Two atoms can share a proton number and still weigh differently. Isotopes have the same electrons, so they react identically, but their extra neutrons show up in every physical measurement and in the decimals of the Periodic Table.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, so the same proton number Z and a different nucleon number A. They have the same electron configuration and therefore the same chemical properties. Physical properties that depend on mass, such as density and rate of diffusion, differ slightly. Relative atomic mass is the weighted mean of the isotopic masses, which is why it is rarely a whole number.
Set the abundance of each isotope of chlorine in the simulator and watch the relative atomic mass move. Then switch element and predict, before you look, whether the average mass sits nearer the light or the heavy isotope.
Chemical reactions involve the electrons: losing them, gaining them, sharing them. The neutrons in the nucleus take no part.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Isotopes of the same element differ in their number of:
Isotopes of an element have the same chemical properties because they have the same:
An atom of 37Cl compared with an atom of 35Cl has:
The nuclide 146C contains:
Write the nucleon number top left and the proton number bottom left. Relative atomic mass, Ar, is the weighted mean of the isotopic masses, treated fully in Topic 2.
Chlorine is about 75% 35Cl and 25% 37Cl, so Ar = (75 × 35 + 25 × 37) / 100 = 35.5. The average sits three times closer to 35 than to 37 because the lighter isotope is three times more abundant.
Isotopes have the same number of protons and electrons; only the neutron number differs. Saying that isotopes have a different atomic number, or a different number of electrons, loses the mark. And the mass number is a property of one atom; the relative atomic mass is a property of the natural mixture.
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Which pair of species are isotopes of each other?
A sample of chlorine is 75.0% 35Cl and 25.0% 37Cl. Its relative atomic mass is:
Which property differs between 35Cl2 and 37Cl2?
Boron has two isotopes, 10B and 11B, and a relative atomic mass of 10.8. The percentage abundance of 11B is:
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