Common Mistakes
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Understanding common errors helps you avoid them on tests. This lesson identifies frequent mistakes students make with atomic theory and provides strategies to prevent them.
Top 5 Common Mistakes
Mistake #1: Confusing Atomic Number with Mass Number
The Error: Using mass number to determine electron configuration instead of atomic number.
Correct Understanding:
- Atomic number = number of protons = number of electrons (in neutral atom)
- Mass number = protons + neutrons
- Electron configuration is based on atomic number only
Prevention Strategy: Always look for atomic number (Z) on the periodic table, usually the smaller number above the element symbol.
Mistake #2: Wrong Orbital Filling Order
The Error: Filling 3d before 4s, or getting the order wrong for transition metals.
Correct Understanding:
- 4s fills before 3d (lower energy)
- Correct order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d...
- Use the diagonal rule or aufbau diagram
Prevention Strategy: Memorize the filling order or draw the diagonal filling diagram before writing configurations.
Mistake #3: Reversing Periodic Trend Directions
The Error: Saying atomic radius increases across a period, or ionization energy decreases.
Correct Understanding:
- Atomic radius: DECREASES across (more protons, same shell), INCREASES down (more shells)
- Ionization energy: INCREASES across (smaller atoms, electrons held tighter), DECREASES down
- Electronegativity: INCREASES across, DECREASES down
Prevention Strategy: Remember "Across = Attract more" (smaller atoms, higher IE and EN).
Mistake #4: Forgetting Exceptions in Electron Configuration
The Error: Writing Cr as [Ar] 4s2 3d4 instead of [Ar] 4s1 3d5.
Correct Understanding:
- Chromium (Cr) and Copper (Cu) are common exceptions
- Cr: [Ar] 4s1 3d5 (half-filled d is more stable)
- Cu: [Ar] 4s1 3d10 (full d is more stable)
Prevention Strategy: Memorize "Cr and Cu are exceptions" - they prefer half-filled or full d orbitals.
Mistake #5: Confusing Period and Group Trends
The Error: Applying the period trend when comparing elements in the same group, or vice versa.
Correct Understanding:
- Period = horizontal row (elements have same number of shells)
- Group = vertical column (elements have same valence electrons)
- Different trends apply depending on whether you go across or down
Prevention Strategy: Always identify whether you're comparing across a period or down a group before applying a trend.
Practice: Find and Fix the Errors
Problem 1: Student writes the electron configuration of Fe (atomic number 26) as 1s2 2s2 2p6 3s2 3p6 3d8. Is this correct?
Error: The student filled 3d before 4s and miscounted.
Correct: 1s2 2s2 2p6 3s2 3p6 4s2 3d6 (4s fills before 3d)
Problem 2: Student says: "Na has a larger atomic radius than Mg because Na has more electrons." Is this reasoning correct?
Error: Na actually has fewer electrons than Mg (11 vs 12). The correct reason is that Mg has more protons pulling on the same number of shells, making it smaller.
Correct reasoning: Na is larger because it has fewer protons in the nucleus, so electrons are less strongly attracted.
Problem 3: Student says: "Ionization energy increases as you go down a group because there are more electrons." Is this correct?
Error: Ionization energy DECREASES down a group.
Correct: Going down, electrons are in higher energy levels farther from the nucleus, making them easier to remove (lower ionization energy).
Problem 4: Student writes the electron configuration of Cu as [Ar] 4s2 3d9. Is this correct?
Error: Copper is an exception.
Correct: [Ar] 4s1 3d10. A full d orbital is more stable than 4s2 3d9.
Problem 5: Student says: "Cl has a smaller atomic radius than S because Cl is below S on the periodic table." Is this correct?
Error: Cl is not below S; they are in the same period (period 3). Cl is to the right of S.
Correct reasoning: Cl is smaller because it's to the right of S in the same period (more protons, stronger pull on electrons).
Problem 6: Student determines that K has more valence electrons than Na because K has a higher atomic number. Is this correct?
Error: Both K and Na are in Group 1 and have 1 valence electron each.
Correct: Valence electrons are determined by group number, not atomic number. Both have 1 valence electron.
Problem 7: Student writes the noble gas configuration of sulfur as [Ar] 3s2 3p4. Is this correct?
Error: The student used the wrong noble gas. Argon has 18 electrons, but sulfur only has 16.
Correct: [Ne] 3s2 3p4. Neon (10 electrons) is the noble gas before sulfur.
Problem 8: Student says: "F is more electronegative than Cl because F has more electron shells." Is this correct?
Error: F has fewer electron shells than Cl, not more.
Correct reasoning: F is more electronegative because it's higher in Group 17 (fewer shells, electrons closer to nucleus, stronger attraction for bonding electrons).
Problem 9: Student says carbon has 6 valence electrons because its atomic number is 6. Is this correct?
Error: Atomic number does not equal valence electrons.
Correct: Carbon has 4 valence electrons (it's in Group 14). Its configuration is 1s2 2s2 2p2, with 4 electrons in the outermost shell.
Problem 10: Student predicts that Ca has higher ionization energy than K because Ca has more electrons. Is this correct?
Partially correct: Ca does have higher ionization energy than K, but the reasoning is incomplete.
Better reasoning: Ca and K are in the same period. Ca is to the right with more protons, creating a stronger nuclear charge that holds electrons more tightly.
Check Your Understanding
- What is the most common mistake students make with electron configurations?
- How can you remember which way periodic trends go?
- Why are Cr and Cu exceptions to the normal filling order?
- What should you check before comparing atomic properties of two elements?
Next Steps
- Keep a personal "error log" of mistakes you've made
- Before submitting any answer, check for these common errors
- Take the Unit Quiz to test your mastery