Lesson 13 of 13 · 15 min
Chapter review
Watch a class
The whole chapter on YouTube
Loading the full lesson
Must-know facts
19 facts
- 01Mendeleev: properties are periodic in atomic weight. Modern law (after Moseley, 1913): periodic in atomic number.
- 02Dobereiner's triads: middle atomic weight ≈ average of the ends (Li 7, Na 23, K 39). Newlands' octaves worked only up to calcium.
- 03Eka-aluminium = gallium; eka-silicon = germanium.
- 04Seven periods of 2, 8, 8, 18, 18, 32 and 32 (theoretical) elements; 18 IUPAC groups.
- 05Period number = n of the valence shell; elements per period = 2 × orbitals being filled.
- 06IUPAC roots: nil 0, un 1, bi 2, tri 3, quad 4, pent 5, hex 6, sept 7, oct 8, enn 9, then -ium. Z = 120 is unbinilium, Ubn.
- 07Lanthanoids Ce (58) to Lu (71); actinoids Th (90) to Lr (103).
- 08d-block general configuration (n−1)d¹⁻¹⁰ns⁰⁻²; Pd is 4d¹⁰5s⁰. Zn, Cd, Hg are not typical transition metals.
- 09Metalloids: Si, Ge, As, Sb, Te. Metals exceed 78% of the elements.
- 10Covalent radius of Cl = 198/2 = 99 pm; metallic radius of Cu = 256/2 = 128 pm.
- 11Cation < parent atom < anion: Na⁺ 95 pm < Na 186 pm; F 64 pm < F⁻ 136 pm.
- 12Isoelectronic size order: O²⁻ > F⁻ > Na⁺ > Mg²⁺.
- 13Period 2 ionization enthalpy: Li < B < Be < C < O < N < F < Ne.
- 14Most negative electron gain enthalpy of all: Cl (−349 kJ mol⁻¹), not F (−328).
- 15Noble gases: positive electron gain enthalpy (Ne +116 kJ mol⁻¹).
- 16Pauling scale: F = 4.0, the highest; Cs = 0.7 among group 1.
- 17Diagonal pairs: Li–Mg and Be–Al.
- 18Second-period maximum covalency 4 (BF₄⁻); Al reaches 6 (AlF₆³⁻).
- 19Oxides: Na₂O basic, Cl₂O₇ acidic, Al₂O₃ and As₂O₃ amphoteric, CO, NO and N₂O neutral.
Common traps
Where marks are lost
Formulas
6 to know
Covalent radius
r = ½ × (single-bond length between two identical atoms)
Cl₂: 198 pm → 99 pm. Metallic radius uses half the metal-metal distance (Cu: 256 → 128 pm).
First ionization enthalpy
X(g) → X⁺(g) + e⁻; ΔᵢH > 0
Second: X⁺(g) → X²⁺(g) + e⁻, always larger than the first.
Electron gain enthalpy
X(g) + e⁻ → X⁻(g); ΔegH
Negative when energy is released (halogens), positive for noble gases.
Electron affinity link
ΔegH = −Aₑ − (5/2)RT
Aₑ is defined at absolute zero with the opposite sign convention.
Per atom to per mole
ΔᵢH (J mol⁻¹) = energy per atom (J) × N_A
H: 2.18 × 10⁻¹⁸ × 6.022 × 10²³ ≈ 1.31 × 10⁶ J mol⁻¹.
Elements in a period
number of elements = 2 × number of orbitals being filled
Period 5: 5s + 4d + 5p = 1 + 5 + 3 = 9 orbitals → 18 elements.
Key terms
18 terms
- Triad
- Dobereiner's set of three similar elements whose middle atomic weight is about the mean of the other two.
- Law of Octaves
- Newlands' observation that, by atomic weight, every eighth element repeats the first element's properties.
- Eka-aluminium
- Mendeleev's name for the missing element below aluminium, later found as gallium.
- Modern Periodic Law
- Properties of elements repeat periodically with atomic number.
- Period
- A horizontal row; its number equals the principal quantum number of the valence shell.
- Group
- A vertical column of elements with similar outer electron configuration, numbered 1 to 18.
- Representative elements
- The s- and p-block elements together, also called main group elements.
- Transition elements
- d-block elements of groups 3 to 12, filling (n−1)d orbitals.
- Inner transition elements
- f-block lanthanoids and actinoids, filling (n−2)f orbitals.
- Metalloid
- An element along the zig-zag border, such as Si or As, with both metallic and non-metallic properties.
- Covalent radius
- Half the single-bond distance between two identical bonded atoms.
- Isoelectronic species
- Atoms or ions that carry the same number of electrons, such as Na⁺ and F⁻.
- Effective nuclear charge
- The net pull felt by an outer electron after inner electrons screen part of the nuclear charge.
- Ionization enthalpy
- Energy to remove the most loosely held electron from an isolated gaseous atom in its ground state.
- Electron gain enthalpy
- Enthalpy change when a gaseous atom accepts one electron to become a negative ion.
- Electronegativity
- The pull an atom in a compound exerts on a shared electron pair; a relative, not measured, quantity.
- Diagonal relationship
- Resemblance between a second-period element and the element one down and one to the right, as Li and Mg.
- Amphoteric oxide
- An oxide that reacts with both acids and bases, such as Al₂O₃.