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Isotopes, Ions and Average Atomic Mass
How to use: pick a mode, answer, then press Check. Keep a periodic table and a calculator next to you.
Quick rules
- Protons = atomic number (the bottom-left number). Protons tell you the element.
- Neutrons = mass number − atomic number.
- Electrons = protons − charge. In a neutral atom, electrons = protons.
- A positive ion (cation) has lost electrons. A negative ion (anion) has gained electrons.
Quick rules
- Isotopes: same protons, different neutrons, so different mass numbers.
- Ions: same protons, different electrons.
- Different protons always means a different element.
- A positive ion keeps the element name: sodium ion. A negative ion ends in -ide: chloride ion.
- Hyphen notation: element name-mass number, like carbon-14.
Question type
Quick rules
- Average atomic mass = sum of (isotope mass × fraction). Fraction = percent ÷ 100. Unit: amu.
- The percents add to 100, so the fractions add to 1.
- Two isotopes with unknown percents: let the fractions be x and (1 − x).
- It is a weighted average, so it sits closest to the mass of the most abundant isotope.
- Answers are accepted within 0.02 amu or 0.3 percentage points.
Element
| Isotope | Mass (amu) | Percent (%) |
|---|
Dot size shows abundance. The line marks the average. Change a percent and watch the line move toward the bigger dot.