Chapter 5 NotesChapter 5 Notes

The atomic mass of an element is a The atomic mass of an element is a weighted average mass of the atoms weighted average mass of the atoms found in nature.found in nature.

If you were to mass an oxygen atom, If you were to mass an oxygen atom, would it weigh 15.9994 amu?would it weigh 15.9994 amu?

NO! 15.999 amu – THIS IS ONLY AN NO! 15.999 amu – THIS IS ONLY AN AVERAGE OF ALL THE ISOTOPES OF AVERAGE OF ALL THE ISOTOPES OF OXYGEN OXYGEN 1616O, O, 1717O, O, 1818OO

The relative abundance of each The relative abundance of each isotope of an element isotope of an element determines its atomic mass.determines its atomic mass.

Naturally occurring Naturally occurring isotopeisotope

Fractional Fractional abundanceabundance

Mass (amu)Mass (amu)

1111C C 0 0 11.011430 11.011430

1212C C 0.989 0.989 12.000000 12.000000

1313C C 0.011 0.011 13.003355 13.003355

1414C C 0 0 14.003241 14.003241

Three isotopes of oxygen occur in Three isotopes of oxygen occur in nature: oxygen-16, oxygen-17, and nature: oxygen-16, oxygen-17, and oxygen-18. The atomic mass of oxygen-18. The atomic mass of oxygen is 15.9994 because oxygen-16 oxygen is 15.9994 because oxygen-16 is the most abundant.is the most abundant.

QUESTIONQUESTION

If element Z has two naturally occurring If element Z has two naturally occurring isotopes: Z-20 and Z-22 and the atomic isotopes: Z-20 and Z-22 and the atomic mass of Z is 21.5 amumass of Z is 21.5 amu

Which isotope occurs most often in Which isotope occurs most often in nature?nature?

Z-22Z-22 the relative abundance of each isotope the relative abundance of each isotope

in nature is one Z-20 to two Z-22.in nature is one Z-20 to two Z-22.

Thallium has two isotopes, thallium-203 and Thallium has two isotopes, thallium-203 and thallium-205. Thallium’s atomic number is thallium-205. Thallium’s atomic number is 81 and its atomic mass is 204.38 amu.81 and its atomic mass is 204.38 amu.

So, is there more Thallium-203 or Thallium-So, is there more Thallium-203 or Thallium-205?205?

There is more Thallium-205 found in nature.There is more Thallium-205 found in nature. Atoms of both isotopes have 81 protons. Atoms of both isotopes have 81 protons.

Thallium-205 atoms have more neutrons.Thallium-205 atoms have more neutrons.

A sample of oxygen contains three A sample of oxygen contains three naturally occurring isotopes:naturally occurring isotopes:

O168 O17

8 O188

The relative abundances and The relative abundances and atomic masses areatomic masses are::

99.7759% (mass = 15.995 amu)99.7759% (mass = 15.995 amu)

0.037% (mass = 16.995 amu)0.037% (mass = 16.995 amu)

0.204% (mass = 17.999 amu)0.204% (mass = 17.999 amu)

Calculate the average atomic mass of the oxygen.Calculate the average atomic mass of the oxygen.

O168

O178

O188

(percent x mass) + (percent x mass) and (percent x mass) + (percent x mass) and so on….so on….

0.99759(15.99491) + 0.99759(15.99491) + 0.00037(16.99913) +0.00037(16.99913) + 0.00204(17.99916) =0.00204(17.99916) = 15.999 amu15.999 amu

Radioactivity (Ch 25)Radioactivity (Ch 25)

An alpha particle (radiation) is a An alpha particle (radiation) is a helium nucleus.helium nucleus. He4

2

HeThU 42

23490

23892

A beta particle (radiation) is an A beta particle (radiation) is an electron.electron.

e01

eNC 01

147

146

Gamma RadiationGamma Radiation

Gamma radiation is high-frequency Gamma radiation is high-frequency electromagnetic radiation. Gamma electromagnetic radiation. Gamma radiation has no mass or charge so radiation has no mass or charge so there is no change in the atomic there is no change in the atomic number caused by the emission of number caused by the emission of gamma radiation.gamma radiation.

Nuclear equations are balanced.Nuclear equations are balanced.

Complete these nuclear Complete these nuclear equations.equations.

eH 01

31

eAl 01

2813

AcPa 22789

23191

eSe 01

7534

HeNp 42

23793