The symbol −10X used to represent which of the particles?

Analyze the question: The symbol is written in nuclear notation, ᴬ_Z X, where A is the mass number (top) and Z is the charge/atomic number (bottom). The question is reproduced as "−10X", which most likely means ⁰₋₁X: mass number 0 and charge −1.

Concept:

• Proton: ¹₁p (mass number 1, charge +1)
• Neutron: ¹₀n (mass number 1, charge 0)
• Electron (beta particle): ⁰₋₁e (mass number about 0, charge −1)
• Atom: has a mass number and an atomic number, and is electrically neutral overall

Solution:

1. The top number is 0, so the particle has almost no mass number. This rules out the proton and the neutron, which both have a mass number of 1.
2. The bottom number is −1, so the particle carries a negative charge of one unit. An atom is neutral and has a nonzero mass number, so it is ruled out too.
3. The only particle with a negligible mass number and a charge of −1 is the electron. This is the same notation used for a beta particle (β⁻) in nuclear equations, for example:

¹⁴₆C → ¹⁴₇N + ⁰₋₁e

Answer: The symbol represents an electron, which is option C.

Exam tip: Memorize the notation for each particle:

• Proton: ¹₁p
• Neutron: ¹₀n
• Electron: ⁰₋₁e
• Alpha particle: ⁴₂He

If the top number is 0, think "electron" (or positron, if the bottom number is +1). If the bottom number is 0 and the top is 1, think "neutron".