GoalKnow isotopes, isobars and isotones; calculate the average relative atomic mass from a mixture of isotopes.
New words
isotopes · izotoplarisobars · izobarlarisotones · izotonlaraverage (mean) value · o‘rtacha arifmetik qiymat
Explanation
Isotopes are kinds of atoms with the same number of protons (same nuclear charge) but different numbers of neutrons; they occupy one cell of the periodic table and have the same chemical properties. Hydrogen has three isotopes: protium ¹H, deuterium ²H and tritium ³H (tritium is radioactive and is used only in special laboratories). Because natural elements are mixtures of isotopes, the relative atomic mass is fractional: Aᵣ = A₁ · w₁ + A₂ · w₂ + …, where w is the fraction of an isotope. For example, copper is a mixture of ⁶³Cu (69 %) and ⁶⁵Cu (31 %), giving Aᵣ ≈ 63.6. Isobars have the same mass number but different nuclear charge (¹⁴C and ¹⁴N), and isotones have the same number of neutrons (¹⁴C and ¹⁶O, both with 8 neutrons).
“Isotope cards”: cards show ¹H, ²H, ³H, ³⁵Cl, ³⁷Cl, ¹⁴C, ¹⁴N, ¹⁶O; label pairs as isotopes, isobars or isotones and write the numbers of protons and neutrons. Paper only.
Practice
1
How do isotopes differ and what do they share?
They differ in the number of neutrons (mass); the number of protons and chemical properties are the same.
2
How many neutrons are in the ³⁷Cl isotope (Z = 17)?
20
3
An element has two isotopes: mass number 79 (50 %) and 81 (50 %). Find the average Aᵣ.
80
4
Why are the relative atomic masses of elements often fractional?
A natural element is a mixture of isotopes of different mass; Aᵣ is their average.