Chemistry · Ch 2 — Structure of Atom
Isobars and Isotopes
Isobars and Isotopes
Representing the Composition of an Atom
Every atom can be described completely by two numbers: its atomic number and its mass number. The standard notation places the mass number as a superscript and the atomic number as a subscript, both on the left side of the element symbol:
Here, X is the element symbol, Z is the atomic number (number of protons), and A is the mass number (total number of protons and neutrons). This notation tells you exactly what the nucleus contains.
Isobars — Same Mass, Different Elements
Isobars are atoms that have the same mass number but different atomic numbers. Because the atomic numbers differ, these atoms belong to different elements. They share the same total number of nucleons (protons + neutrons), but the split between protons and neutrons is different.
A classic example from the textbook is carbon-14 and nitrogen-14:
Both have A = 14, but carbon has Z = 6 while nitrogen has Z = 7. The carbon nucleus contains 6 protons and 8 neutrons; the nitrogen nucleus contains 7 protons and 7 neutrons.
The word "isobar" comes from the Greek isos (equal) and baros (weight). The mass numbers are equal, but the elements are different.
Isotopes — Same Element, Different Mass
Isotopes are atoms of the same element that have the same atomic number but different mass numbers. Since Z is identical, they have the same number of protons (and therefore the same number of electrons in a neutral atom). The difference in mass number comes entirely from a different number of neutrons in the nucleus.
From equation (2.4) in the textbook — which states that the number of neutrons N = A − Z — it follows directly that if Z is fixed but A varies, the number of neutrons must be what changes. This is the defining feature of isotopes.
The Three Isotopes of Hydrogen
Hydrogen provides the clearest illustration. The textbook gives these exact percentages:
| Isotope | Symbol | Protons | Neutrons | Natural Abundance |
|---|---|---|---|---|
| Protium | 1 | 0 | 99.985% | |
| Deuterium | 1 | 1 | 0.015% | |
| Tritium | 1 | 2 | Trace amounts |
Protium is the ordinary hydrogen atom — just a single proton with no neutron. Deuterium has one proton and one neutron; its symbol is sometimes written as D instead of H. Tritium has one proton and two neutrons; it is radioactive and found only in trace amounts on Earth.
Other Common Examples
Carbon has three naturally occurring isotopes. All have 6 protons, but the number of neutrons varies:
Chlorine has two stable isotopes. Both have 17 protons, but the neutron count differs:
Do not confuse the notation. In , the superscript 35 is the mass number (protons + neutrons), not the number of neutrons. The number of neutrons is 35 − 17 = 18.