Chemistry worksheet focusing on isotopes, atomic number, mass number, and electronic configuration.

Let’s go through each question one by one.

---

Question 1:
> The atoms of an element which have the same number of protons and different number of neutrons are called ______

- Atoms of the same element must have the same number of protons (that’s what makes them that element).
- If they have different numbers of neutrons, their mass changes, but they’re still the same element.
- These are called isotopes.
- Isobars = same mass number, different atomic number.
- Isotones = same number of neutrons.
- Isomers = same molecular formula, different arrangement (usually in chemistry/organic).

✔ Correct answer: (a) isotopes

---

Question 2:
> Isotopes of an element have ______ atomic number but ______ mass numbers.

- Atomic number = number of protons → same for all isotopes of an element.
- Mass number = protons + neutrons → different because neutrons differ.
→ So: same atomic number, different mass numbers.

✔ Correct answer: (c) same, different

---

Question 3:
> The electronic configuration of isotopes of an element is ______.

- Electronic configuration depends on the number of electrons.
- In neutral atoms, number of electrons = number of protons.
- Isotopes have same protons → same electrons → same electron configuration.
→ So it’s same.

✔ Correct answer: (a) same

---

Question 4:
> An element with atomic number equal to one, exists in three isotopes namely ¹H₁, ²H₁ and ³H₁. Which out of these has only one electron in its outermost shell?

- Atomic number = 1 → Hydrogen.
- All hydrogen isotopes have 1 proton → in neutral atom, 1 electron.
- That 1 electron is in the first (and only) shell → so it’s the “outermost” shell.
- This is true for ALL three: ¹H, ²H (deuterium), ³H (tritium).
→ So answer is all the three.

✔ Correct answer: (d) all the three

---

Question 5:
> The atoms which have the same mass number but different atomic numbers are called ______

- Same mass number → total protons + neutrons is same.
- Different atomic number → different number of protons → different elements.
- These are called isobars.
- Example: Argon-40 and Calcium-40 — both mass 40, but different elements.

✔ Correct answer: (a) isobars

---

Final Answer:
1. (a) isotopes
2. (c) same, different
3. (a) same
4. (d) all the three
5. (a) isobars