Atomic Number and Mass Number - UNSOLVED PRACTICE SET

Q7. Define atomic number and mass number. Write the relationship between them using a formula.

Q8. An element is represented as ⁴⁰₂₀Ca. Identify the atomic number, mass number, number of protons, number of neutrons, and number of electrons in a neutral atom of this element.

Q9. Your Aadhaar card has a unique 12-digit number that identifies you. Your teacher says the atomic number is like the Aadhaar number of an element. Explain this analogy and why no two elements can have the same atomic number.

Q10. If the atomic number of an element is 8 and its mass number is 16, write its representation in the ᴬZX format. How many electrons does a neutral atom of this element have?

Q11. Why does the atomic number determine the chemical identity of an element, while the mass number can vary? Explain with reference to the role of protons and neutrons.

Q12. Complete the following table:

Table

Symbol Atomic Number Mass Number Protons Neutrons Electrons

¹⁶₈O ? ? ? ? ?

²³₁₁Na ? ? ? ? ?

? 17 35 ? ? ?

? 26 56 ? ? ?

Q13. Define atomic number and mass number with examples. Explain the standard notation used to represent an atom (ᴬZX). For the element ³⁵₁₇Cl, determine the number of protons, neutrons, and electrons. Explain why the atomic number is more fundamental than the mass number in determining the identity of an element.

Q14. A student argues: "If I add neutrons to an atom, I change its atomic number, so I create a new element." Another student disagrees. Settle this debate by:

(a) Explaining what happens when neutrons are added to a nucleus.

(b) Defining isotopes and giving two examples.

(c) Explaining why adding neutrons does NOT change the atomic number.

(d) Describing what would actually change the atomic number and create a new element.

Q15. "The atomic number and mass number are like two sides of a coin — together they give us the complete identity of an atom." Discuss this statement by explaining: (i) how the atomic number determines the element's position in the periodic table, (ii) how the mass number helps identify isotopes, (iii) the relationship between these two numbers and the subatomic particles, and (iv) why both numbers are essential in nuclear chemistry and medicine.

Q16. The following data is given for several atoms:

Table

Atom Protons Neutrons Electrons

A 6 6 6

B 6 7 6

C 6 8 6

D 7 7 7

E 8 8 8

F 8 9 8

G 8 10 8

(a) Write the atomic number and mass number for each atom.

(b) Identify which atoms are isotopes of the same element.

(c) Identify which atoms are isobars (same mass number, different atomic number).

(d) If Atom A is carbon-12, what are Atoms B and C called?

(e) Calculate the average atomic mass of the element represented by Atoms A, B, and C if their natural abundances are 98.9%, 1.1%, and trace amounts respectively.

Q17. In a chemistry laboratory, a technician needs to prepare samples of different isotopes for analysis.

(a) How many protons, neutrons, and electrons are in: (i) ¹H (protium), (ii) ²H (deuterium), (iii) ³H (tritium)?

(b) Calculate the mass number of each isotope.

(c) Heavy water (D₂O) contains deuterium instead of normal hydrogen. Calculate the molecular mass of heavy water.

(d) If normal water (H₂O) has a molecular mass of 18 u, how much heavier is heavy water?

(e) Why is heavy water used in some nuclear reactors in India?

Q18. A forensic scientist uses atomic number and mass number to identify unknown substances at a crime scene.

(a) An unknown sample contains atoms with 79 protons and 118 neutrons. Identify the element and write its symbol in ᴬZX notation.

(b) Another sample contains atoms with 47 protons and 60 neutrons. Identify this element.

(c) The scientist finds that a bullet fragment has the same atomic number as lead (Z = 82) but a different mass number. What does this tell her about the bullet?

(d) If the bullet fragment has 82 protons and 124 neutrons, write its complete symbol.

(e) How does understanding atomic number and mass number help forensic scientists solve crimes?