Egypt Secondary 3 Chemistry, Week 2: Who counts as a transition element

Configurations, oxidation states and who counts as a transition element

Week 2 · Chapter 1 (B): electron configurations of the first series with the chromium and copper exceptions, oxidation states (why iron(II) oxidises easily and manganese(II) does not), the definition of a transition element, coinage metals in and zinc, cadmium and mercury out · 3 lessons

Who counts as a transition element

Chemistry Sec 3 Term 1, Chapter 1: pp. 8–11

GoalState the test for a transition element and apply it to copper, gold, zinc, cadmium and mercury.

New words

transition element · an element whose atom, or at least one of its common ions, has a partly filled d sublevelpartly filled · some but not all ten places in the sublevel taken, 1 to 9 electrons in dcoinage metals · copper, silver and gold (group IB), used for coins and jewellery

Explanation

Chemists use a test, not only the position in the table: an element is a transition element if its atom, or at least one of its common ions, has a partly filled d sublevel (1 to 9 electrons in d). Apply it. Iron: the atom is 3d⁶, so yes. Scandium: the atom is 3d¹, so yes, although Sc³⁺ has no d electrons; one case is enough. Copper: the atom is 3d¹⁰, which is full, but Cu²⁺ is 3d⁹, partly filled, so copper counts. Gold: Au³⁺ is 5d⁸, so gold counts. Zinc: the atom is 3d¹⁰ and its only common ion Zn²⁺ is also 3d¹⁰, so zinc is in the d block but is not a transition element; the same holds for cadmium and mercury. Silver is a borderline case: Ag⁺ is 4d¹⁰, and only the rare Ag²⁺ (4d⁹) makes it a transition element, which is why courses treat it with care (extra). So the d block and the transition elements are nearly, but not exactly, the same set.

Element	Atom’s d electrons	Common ion	Transition element?
Fe	3d⁶	Fe²⁺ 3d⁶	yes
Cu	3d¹⁰	Cu²⁺ 3d⁹	yes
Zn	3d¹⁰	Zn²⁺ 3d¹⁰	no
Sc	3d¹	Sc³⁺ none	yes

Worked examples

Cadmium (Z = 48) is [Kr] 4d¹⁰ 5s² and forms only Cd²⁺, which is 4d¹⁰. The atom has a full d sublevel and so does the ion, so cadmium is not a transition element, like zinc above it in the same column.

Particle	d electrons	Partly filled?
Au atom	5d¹⁰	no
Au³⁺	5d⁸	yes

Gold is [Xe] 4f¹⁴ 5d¹⁰ 6s¹. The atom’s 5d is full, so look at the ions: Au³⁺ is 5d⁸, partly filled. Gold passes the test, which is why it is classed with copper as a transition element.

Titanium has Z = 22. The atom is [Ar] 3d² 4s², partly filled, so the test is passed at once. It does not matter that Ti⁴⁺ is [Ar] with no d electrons: the definition asks for the atom or at least one ion.

Class activity

A sorting game on cards. Write on twelve cards: Sc, Ti, Mn, Fe, Ni, Cu, Zn, Ca, Ga, Cd, Au, Hg. On the back write the d electrons of the atom and of the common ion (use the lesson, a periodic table and the rules of the week). Shuffle the cards, deal them out and sort them into two piles: transition elements and not. Then stack coins as electrons in five boxes to show Cu and Cu²⁺ and check that one of the two is partly filled.

Practice

Mercury is [Xe] 4f¹⁴ 5d¹⁰ 6s² and forms Hg²⁺ (5d¹⁰). Is it a transition element?

Why is calcium not a transition element, although it comes just before the d block?

Nickel forms Ni²⁺. Write the d configuration of the ion and decide whether it supports nickel as a transition element.

Which ion makes copper a transition element, Cu⁺ or Cu²⁺, and why?