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Must-know facts

19 facts

1Mendeleev: properties are periodic in atomic weight. Modern law (after Moseley, 1913): periodic in atomic number.
2Dobereiner's triads: middle atomic weight ≈ average of the ends (Li 7, Na 23, K 39). Newlands' octaves worked only up to calcium.
3Eka-aluminium = gallium; eka-silicon = germanium.
4Seven periods of 2, 8, 8, 18, 18, 32 and 32 (theoretical) elements; 18 IUPAC groups.
5Period number = n of the valence shell; elements per period = 2 × orbitals being filled.
6IUPAC 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.
7Lanthanoids Ce (58) to Lu (71); actinoids Th (90) to Lr (103).
8d-block general configuration (n−1)d¹⁻¹⁰ns⁰⁻²; Pd is 4d¹⁰5s⁰. Zn, Cd, Hg are not typical transition metals.
9Metalloids: 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

Saying Mendeleev arranged strictly by atomic weight.

He broke the order when properties demanded it: iodine went after the heavier tellurium, into Group VII.

Saying ionization enthalpy rises steadily from Li to Ne.

It dips twice: B is below Be (2p vs 2s) and O is below N (paired 2p electrons repel).

Naming fluorine as the most negative electron gain enthalpy.

Chlorine is: −349 against −328 kJ mol⁻¹. The small n = 2 shell of F repels the added electron.

Assuming a noble gas releases energy when it gains an electron.

Its ΔegH is positive; the electron must start a new shell.

Treating electronegativity as a measured quantity with fixed values.

It is qualitative, depends on the bonding partner, and the numbers come from scales such as Pauling's.

Comparing noble gas radii with covalent radii of other elements.

Noble gases only have van der Waals radii; compare them with van der Waals radii.

Thinking a greater nuclear charge always makes an ion larger among isoelectronic species.

More protons pull the same electrons closer: Mg²⁺ is smaller than Na⁺, which is smaller than F⁻.

Placing helium in the s-block column in the table.

It is s-block by configuration but sits in group 18 because its shell is full and it behaves as a noble gas.

Equating oxidation state with covalency.

In [AlCl(H₂O)₅]²⁺ aluminium is +3 but forms 6 bonds.

Writing the IUPAC name for 120 as unbinillium or ubinilium.

un (1) + bi (2) + nil (0) + ium = unbinilium, symbol Ubn.

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.

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