Why is the atomic mass on the periodic table almost never a whole number?
Isotopes and average atomic mass: define isotopes, write nuclide notation, and calculate the weighted average atomic mass of an element from its isotopes.
A focused Virginia SOL Chemistry answer on isotopes under CH.2: what isotopes are, how to read nuclide notation, and how to calculate the weighted average atomic mass of an element from the masses and natural abundances of its isotopes.
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What this topic is asking
Continuing standard CH.2, Virginia expects you to explain what isotopes are, to read and write nuclide notation, and to calculate the weighted average atomic mass of an element from the masses and natural abundances of its isotopes. This is why the atomic masses on the periodic table are rarely whole numbers, and the calculation is a recurring SOL skill.
What isotopes are
Carbon, for example, exists as carbon-12 (6 protons, 6 neutrons) and carbon-13 (6 protons, 7 neutrons), plus trace carbon-14 (6 protons, 8 neutrons). All are carbon and all react the same way, because reactions involve electrons, and isotopes of an element have identical electron arrangements. Nuclide notation names an isotope by its mass number, as in "chlorine-37" or with the mass number as a leading superscript.
Why the periodic-table mass is a decimal
The atomic mass printed on the periodic table is not the mass of a single atom. It is the average atomic mass, a weighted mean across all the naturally occurring isotopes of the element, each weighted by how common it is.
A weighted average is not the same as a simple average. Each isotope contributes in proportion to how common it is, so the value lands closest to the most abundant isotope. This is why the average sits at for chlorine (mostly chlorine-35) rather than at , the midpoint of and .
Reading an abundance problem
A typical SOL item gives you each isotope's mass and its percentage abundance and asks for the average. The method is always the same: turn each percentage into a decimal fraction, multiply each isotope mass by its fraction, and add the products. Check that your fractions add up to (or your percentages to ) before you start.
Try this
Q1. Magnesium-24 (mass u) makes up of natural magnesium, magnesium-25 (mass u) makes up , and magnesium-26 (mass u) makes up . Set up the average-atomic-mass calculation. [1 point]
- Cue. .
Q2. Explain why two isotopes of oxygen react the same way chemically. [1 point]
- Cue. They have the same number of electrons and the same electron arrangement, and chemistry depends on electrons, not on the number of neutrons.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SOL (multiple choice)1 marksTwo isotopes of the same element must have the same number of (A) neutrons (B) protons (C) nucleons (D) mass numberShow worked answer →
The answer is (B) protons.
Isotopes are atoms of the same element, so they have the same number of protons (the same atomic number); that is what makes them the same element. They differ in the number of neutrons, which is why they have different mass numbers and different numbers of nucleons (protons plus neutrons). Options (A), (C) and (D) all describe properties that differ between isotopes.
The trap is choosing neutrons or mass number; those are exactly the quantities that differ between isotopes.
SOL (tech-enhanced, fill in blank)2 marksChlorine has two isotopes: chlorine-35 (mass u, abundance ) and chlorine-37 (mass u, abundance ). (a) Set up the calculation for the average atomic mass. (b) Calculate the average atomic mass to one decimal place.Show worked answer →
A 2-point weighted-average item.
(a) Setup (1 point): .
(b) Calculation (1 point): u.
Markers reward converting the percentages to decimals and multiplying each isotope mass by its fraction, then summing. The answer is closer to 35 than to 37 because chlorine-35 is the more abundant isotope, which matches the periodic-table value of about .
Related dot points
- Structure of the atom: describe protons, neutrons and electrons, atomic number and mass number, and the historical development of the atomic model from Dalton to the modern view.
A focused Virginia SOL Chemistry answer on standard CH.2: the subatomic particles, atomic number and mass number, how they define an element and its ions, and the development of the atomic model from Dalton, Thomson and Rutherford to Bohr and the modern model.
- Electron configuration and energy levels: describe how electrons occupy energy levels, write electron configurations, identify valence electrons, and relate ground and excited states to spectra.
A focused Virginia SOL Chemistry answer on electron arrangement under CH.2: energy levels and sublevels, writing electron configurations, counting valence electrons, and the difference between ground state and excited state and how it produces line spectra.
- The periodic table and periodic trends: describe the organization of the periodic table and the trends in atomic radius, ionization energy, electronegativity and reactivity across periods and down groups.
A focused Virginia SOL Chemistry answer on the periodic table under CH.2: how it is organized into groups, periods, metals, nonmetals and metalloids, and the trends in atomic radius, ionization energy, electronegativity and reactivity and why each runs the way it does.
- The mole and molar mass: use the mole, molar mass and Avogadro's number to convert between mass, moles and number of particles.
A focused Virginia SOL Chemistry answer on the mole under CH.3: Avogadro's number, finding the molar mass from the periodic table, and converting between mass, moles and number of particles, the master skill behind all chemical calculations.
- Measurement, significant figures and dimensional analysis: use SI units, significant figures and scientific notation, convert units by dimensional analysis, and calculate density and percent error.
A focused Virginia SOL Chemistry answer on measurement under CH.1: SI units, the rules for significant figures, scientific notation, converting units by dimensional analysis (factor-label), and calculating density and percent error.
Sources & how we know this
- 2018 Science Standards of Learning - Chemistry — Virginia Department of Education (2018)
- Chemistry Curriculum Framework — Virginia Department of Education (2018)