An atom X consist of 6 protons, 6 electrons and 7 neutrons. Which of the following representations of the atom is correct?

Analyze the question We are given the particle counts of atom X (6 protons, 6 electrons, 7 neutrons) and must choose the correct notation. The options are written in a flattened format: "136X" means mass number 13 and atomic number 6, i.e. ¹³₆X. Likewise "137X" is mass number 13, atomic number 7, "196X" is mass number 19, atomic number 6, and "197X" is mass number 19, atomic number 7.

Concept

• Atomic number (Z) = number of protons
• Mass number (A) = protons + neutrons
• Notation: ᴬ_ZX (mass number on top, atomic number at the bottom)

Solution

• Atomic number Z = 6 (protons)
• Mass number A = 6 + 7 = 13
• Representation: ¹³₆X (this is carbon-13)

Checking the options:

• A. ¹³₆X: A = 13, Z = 6. Matches.
• B. ¹³₇X: Z = 7. Wrong, since it would have 7 protons.
• C. ¹⁹₆X: A = 19. Wrong, since it would have 13 neutrons.
• D. ¹⁹₇X: both values are wrong.

Answer: A

Tip: Remember "Top is Total, bottom is Basic": the mass number on top is the total of protons and neutrons, and the atomic number at the bottom is the number of protons. The number of electrons equals the number of protons in a neutral atom, so it does not change the notation.