define atomic weight Calculate atomic weight from percent abundance Manipulate the atomic weight equation to calculate miscellaneous unknown variables Distinguish between atomic weight, atomic number, and also mass number


As pointed out in the ahead section, atoms that have the very same atomic number (number that protons), but different mass number (number the protons and also neutrons) are called isotopes(nuclides). There space naturally arising isotopes and also isotopes that room artificially produced. Of every the aspects on the periodic table, just 21 room pure elements. Pure, or monotopic, elements are those aspects with only one naturally emerging isotope. The adhering to lists the 21 pure elements:

(^27_13Al) (^75_33As) (^9_4Be) (^209_83Bi) (^133_55Cs) (^59_27Co) (^19_9F)
(^197_79Au) (^165_67Ho) (^127_53I) (^55_25Mn) (^93_41Nb) (^31_15P) (^141_59Pr)
(^103_45Rh) (^45_21Sc) (^23_11Na) (^159_65Tb) (^232_90Th) (^169_69Tm) (^89_39Y)

Table of the 2.3.1 Monotopic Elements

Isotopic Abundance

Isotopes of a given facet do no all exist in equal ratios. Mercury, for example, has seven naturally developing isotopes: (^196Hg), (^198Hg), (^199Hg), (^200Hg), (^201Hg), (^202Hg), (^204Hg); these have actually the percent natural abundances that 0.146%, 10.02%, 16.84%, 23.13%, 13.22%, 29.80%, and 6.85%, respectively. The is clear the (^202Hg) occurs with biggest abundance, and (^200Hg) is the next many abundant, but the various other isotopes only happen in little traces.

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Note: The amount of the percent natural abundances of every the isotope of any type of given aspect must complete 100%.

Some normally occurring and also artificially produced isotopes room radioactive. Every atoms heavier than Bismuth ((^209_83Bi)) room radioactive. However, there are countless lighter nuclides that are radioactive. Because that example, hydrogen has actually two naturally developing stable isotopes, (^1H) and also (^2H) (deuterium), and a third naturally arising radioactive isotope, (^3H) (tritium).

It must not it is in surprising, however isotopic abundances (% of each isotope) have the right to vary between samples. Below is an exciting IUPAC technological report, "Isotope-Abundance sport of Selected Elements," which explains this,

How carry out we recognize what the percent abundance for each the the isotope of a given element? Isotopes are separated v mass spectrometry; multiple sclerosis traces display the family member abundance of isotopes vs. Mass number (mass : fee ratio).

Measuring Isotopic Abundances

Although us cannot straight measure the fixed of atoms, we have the right to use fixed Spectrometer, an tool that enables us to measure the fixed to fee ratio. In figure 2.3.2 you can see chlorine gas entering an mass spectrometer. The chlorine has multiple isotopes and is hit v a currently of ionization electrons which break the shortcut of Cl2 and also strips electrons off the chlorine causing ions to form. These are then increased down the chamber until they reach a magnetic ar that deflects the particles. The angle of deflection counts on both the fixed of the particle and also the magnetic field strength, v the lighter particles gift deflected much more (the lighter 35Cl+ ions room deflected more than the heavier 37Cl+ ions.) at the finish of the chamber is an departure hole through a detector, and as the magnetic ar intensity is boosted the deflection angle changes, i m sorry separates the particles. Note, the mass spectrum in figure 2.3.2 (b) gives the relative abundance of each isotope, with the optimal normalized to the isotope v the highest abundance. So if this ratio was 3:1 that means there space 3 particles of 35Cl because that every particle of 37Cl, and also the percent abundance would certainly be 75% 35Cl and also 25% 37Cl.


Exercise (PageIndex1)

A fictional facet has two isotopes and also an atomic mass of 131.244 amu. If the first isotope (Isotope 1) has actually a mass of 129.588amu and also the 2nd isotope (Isotope 2) has actually a massive of 131.912 amu, i beg your pardon isotope has the greatest natural abundance?

A) Isotope 1

B) Isotope 2

C) There are equal amounts.

D) Not enough information provided.

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E) nobody of the above


B) Isotope 2. Although it is algebraically feasible to calculate the details percent abundances for both isotopes, over there is not have to spend that much time on this difficulty if you recognize the rule behind it. The mean is 131.244 amu. That looks favor the fixed of Isotope 2 (131.912amu) is closer to the average than the massive of isotope 1 (129.588 amu). This suggests that isotope 2 influenced the average much an ext than isotope 1 and also has a better percent abundance.