Inside the Atom

A Z

X

Nuclear Symbol

A = Mass Number: Total sum of protons + neutrons in the nucleus.

Z = Atomic Number: Total count of protons. Uniquely identifies the element!

X = Element Symbol: 1 or 2 letter chemical symbol from Periodic Table.

Number of Protons

\(p^+ = Z\)

Number of Neutrons

\(n^0 = A - Z\)

Number of Electrons

\(e^- = p^+\) (neutral)

Guided Practice: Fill In The Blanks

Example 1: Carbon-14 \(^{14}_6\text{C}\)

Mass Number (\(A\)): 14

Atomic Number (\(Z\)): 6

Protons (\(p^+\)):

Neutrons (\(n^0\)):

Electrons (\(e^-\)):

Example 2: Potassium-40 \(^{40}_{19}\text{K}\)

Mass Number (\(A\)): 40

Atomic Number (\(Z\)): 19

Protons (\(p^+\)):

Neutrons (\(n^0\)):

Electrons (\(e^-\)):

Unit 2: Inside the Atom Page 2 of 2

Part 4: Comparative Analysis & Applications

9. Isotope Comparison (Chlorine-35 vs. Chlorine-37):

Both Chlorine-35 and Chlorine-37 react vigorously with sodium metal to form common table salt (\(\text{NaCl}\)).

a) What subatomic property makes their chemical reactions identical?

b) What physical property is different between them?

10. Average Atomic Mass Reasoning:

On the periodic table, chlorine's atomic mass is listed as \(35.45\text{ amu}\). If an individual chlorine atom must have a whole-number mass (either 35 amu or 37 amu), explain why the periodic table value is a decimal.

11. Isotope Identification Challenge:

An unknown neutral atom has 26 protons, 30 neutrons, and 26 electrons. Write its complete isotopic symbol (\(^A_Z\text{X}\)) and identify the element.

Element Name:

Nuclide Symbol (\(^A_Z\text{X}\)):

Isotope Investigators • Practice Worksheet Page 2 of 2

Part 4: Scientific Reasoning & Explanation (7 pts) Claim • Evidence • Reasoning

5. The Nature of Scientific Models (4 pts):

When Ernest Rutherford published his nuclear model in 1911, he proved that Thomson's Plum Pudding model was inaccurate. Does this mean Thomson was a "bad scientist"? Explain how scientific models function and why replacing an older model represents progress rather than failure.

6. Medical Isotopes & Identity (3 pts):

Cobalt-60 is a radioactive isotope used in cancer radiotherapy, while standard Cobalt-59 is non-radioactive. Explain why both isotopes behave identically in the human body's biochemical pathways despite one having an unstable nucleus.

Atomic Mastery Assessment • Form A Page 2 of 2

Q9b (Different Property): Different mass / density / nuclear stability (2 extra neutrons in Cl-37).

Q10 (Decimal Atomic Mass): \(35.45\text{ amu}\) is a weighted average of all naturally occurring isotopes based on their percent relative abundance (~75.77% Cl-35 and ~24.23% Cl-37).

Q11 (Identity Challenge): Element is Iron (Fe); Nuclide Symbol is \(^{56}_{26}\text{Fe}\) (Mass = \(26 + 30 = 56\)).

Teacher Answer Key • Inside the Atom Page 1 of 2

Teacher Resource • Assessment Key

Atomic Mastery Exam • Answer Key

Total: 35 Points

Part 1: Multiple Choice (12 pts)

1. C: Majority passed straight through (mostly empty space)

2. B: Same protons (\(Z\)), different neutrons (\(A\))

3. A: Emission line spectra from quantized electron transitions

4. C: 35 protons, 45 neutrons (\(80-35\)), 35 electrons

Part 2: Model ID Key (8 pts)

Feature A: Thomson Plum Pudding

Feature B: Bohr Planetary Model

Feature C: Dalton Solid Sphere

Feature D: Quantum Mechanical Cloud

Part 3: Quantitative Calculation Key (8 pts)

Part 4: Scoring Rubric & Common Student Misconceptions

Q5 (4 pts) Rubric:

• 1 pt: Recognizes Thomson was not a "bad scientist".
• 1 pt: Explains Thomson used the best data of his era.
• 2 pts: Articulates that scientific models are revised when new evidence (gold foil experiment) emerges.

Watch for Misconceptions:

• Confusing Mass # with Atomic Mass: Mass number is a whole integer (\(p+n\)); atomic mass is a decimal average.
• Altering Protons for Isotopes: Changing proton count changes the element altogether!

Teacher Answer Key • Inside the Atom Page 2 of 2