Isotopes Worksheet - Free Printable

Here's the complete solution to your Isotopes Worksheet, with clear explanations for each part.

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## ✔ Part 1: Short Answer Questions

1. What is an isotope?

> An isotope is a variant of a chemical element that has the same number of protons (same atomic number) but a different number of neutrons, resulting in a different mass number.

🔹 *Explanation:* All atoms of a given element have the same number of protons — this defines the element. Isotopes differ only in their neutron count, which changes the atom’s mass but not its chemical identity.

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2. What does the number next to isotopes signify?

> The number next to an isotope (e.g., Carbon-12, Chromium-58) is the mass number — the total number of protons and neutrons in the nucleus.

🔹 *Explanation:* The mass number = protons + neutrons. It’s written as a superscript before the element symbol (like ¹²C), or after the name (Carbon-12). This number helps distinguish between isotopes of the same element.

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3. How can you tell isotopes apart?

> You can tell isotopes apart by their mass number (or atomic mass), since they have the same number of protons but different numbers of neutrons.

🔹 *Explanation:* Because isotopes have identical proton counts (so same atomic number and chemical behavior), the only way to distinguish them is by measuring their mass — either via mass spectrometry or by knowing their mass number from notation.

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## ✔ Part 2: Fill in the Table — Protons, Neutrons, Electrons

To solve these, remember:

- # of protons = atomic number (from periodic table)
- # of electrons = # of protons (in neutral atoms — assumed unless ion is specified)
- # of neutrons = mass number – atomic number

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🟢 Chromium-58 & Chromium-63

*(Atomic number of Cr = 24)*

| | Chromium-58 | Chromium-63 |
|---------------|-------------|-------------|
| # of protons | 24 | 24 |
| # of neutrons | 58 - 24 = 34 | 63 - 24 = 39 |
| # of electrons| 24 | 24 |

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🟢 Carbon-12 & Carbon-16

*(Atomic number of C = 6)*

| | Carbon-12 | Carbon-16 |
|---------------|-----------|-----------|
| # of protons | 6 | 6 |
| # of neutrons | 12 - 6 = 6 | 16 - 6 = 10 |
| # of electrons| 6 | 6 |

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🟢 Nitrogen-15 & Nitrogen-20

*(Atomic number of N = 7)*

| | Nitrogen-15 | Nitrogen-20 |
|---------------|-------------|-------------|
| # of protons | 7 | 7 |
| # of neutrons | 15 - 7 = 8 | 20 - 7 = 13 |
| # of electrons| 7 | 7 |

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🟢 Sulfur-23 & Sulfur-25

*(Atomic number of S = 16)*

| | Sulfur-23 | Sulfur-25 |
|---------------|-----------|-----------|
| # of protons | 16 | 16 |
| # of neutrons | 23 - 16 = 7 | 25 - 16 = 9 |
| # of electrons| 16 | 16 |

> ⚠️ Note: Sulfur-23 and Sulfur-25 are not stable isotopes — they’re very rare or synthetic — but for worksheet purposes, we calculate based on given mass numbers.

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🟢 Sodium-12 & Sodium-20

*(Atomic number of Na = 11)*

| | Sodium-12 | Sodium-20 |
|---------------|-----------|-----------|
| # of protons | 11 | 11 |
| # of neutrons | 12 - 11 = 1 | 20 - 11 = 9 |
| # of electrons| 11 | 11 |

> ⚠️ Sodium-12 is extremely unstable (not naturally occurring), but again — we follow the math.

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🟢 Selenium-30 & Selenium-35

*(Atomic number of Se = 34)*

| | Selenium-30 | Selenium-35 |
|---------------|-------------|-------------|
| # of protons | 34 | 34 |
| # of neutrons | 30 - 34 = –4? ✘ → Wait! That’s impossible!

⚠️ Problem Alert!

Mass number cannot be less than atomic number — because neutrons can’t be negative.

→ Selenium-30 is NOT possible. The lightest known selenium isotope is Selenium-70 (or around there).

Likewise, Selenium-35 is also unrealistic — actual stable isotopes start at ~Se-74.

But for educational purposes, if the worksheet says “Selenium-30”, we assume it’s a typo or hypothetical.

➡️ If we proceed with the math (ignoring physical impossibility):

| | Selenium-30 | Selenium-35 |
|---------------|-------------|-------------|
| # of protons | 34 | 34 |
| # of neutrons | 30 - 34 = –4 | 35 - 34 = 1 |
| # of electrons| 34 | 34 |

⛔ But scientifically, Selenium-30 doesn't exist — it would require negative neutrons, which is impossible.

✔ Recommendation: Point out this error to your teacher — likely meant Selenium-70 & Selenium-75 or similar.

If forced to fill the table as-is, write:
- Selenium-30: neutrons = –4 (but note: impossible)
- Selenium-35: neutrons = 1

But better to flag it as invalid.

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## ✔ Final Summary Table (Corrected Where Possible)

| Element | Isotope | Protons | Neutrons | Electrons |
|---------------|-------------|---------|----------|-----------|
| Chromium | Cr-58 | 24 | 34 | 24 |
| Chromium | Cr-63 | 24 | 39 | 24 |
| Carbon | C-12 | 6 | 6 | 6 |
| Carbon | C-16 | 6 | 10 | 6 |
| Nitrogen | N-15 | 7 | 8 | 7 |
| Nitrogen | N-20 | 7 | 13 | 7 |
| Sulfur | S-23 | 16 | 7 | 16 |
| Sulfur | S-25 | 16 | 9 | 16 |
| Sodium | Na-12 | 11 | 1 | 11 |
| Sodium | Na-20 | 11 | 9 | 11 |
| Selenium | Se-30 | 34 | –4* | 34 |
| Selenium | Se-35 | 34 | 1 | 34 |

> *Note: Selenium-30 is physically impossible — likely a worksheet error.*

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## 💡 Tips for Future Problems:
- Always look up the atomic number of the element first.
- Mass number = protons + neutrons → so neutrons = mass number – atomic number.
- Electrons = protons (for neutral atoms).
- If mass number < atomic number → impossible (negative neutrons).

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Let me know if you’d like a printable version or flashcards for memorizing atomic numbers! 🧪📚