Electron Configuration Practice Problems- Chemistry Guide

Electron Configuration Practice Problems: What You Actually Need to Know

Most students mess up electron configurations because they memorize rules without understanding why those rules exist. This guide cuts through the B.S. and gives you real practice problems with actual explanations—not the vague "here's how it works" stuff your textbook throws at you.

You'll learn by doing, not by reading. Let's get started.

The Basics: What You're Working With

Electron configuration tells you how electrons are arranged in an atom. That's it. The notation looks like this: 1s² 2s² 2p⁶. Each number-letter-number combo represents electrons in a specific orbital.

The Orbital Filling Order (Memorize This)

Forget the diagonal rule diagrams your teacher drew. Here's the simple order:

• 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p

Or use the "Aufbau" method: fill orbitals in order of increasing energy. The 4s orbital fills before 3d because it has lower energy—this trips up almost everyone.

Electron Capacity Rules

• s orbital: max 2 electrons
• p orbital: max 6 electrons
• d orbital: max 10 electrons
• f orbital: max 14 electrons

Practice Problems With Solutions

Work through these yourself before checking the answers. No peeking.

Problem 1: Carbon (Atomic Number 6)

Carbon has 6 electrons. Write its electron configuration.

Answer: 1s² 2s² 2p²

How you get there:

• 1s² fills first (2 electrons)
• 2s² fills next (2 electrons) — total: 4
• 2p² takes the last 2 electrons — total: 6 ✓

Problem 2: Iron (Atomic Number 26)

Iron has 26 electrons. This one's trickier because of the d orbital.

Answer: [Ar] 4s² 3d⁶

Or written out: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶

Common mistake: students write [Ar] 3d⁶ 4s². The order in the notation doesn't change—4s always fills first, even though we write d orbitals after s in noble gas notation. Electrons in 4s get removed first during ionization, which is why iron commonly forms Fe²⁺ by losing those two 4s electrons.

Problem 3: Sulfur (Atomic Number 16)

Answer: 1s² 2s² 2p⁶ 3s² 3p⁴

Simple. 2 + 2 + 6 + 2 + 4 = 16 electrons. The 3p subshell has 4 electrons, not 6, because sulfur only has 16 electrons total.

Problem 4: Write the shorthand notation for Bromine (Atomic Number 35)

Answer: [Ar] 4s² 3d¹⁰ 4p⁵

Argon is atomic number 18. Bromine has 35 electrons, so you add 17 more: 2 in 4s, 10 in 3d, 5 in 4p.

Problem 5: Identify the element from 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁵

Count the electrons: 2+2+6+2+6+2+10+6+2+10+5 = 53 electrons.

Answer: Iodine (I)

The Exceptions You Need to Watch

Chromium (Cr) and Copper (Cu) break the rules. Don't panic—memorize these two cases:

• Chromium (24 electrons): [Ar] 4s¹ 3d⁵ — NOT 4s² 3d⁴. The half-filled d subshell is more stable.
• Copper (29 electrons): [Ar] 4s¹ 3d¹⁰ — NOT 4s² 3d⁹. The filled d subshell is more stable.

Other elements show similar behavior (molybdenum, silver, gold). The pattern: when a d subshell can reach half-full or fully-filled status, electrons rearrange to get there.

Orbital Diagrams: When Notation Isn't Enough

Sometimes you need to show individual electron spins. Use boxes with arrows:

For nitrogen (7 electrons), the 2p orbitals look like this:

• 2pₓ: ↑
• 2pᵧ: ↑
• 2pᵤ: ↑

Each electron in the 2p subshell gets its own orbital and spins in the same direction (Hund's Rule). Don't pair electrons in the same orbital until you have no choice.

Quick Reference: Orbital Filling Cheat Sheet

How To: Solving Any Electron Configuration Problem

Step 1: Find the atomic number. That's your electron count.

Step 2: Fill orbitals in order of increasing energy. Use the order list above.

Step 3: Don't exceed the max electrons for each orbital type.

Step 4: Watch for exceptions (Cr, Cu, and their heavier analogs).

Step 5: Use noble gas shorthand if the full notation gets long. Find the nearest noble gas and write [He], [Ne], [Ar], [Kr], [Xe], or [Rn], then add what's left.

Common Mistakes That Cost You Points

• Putting 3d before 4s: 4s fills first, always. It's lower in energy.
• Forgetting Hund's Rule: Don't pair electrons until you must.
• Writing exceptions for every element: Cr and Cu are the only common ones you need to memorize for general chemistry.
• Confusing notation: 3p⁶ means 6 electrons in the 3p subshell. The superscript is not an exponent—it's just a count.
• Skipping the noble gas shortcut: Writing out 1s² 2s² 2p⁶... for every problem wastes time on exams.

Why This Matters

Electron configuration predicts chemical behavior. Elements with similar outer electron arrangements react similarly. This is the foundation for understanding periodic trends, bonding, and reactivity.

If you can't write electron configurations reliably, you'll struggle with everything that comes after. That's the bitter truth.

Go practice. The problems above are a start. Find elements from the periodic table and write their configurations until it's automatic.