all comes down to how atoms hold on to electrons.
💡 Key idea
Ionic bonds transfer electrons, covalent bonds share them, and a balanced equation has the same atoms on both sides.
🧠 The idea
In an IONIC BOND, a metal transfers electrons to a nonmetal, and the resulting cation and anion attract each other, as in sodium chloride (NaCl). In a COVALENT BOND, two nonmetals share electron pairs, as in water (H₂O) or methane (CH₄). If the atoms pull on the shared electrons equally, the bond is nonpolar; if one atom is more electronegative, the bond is POLAR, with partial charges. Water is polar because oxygen pulls electrons away from hydrogen. A HYDROGEN BOND is a weaker attraction between molecules: a hydrogen attached to oxygen, nitrogen or fluorine is drawn to a nearby O, N or F. Hydrogen bonds hold water molecules together and pair DNA bases. In a chemical equation, mass is conserved: atoms are rearranged, never created or destroyed. To balance an equation, change only the COEFFICIENTS in front of formulas, never the subscripts, until each element has the same count on both sides.
✏️ Worked example
Balance H₂ + O₂ → H₂O. Step 1: count atoms. Left: H 2, O 2. Right: H 2, O 1. Step 2: oxygen is short on the right, so write 2H₂O. Now right: H 4, O 2. Step 3: hydrogen is short on the left, so write 2H₂. Left: H 4, O 2. Step 4: 2H₂ + O₂ → 2H₂O. Both sides have 4 H and 2 O, so it is balanced.
📌 Takeaways
- Metal + nonmetal → ionic; nonmetal + nonmetal → covalent.
- Unequal sharing makes a polar bond; water is polar.
- Balance with coefficients only; atoms in = atoms out.
📖 Terms in this lesson
Covalent bond: A bond in which atoms share electrons (an ionic bond transfers them); for example, the bonds in a water molecule.
✅ Check yourself
Question 1 of 3
Which pair of elements is most likely to form an ionic bond?
- Sulfur and oxygen
- Potassium and bromine
- Nitrogen and hydrogen
- Carbon and oxygen
Show the answer and rationale
Answer: B
Potassium is a metal and bromine is a nonmetal, so potassium transfers an electron to bromine, forming an ionic bond. The other pairs are two nonmetals, which share electrons in covalent bonds.
Question 2 of 3
When balancing a chemical equation, you may change only the
- subscripts within the chemical formulas
- coefficients in front of formulas
- symbols of the elements
- charges on the ions
Show the answer and rationale
Answer: B
Coefficients change how many units of a substance react without changing what the substance is. Changing a subscript changes the compound itself (H₂O to H₂O₂ is a different chemical), and symbols and charges are fixed.
Question 3 of 3
What is the coefficient of O₂ when the equation CH₄ + __O₂ → CO₂ + 2H₂O is balanced?
- 3
- 1
- 2
- 4
Show the answer and rationale
Answer: C
Right side oxygen: CO₂ has 2 and 2H₂O has 2, so 4 O atoms. Two O₂ molecules supply 4 O atoms. A coefficient of 1 supplies only 2, and 3 or 4 would put 6 or 8 O atoms on the left.
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