Atomic Structure: Question 1

Worked solution

Step 1: Read the nuclide notation

In $^{79}_{35}\text{Br}^-$:

• the top-left number, $79$, is the mass (nucleon) number, the total number of protons and neutrons;
• the bottom-left number, $35$, is the proton (atomic) number, the number of protons.

The proton number of an element never changes, so this ion has $\boxed{35}$ protons.

Step 2: Find the number of neutrons

$\text{neutrons} = \text{mass number} - \text{proton number} = 79 - 35 = 44$

Step 3: Find the number of electrons

A neutral bromine atom has the same number of electrons as protons, i.e. 35. The species here is $\text{Br}^-$, a $1-$ ion, which means the atom has gained one extra electron to acquire the negative charge:

$\text{electrons} = 35 + 1 = 36$

Step 4: Why the other options are wrong

• B correctly finds 35 protons and 44 neutrons, but subtracts an electron instead of adding one. This would be the electron count for $\text{Br}^+$, not $\text{Br}^-$.
• C mistakes the mass number (79) for the neutron number, forgetting to subtract the 35 protons already included in that total.
• D swaps the proton and neutron counts, reading the notation upside down.

Final answer

• $\boxed{35}$ protons, $\boxed{44}$ neutrons, $\boxed{36}$ electrons, option A.