Atomic Structure and Isotopes: Question 6

Worked solution

Step 1: Read the nuclide symbol

In ${}^{19}_{9}\text{F}$, the top number, $19$, is the nucleon (mass) number and the bottom number, $9$, is the proton (atomic) number.

Step 2: Count each particle

• Protons $= 9$ (the proton number)
• Neutrons $= 19 - 9 = 10$ (nucleon number minus proton number)
• Electrons $= 9$ (equal to the number of protons in a neutral atom)

Why the other options are wrong

• A: incorrectly sets neutrons equal to protons (9), ignoring the nucleon number entirely.
• C: swaps the proton number and nucleon number, wrongly using 19 as the proton number.
• D: gives one extra electron, as if the atom were an ion, rather than a neutral atom.

Final answer

• Protons $=9$, neutrons $=10$, electrons $=9$, $\boxed{\text{option B}}$.