Unit 9 Quick Review

Key Terms

• Matter: anything with mass & volume.
• Element: pure substance of one atom type.
• Compound: two + elements in fixed ratio; smallest unit = molecule.
• Atom: smallest particle of an element able to enter reactions.
• Molecule: smallest free-existing particle of element/compound.

Sub-Atomic Particles

• Proton $p^+$: + charge, $1\,amu$, nucleus.
• Neutron $n^0$: 0 charge, $1\,amu$, nucleus.
• Electron $e^-$: – charge, $\tfrac{1}{1837}\,amu$, shells.
• Atoms neutral: #p^+ = #e^-.
• $1\,amu = \tfrac{1}{12}$ mass of $^{12}C$ atom.

Atomic Number & Mass

• Atomic number Z = #p^+ = #e^- (neutral).
• Mass number A = #p^+ + #n^0.
• #n^0 = A - Z.

Electronic Configuration

• Electrons occupy shells $K,L,M,N,O,P,Q$.
• Max electrons per shell: $2n^2$ (valid to $N$); practical maxima $O=32$, $P=18$, $Q=8$.
• Fill from lower to higher energy levels.

Valence Shell, Electrons & Valency

• Valence shell: outermost shell.
• Valence electrons determine combining capacity (valency).
• Valency = electrons lost, gained, or shared to attain stability.
  • In period: increases $1\rightarrow4$ then decreases $4\rightarrow0$.
  • In group: constant.

Duplet & Octet

• Duplet state: $2$ electrons in $K$-shell (He).
• Octet state: $8$ valence electrons (Ne, Ar, …).
• Duplet/octet rules: atoms transfer/share e⁻ to achieve these stable configurations.

Radicals

• Charged species acting as single unit.
• Electropositive (basic) radicals: positive charge, e.g.
  • $\text{Na}^+$, $\text{NH}_4^+$, $\text{Ca}^{2+}$.
• Electronegative (acidic) radicals: negative charge, e.g.
  • $\text{Cl}^-$, $\text{SO}<em>4^{2-}$, $\text{PO}</em>4^{3-}$.

Molecular Formula & Weight

• Shows actual number of atoms per molecule (e.g. $\text{H}_2\text{O}$).
• Writing steps: write radicals → assign valencies → cross-swap → reduce if needed.
• Molecular weight = sum of constituent atomic weights (e.g. $\text{H}_2\text{O}=2\times1+16=18\,amu$).

Periodic Table

• Mendeleev’s law: properties periodic w.r.t. atomic weight.
• Modern law: properties periodic w.r.t. atomic number.
• Modern table: 7 periods (rows), 18 groups (columns).
• Position highlights:
  • Group $1$: alkali metals.
  • Group $2$: alkaline-earth metals.
  • Groups $3$–$12$: transition metals.
  • Group $17$: halogens.
  • Group $18$: noble gases (valency $0$).
  • Lanthanides & actinides: $f$-block, separate rows.

Periodic Trends

• Atomic radius: ↓ left→right; ↑ top→bottom.
• Metallic character: ↓ left→right; ↑ top→bottom.
• Non-metallic character: opposite of metallic.
• Number of shells: constant across period; increases down group.

Chemical Reactions & Equations

• Reactants → Products; expressed as word or chemical equations.
• Balanced equation: equal atoms each side (obey conservation of mass).
  • Balance using coefficients; largest molecule usually first.
• Example: $2\text{H}<em>2 + \text{O}</em>2 \rightarrow 2\text{H}_2\text{O}$.
• Reversible reaction: products can reform reactants (double arrow $\rightleftharpoons$).
• Irreversible reaction: proceeds one direction (single arrow).

Quick Facts

• $6\times10^{23}$ amu = $1\,g$.
• $1\,C$ of charge ≈ $6.25\times10^{18}$ e⁻.
• First 20 elements: remember electronic configurations (e.g. $\text{Ca}: 2\,8\,8\,2$).
• Metals become more reactive down a group; non-metals less reactive.