Egypt Secondary 3 Chemistry, Week 3: Metallic character and magnetism: counting unpaired electrons

Properties, magnetism and catalysts of the first series

Week 3 · Chapter 1 (C): general properties of the first series (atomic mass, radius, density, metallic character), paramagnetic and diamagnetic substances and counting unpaired electrons, transition metals as catalysts · 3 lessons

Metallic character and magnetism: counting unpaired electrons

Chemistry Sec 3 Term 1, Chapter 1: pp. 12–15

GoalDescribe the metallic character of the series in brief, define paramagnetic and diamagnetic, and decide which ions are which by counting unpaired electrons.

New words

paramagnetic · drawn into a magnetic field, because the particle has unpaired electronsdiamagnetic · pushed out very slightly by a magnetic field, because all the particle’s electrons are pairedmagnetic moment · a measure of how strongly a particle behaves as a magnet; it grows with the number of unpaired electrons

Explanation

The metals of the first series are shiny solids that conduct heat and electricity, with high melting points and high densities. Both the 4s and the 3d electrons of neighbouring atoms are shared in the metallic bond, which makes it strong and the melting points high. Their chemical activity varies widely: copper is hardly active, iron rusts, scandium is active enough to react with water (a fact in words only). Magnetism comes from electron spin. A spinning electron is a tiny magnet, and two electrons in one orbital spin opposite ways so that their effects cancel. A particle with unpaired electrons is therefore paramagnetic: it is drawn into a magnetic field, and the more unpaired electrons it has, the larger its magnetic moment and the stronger the pull. A particle with all its electrons paired has zero magnetic moment and is diamagnetic, pushed out very slightly. Most compounds of transition elements are paramagnetic because their ions have partly filled d sublevels. To decide: write the ion’s d configuration, draw the boxes by Hund’s rule and count the single arrows.

Type	Electrons	In a magnetic field	Magnetic moment
paramagnetic	at least one unpaired	drawn in	greater than zero
diamagnetic	all paired	pushed out slightly	zero

Worked examples

Co²⁺. Cobalt is [Ar] 3d⁷ 4s²; Co²⁺ loses the two 4s electrons and is 3d⁷. In the five boxes, 5 electrons stand alone and 2 are added as pairs, so 3 are unpaired. The ion is paramagnetic.

Ion	d configuration	Unpaired	Behaviour
V³⁺	3d²	2	paramagnetic
Cu⁺	3d¹⁰	0	diamagnetic

Two more ions. V³⁺ is 3d², with 2 unpaired electrons: paramagnetic. Cu⁺ is 3d¹⁰, a full sublevel with every electron paired: diamagnetic, magnetic moment zero.

A ranking. Put Mn²⁺, Cr³⁺ and Cu²⁺ in order of increasing magnetic moment. Count first: Cu²⁺ is 3d⁹ (1 unpaired), Cr³⁺ is 3d³ (3), Mn²⁺ is 3d⁵ (5). The order is Cu²⁺ < Cr³⁺ < Mn²⁺.

Class activity

A fair magnet test, with a grown-up. Lay out an iron nail, a paper clip, a coin, a ball of aluminium foil, a steel fork and a plastic spoon. Predict for each whether a fridge magnet will pick it up, then test and fill in a table. The objects that are attracted contain iron, and the pull is far stronger than the weak paramagnetism of ions, which you cannot notice at home. Write one sentence that tells apart “drawn into a field because of unpaired electrons” from “picked up by a magnet in the kitchen”. Do not use any magnet near a phone or a bank card.

Practice

Decide whether Ni²⁺ is paramagnetic or diamagnetic. (Nickel atom: [Ar] 3d⁸ 4s².)

Titanium(IV), Ti⁴⁺, has the configuration of argon. Is it paramagnetic?

Rank Fe²⁺, Ni²⁺ and Zn²⁺ from the smallest to the largest magnetic moment.

Why does a diamagnetic particle have zero magnetic moment?