Mixtures of Isotopes In nature, elements occur as a mixture of isotopes.In nature, elements occur as a mixture of isotopes. Average atomic mass = weighted.

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Presentation transcript:

Mixtures of Isotopes In nature, elements occur as a mixture of isotopes.In nature, elements occur as a mixture of isotopes. Average atomic mass = weighted average of isotope massesAverage atomic mass = weighted average of isotope masses On the periodic table, the average atomic mass is below the element name.On the periodic table, the average atomic mass is below the element name.

Natural abundances: the proportion of each isotope of an element found in nature.Natural abundances: the proportion of each isotope of an element found in nature. Atomic mass is calculated using the natural abundance of each isotope.Atomic mass is calculated using the natural abundance of each isotope.

The Mass Spectrometer

Mass spectrometer: an instrument that separates particles by mass, and records the relative proportions of each size particle.Mass spectrometer: an instrument that separates particles by mass, and records the relative proportions of each size particle.

Use: to determine the natural abundances of the isotopes of a particular element and the atomic mass of an element.Use: to determine the natural abundances of the isotopes of a particular element and the atomic mass of an element.

A Thin tube B C D E F

A: vaporizes (gas) substanceA: vaporizes (gas) substance B: particles converted from neutral atoms to positive ions by bombarding (hitting repeatedly) with electrons.B: particles converted from neutral atoms to positive ions by bombarding (hitting repeatedly) with electrons. C: particles accelerated by an electrical fieldC: particles accelerated by an electrical field D: fast-moving ions enter a magnetic field and are deflectedD: fast-moving ions enter a magnetic field and are deflected –smaller mass: deflected more –greater mass: deflected less

E: detector records ions of a particular massE: detector records ions of a particular mass F: vacuum prevents collisions with gas molecules.F: vacuum prevents collisions with gas molecules.

Strength of magnetic field (D) is varied to bring particles of different masses to the detector.Strength of magnetic field (D) is varied to bring particles of different masses to the detector. The abundance of each ion in the sample can be determined because of the different masses of each kind of particle.The abundance of each ion in the sample can be determined because of the different masses of each kind of particle. Mass spectrum: the relative abundance of each mass particleMass spectrum: the relative abundance of each mass particle

Calculating Atomic Mass (percent A x mass number A) + (percent B x mass number B) 100 Ex: Natural chlorine contains 75% 35 Cl and 25% 37 Cl. (75 x 35) + (25 x 37) 100 atomic mass = 35.5 atomic mass = 35.5

Most abundant assigned value of 100, others given a value in proportion to this.

relative atomic mass = (24 x 0.786) + (25 x 0.100) + (26 x 0.113) = 24.3

Next week Where are the electrons around the nucleus?Where are the electrons around the nucleus? How do we know?How do we know? Notation for writing about electron location.Notation for writing about electron location. Also: turning research into a paragraph.Also: turning research into a paragraph.