Mistake Bank: Periodic Classification | Chemca

The Mistake Bank

Chapter 3: Classification of Elements

Trends have exceptions. Don't let the exceptions become your errors.

The Nitrogen vs. Oxygen IE

Ionization Enthalpy

Scenario: Compare the First Ionization Enthalpy (\(IE_1\)) of Nitrogen (Z=7) and Oxygen (Z=8).

✖ What Students Do

Student applies the general trend: "IE increases across a period."

Since O is to the right of N:

$$ IE_1(O) > IE_1(N) $$

(Incorrect!)

✔ The Correct Way

Check for Electronic Stability!

Nitrogen has a half-filled p-orbital (\(2p^3\)), which is extra stable.

Removing an electron from N is harder than removing one from O (\(2p^4\)), which wants to lose one to become half-filled.

$$ IE_1(N) > IE_1(O) $$

Fluorine vs. Chlorine Affinity

Electron Gain Enthalpy

Scenario: Which element has the most negative Electron Gain Enthalpy (highest affinity)? F or Cl?

✖ What Students Do

Student thinks: "Fluorine is the most electronegative element, so it must love electrons the most."

$$ \Delta_{eg}H(F) > \Delta_{eg}H(Cl) $$

(Wrong due to size!)

✔ The Correct Way

Small Size causes Repulsion!

Fluorine is so small (2p) that adding an electron causes high inter-electronic repulsion.

Chlorine (3p) has more space, so it accepts the electron more easily.

$$ Cl > F $$

Order: \( Cl > F > Br > I \)

Isoelectronic Species Radius

Ionic Radius

Scenario: Arrange in increasing order of size: \( N^{3-}, O^{2-}, F^-, Na^+, Mg^{2+}, Al^{3+} \)

✖ What Students Do

Student gets confused by the charges and thinks cations are larger because they have "more positive power".

Or assumes neutral is always largest.

✔ The Correct Way

More Protons = Smaller Size

All have 10 electrons. Look at nuclear charge (Z).

• Anions > Neutral > Cations
• Higher -ve charge = Larger
• Higher +ve charge = Smaller

$$ Al^{3+} < Mg^{2+} < Na^+ < F^- < O^{2-} < N^{3-} $$

The "Neutral" Oxides

Nature of Oxides

Scenario: Identify the acidic oxide: \( CO_2, SO_2, CO, Cl_2O_7 \)

✖ What Students Do

Student applies rule: "Non-metal oxides are acidic."

Since C is a non-metal, student assumes CO (Carbon Monoxide) is also acidic.

✔ The Correct Way

Memorize the 3 Neutral Oxides!

While most non-metal oxides are acidic, there are three important exceptions that are Neutral:

1. CO (Carbon Monoxide)
2. NO (Nitric Oxide)
3. \( N_2O \) (Nitrous Oxide)

Electronegativity vs. Electron Gain

Definitions

Scenario: Which value is measured in kJ/mol?

✖ What Students Do

Student treats Electronegativity (EN) and Electron Gain Enthalpy (EGE) as the same thing.

✔ The Correct Way

One is a Tendency, One is Energy!

Electronegativity: A relative tendency to attract shared pairs. Unitless (Pauling Scale).

Electron Gain Enthalpy: Energy released when an isolated gaseous atom accepts an electron. Measured in kJ/mol or eV.

Lanthanoid Contraction

Atomic Radius

Scenario: Compare the size of Zr (4d series) and Hf (5d series).

✖ What Students Do

Student expects size to increase down the group:

$$ \text{Size of } Hf > \text{Size of } Zr $$

(Wrong! They are almost identical.)

✔ The Correct Way

Poor Shielding of f-orbitals!

Before Hf, 14 electrons fill the 4f orbitals.

f-electrons shield the nucleus very poorly, increasing the effective nuclear charge significantly.

This pulls the outer shell in, cancelling the size increase.

$$ \text{Size of } Zr \approx \text{Size of } Hf $$

Confess Your Sins!

"Chemistry is the study of matter, but mostly the study of exceptions."

Did one of these catch you? Or do you have a different horror story from your last exam?

Scroll down to the comments section below and tell us:

"Which mistake were you making?"

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