An atom W has 17 electrons and 18 neutrons. Which of the following representations of the atom is correct?

Analyze the question

We are told atom W has 17 electrons and 18 neutrons, and we must pick the correct atomic notation. The options are written without superscripts and subscripts, so "3517W" means mass number 35 on top and atomic number 17 at the bottom: ³⁵₁₇W.

Concept

Standard notation is ᴬ_ZX, where:

• Z = atomic number = number of protons
• A = mass number = protons + neutrons
• In a neutral atom, the number of electrons equals the number of protons.

Solution

1. The atom is neutral, so protons = electrons = 17. This means Z = 17.
2. Mass number A = protons + neutrons = 17 + 18 = 35.
3. The notation is therefore ³⁵₁₇W (this is chlorine-35).

Checking the options

• A. ¹⁷₁₈W: mass number 17 and atomic number 18. Wrong, since the mass number can't be smaller than the atomic number here.
• B. ¹⁸₁₇W: mass number 18. Wrong.
• C. ³⁵₁₇W: mass number 35, atomic number 17. Correct.
• D. ³⁵₁₈W: atomic number 18 would mean 18 protons. Wrong.

Tip

Remember "big number on top is the mass (protons + neutrons), small number at the bottom is the proton count." In the options, find the one where top − bottom = neutrons (35 − 17 = 18 ✔) and the bottom equals the electrons (17 ✔).