Chemistry - Atoms, Molecules, Ions, and Chemical Equations
This domain tests atomic structure, ion formation, formula writing, equation balancing, and simple amount calculations. Niigata items often give a reaction in words and ask for a balanced equation or a mass calculation, so link particles to coefficients.
Atoms and molecules
- An atom has a nucleus (protons + neutrons) and electrons. Atomic number = protons; mass number = protons + neutrons.
- Isotopes have the same number of protons but different neutrons.
- Electrons occupy shells; valence electrons determine bonding.
- Covalent molecules share electrons: H₂O, CO₂, O₂. Ionic compounds form crystals of ions, such as NaCl.
Ions
- Metals lose electrons to form cations; nonmetals gain electrons to form anions.
- Common ions: Na⁺, K⁺, Ca²⁺, Mg²⁺, Al³⁺, Cl⁻, O²⁻, OH⁻, NO₃⁻, SO₄²⁻, CO₃²⁻, NH₄⁺.
- Ionic formulas balance charge: CaCl₂, Al₂O₃.
Equations and amount
- Balance equations by changing coefficients only; atoms are conserved.
- Coefficients give mole ratios; molar mass converts mass to moles.
- 1 mol = 6.0 × 10²³ particles; gas molar volume is 22.4 L at STP.
- Diatomic elements are H₂, O₂, N₂, Cl₂.
Worked example
Hydrogen burns: 2H₂ + O₂ → 2H₂O. If 4.0 g of H₂ reacts with excess oxygen, find the mass of water. Molar mass of H₂ is 2.0 g/mol, so 4.0 g = 2.0 mol H₂. The coefficients show 2 mol H₂ gives 2 mol H₂O. Molar mass of H₂O is 18 g/mol, so mass = 2.0 × 18 = 36 g. The reasoning uses the mole ratio from the balanced equation, not a mass ratio guessed from the formula.
Traps
- Changing subscripts to balance an equation.
- Writing Na₂Cl instead of NaCl.
- Confusing atomic number with mass number.
- Ignoring coefficients when finding mole ratios.
- Forgetting that H, O, N, and Cl are diatomic.