Grade: Grade 10 Subject: Science (Chemistry) Unit: Atomic Theory Lesson: 3 of 6 SAT: ProblemSolving+DataAnalysis ACT: Science

Guided Practice

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This lesson provides step-by-step practice with electron configurations and periodic trends. Work through each problem carefully, following the strategies demonstrated.

Writing Electron Configurations - Strategy

  1. Identify the element and its atomic number (number of electrons)
  2. Follow the aufbau principle: fill orbitals in order of increasing energy
  3. Remember the orbital order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p...
  4. Apply Hund's rule: electrons occupy orbitals singly before pairing
  5. Check that total electrons equal the atomic number

Predicting Periodic Trends - Strategy

  1. Identify what trend is being asked about (atomic radius, ionization energy, electronegativity)
  2. Determine if comparing across a period (left to right) or down a group
  3. Apply the trend rule for that property
  4. Consider any exceptions (especially for transition metals)

Worked Examples

Example 1: Writing Electron Configuration

Problem: Write the electron configuration for phosphorus (P, atomic number 15).

Solution:

  1. Phosphorus has 15 electrons
  2. Fill in order: 1s(2) 2s(2) 2p(6) 3s(2) 3p(3)
  3. Check: 2 + 2 + 6 + 2 + 3 = 15 electrons

Answer: 1s2 2s2 2p6 3s2 3p3

Example 2: Comparing Atomic Radius

Problem: Which has a larger atomic radius: sodium (Na) or chlorine (Cl)?

Solution:

  1. Both are in period 3
  2. Across a period, atomic radius decreases (more protons, same energy level)
  3. Na is on the left, Cl is on the right

Answer: Sodium has a larger atomic radius.

Example 3: Predicting Ionization Energy

Problem: Which has higher first ionization energy: lithium (Li) or fluorine (F)?

Solution:

  1. Both are in period 2
  2. Across a period, ionization energy increases
  3. F is to the right of Li

Answer: Fluorine has higher ionization energy.

Practice Problems

Problem 1: Write the electron configuration for oxygen (O, atomic number 8).

Answer: 1s2 2s2 2p4

Check: 2 + 2 + 4 = 8 electrons

Problem 2: Write the electron configuration for calcium (Ca, atomic number 20).

Answer: 1s2 2s2 2p6 3s2 3p6 4s2

Check: 2 + 2 + 6 + 2 + 6 + 2 = 20 electrons

Problem 3: Which has a larger atomic radius: magnesium (Mg) or barium (Ba)?

Answer: Barium (Ba) has a larger atomic radius.

Both are in Group 2. Down a group, atomic radius increases because electrons are in higher energy levels farther from the nucleus.

Problem 4: Arrange in order of increasing electronegativity: C, N, O

Answer: C < N < O

Across a period, electronegativity increases. These are all in period 2, with C on the left and O on the right.

Problem 5: Write the electron configuration for iron (Fe, atomic number 26).

Answer: 1s2 2s2 2p6 3s2 3p6 4s2 3d6

Note: 4s fills before 3d. Check: 2+2+6+2+6+2+6 = 26

Problem 6: Which has higher ionization energy: potassium (K) or calcium (Ca)?

Answer: Calcium has higher ionization energy.

Both are in period 4. Across a period, ionization energy increases.

Problem 7: Write the noble gas shorthand for sulfur (S, atomic number 16).

Answer: [Ne] 3s2 3p4

Neon (Ne) accounts for the first 10 electrons (1s2 2s2 2p6).

Problem 8: Arrange in order of decreasing atomic radius: Li, Na, K

Answer: K > Na > Li

All are in Group 1. Down a group, atomic radius increases.

Problem 9: How many valence electrons does silicon (Si) have?

Answer: 4 valence electrons

Si is in Group 14. Its configuration is [Ne] 3s2 3p2. The valence electrons are in the 3s and 3p orbitals.

Problem 10: Which is more electronegative: bromine (Br) or iodine (I)?

Answer: Bromine is more electronegative.

Both are in Group 17. Down a group, electronegativity decreases.

Check Your Understanding

  1. What is the aufbau principle and why is it important for electron configurations?
  2. How does atomic radius change across a period? Down a group? Why?
  3. How does electronegativity change across a period? Down a group?
  4. What is the relationship between ionization energy and atomic radius?

Next Steps

  • If you scored 8-10 correct, move on to Lab Investigation
  • If you scored 5-7 correct, review examples and try practice again
  • If you scored below 5, revisit Electron Configuration and Periodic Trends lessons