Paramagnetic and diamagnetic substances

Why some substances are pulled into a magnetic field

1. Magnetic properties and catalytic activity

2. Pulled in or pushed out?

FeCl3 FeCl X_{3}

is pulled into a magnetic field.

ZnSO4 ZnSO X_{4}

is pushed out of it.

Pick the right answer

Predict: what makes the difference?

A. Iron is a magnet; zinc is not
B. $Fe³⁺ Fe X^{3 +}$ has unpaired electrons; in $Zn²⁺ Zn X^{2 +}$ all electrons are paired
C. Zinc compounds are not affected by magnets

Explanation

Fe³⁺ Fe X^{3 +}

(

3d⁵ 3 d^{5}

) has unpaired electrons, so

FeCl3 FeCl X_{3}

is paramagnetic and is attracted to the magnetic field. In

Zn²⁺ Zn X^{2 +}

(

3d¹⁰ 3 d^{10}

) all electrons are paired, so

ZnSO4 ZnSO X_{4}

is diamagnetic and repels the field.

3. A spinning electron

Book p.13: an unpaired electron (↑) forms a magnetic field due to its spinning.

Each electron spins. An unpaired electron (↑), spinning on its axis, forms a small magnetic field.

In a pair (↑↓) the two spins are opposite, so their fields cancel.

4. Paramagnetic substances

Unpaired electrons? Then the substance is pulled toward the field.

Paramagnetic substance

A substance that is attracted to the external magnetic field due to the presence of unpaired electrons.

A paramagnetic sample is pulled into the field between two poles.

5. Magnetic moment

Magnetic moment

The strength of the magnetism of a substance's atoms or ions. It increases with the number of unpaired electrons.

Add d electrons one at a time, by Hund's rule. Watch the unpaired electrons.

electrons in the d sublevel 3

↑↑↑

d³

6. Diamagnetic substances

All electrons paired? Then every field cancels.

Diamagnetic substance

A substance that repels the external magnetic field, due to the pairing of all its electrons. Its magnetic moment equals zero.

A diamagnetic sample is pushed out of the field.

7. The book's exercise

Book exercise, p.14: which is paramagnetic, which is diamagnetic? Then order their magnetic moments.

↑↓↑↓↑↓↑↓↑↓

Zn (

d¹⁰ d^{10}

)

↑↓↑↓↑↓↑↑

Ni²⁺ Ni X^{2 +}

(

d⁸ d^{8}

)

↑↓↑↑↑↑

Iron(II) chloride,

Fe²⁺ Fe X^{2 +}

(

d⁶ d^{6}

)

Need a hint?

Count the boxes that hold only one electron (↑).
Zero unpaired electrons means diamagnetic.
More unpaired electrons means a larger magnetic moment.

Reveal

Configuration of (d)	Unpaired electrons	Magnetic property
Zn ( $d¹⁰ d^{10}$ )	0	diamagnetic
$Ni²⁺ Ni X^{2 +}$ ( $d⁸ d^{8}$ )	2	paramagnetic
$Fe²⁺ Fe X^{2 +}$ ( $d⁶ d^{6}$ )	4	paramagnetic

Book's answer, p.14

Order of magnetic moment: Zn <

Ni²⁺ Ni X^{2 +}

< iron(II) chloride.

8. Which pulls hardest?

Pick the right answer

Which ion has the largest magnetic moment?

A. $Cu+ Cu X^{+}$ ( $3d¹⁰ 3 d^{10}$ )
B. $Cr³⁺ Cr X^{3 +}$ ( $3d³ 3 d^{3}$ )
C. $Co²⁺ Co X^{2 +}$ ( $3d⁷ 3 d^{7}$ )
D. $Fe³⁺ Fe X^{3 +}$ ( $3d⁵ 3 d^{5}$ )

Explanation

Unpaired:

3d¹⁰ 3 d^{10}

3d³ 3 d^{3}

3d⁷ 3 d^{7}

3d⁵ 3 d^{5}

5. The magnetic moment follows the unpaired electrons, not the total.

True or false?

A paramagnetic substance attracts iron nails by itself, like a bar magnet.

Explanation

Paramagnetic: attracted to the external magnetic field due to unpaired electrons.

9. Sort the ions

Sort into groups

Sort these ions. Picture their 3d boxes first.

$V²⁺ V X^{2 +}$ ( $3d³ 3 d^{3}$ ) $Zn²⁺ Zn X^{2 +}$ ( $3d¹⁰ 3 d^{10}$ ) $Mn²⁺ Mn X^{2 +}$ ( $3d⁵ 3 d^{5}$ ) $Sc³⁺ Sc X^{3 +}$ ( $3d⁰ 3 d^{0}$ ) $Cu²⁺ Cu X^{2 +}$ ( $3d⁹ 3 d^{9}$ )

Fill in the blanks

A diamagnetic substance _____ the external magnetic field, because all its electrons are _____.

Explanation

Book p.14: a diamagnetic substance repels the external magnetic field, due to the pairing of all its electrons.

10. Summary

A spinning unpaired electron forms a magnetic field; paired spins cancel.
Paramagnetic: attracted to the external field (unpaired electrons). Most compounds of the transition elements are paramagnetic.
Diamagnetic: all electrons paired, magnetic moment zero, repels the external field.
Magnetic moment increases with the number of unpaired electrons.

Back to the hook:

Fe³⁺ Fe X^{3 +}

(

3d⁵ 3 d^{5}

) has unpaired electrons;

Zn²⁺ Zn X^{2 +}

(

3d¹⁰ 3 d^{10}

) has none. Can you explain why

d¹⁰ d^{10}

gives zero magnetic moment?

Reveal

d¹⁰ d^{10}

all five boxes hold a pair (↑↓). In a pair the two spins are opposite, so their fields cancel: no unpaired electrons, magnetic moment zero.

↑↓↑↓↑↓↑↓↑↓

d¹⁰ d^{10}

: 0 unpaired

Concepts in this lesson