Separate proton identity,nuclear mass count and charge

Atomic number is the proton count. Mass number counts protons plus neutrons, while ion charge reflects the imbalance between positive protons and negative electrons. These three descriptions answer different questions. Do not add electrons to mass number or use neutron count as the element’s identity. Mass number is also not the weighted average atomic mass printed for a natural mixture of isotopes. Worked example: an abstract particle record contains 8 protons, 10 neutrons and 10 electrons. Its atomic number is 8, and its mass number is 8+10=18. In elementary-charge units, the net charge is 8−10=−2. The excess electrons explain the negative sign. Nothing in this arithmetic changes the number of protons merely because the particle has gained electrons. Use three labeled rows before looking at an answer: Z=protons, A=protons+neutrons, and charge=protons−electrons. Check each part of a compound answer independently. One correctly calculated field does not rescue another wrong field. The original diagnostic supplies all required counts; you do not need a clinical reference range or a memorized atomic weight. This is a foundational chemistry model, not advice about radioactive materials, chemical handling or human physiology.

WORKED EXAMPLE

A particle has 17 protons, 18 neutrons and 18 electrons. Which record gives its atomic number Z, mass number A and net charge?

Z counts 17 protons. A counts the nucleus: 17+18=35. Net charge in elementary-charge units is 17−18=−1. Changing electrons affects charge without changing the proton-defined element identity.

Answer: Z=17; A=35; charge=−1

Compare the other answer choices

Z=18; A=35; charge=−1

Atomic number counts protons, not neutrons or electrons.

Z=17; A=36; charge=−1

Mass number is protons plus neutrons: 17+18=35. Electrons are not added to mass number.

Z=17; A=35; charge=+1

There is one more electron than proton, so the net charge is negative, not positive.

Try it, then explain it.

Complete a short set without looking at the example. After each answer, explain which evidence or rule supports your decision and why the alternatives do not fit.

Practice this skill

Sources & authorship

Original Slope explanations and examples. References support the concepts; they are not the source of copied questions. Independent educator review is pending.

Averill/LibreTexts—Isotopes and Atomic Masses Harper College/LibreTexts—Atomic Structure

Separate proton identity,nuclear mass count and charge · HESI A2