Lecture 3

# Lecture 3 - Fractions of different isotopes not universal...

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Fractions of different isotopes not universal

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Atomic Masses The mass spectrometer measures atomic and molecular masses by (i) ionizing the sample using electron bombardment (or some other means) (ii) accelerating the ions through a magnetic field, and (iii) measuring the amount of deflection, which depends on the mass/charge ratio.
FIGURE 3.2: Mass Spectrum of Neon

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Atomic masses are reported relative to the mass of 12 C being defined as exactly 12 atomic mass units (amu). However, the atomic mass of carbon listed in tables is 12.01 amu. The discrepancy is a consequence of the fact that natural abundance carbon exists as a mixture of isotopes: 12 C (98.89%) & 13 C (1.11%). Therefore, for stoichiometry purposes, the atomic mass that is used is the weighted average for 12 C (12.0000 amu) and 13 C (13.0034 amu). Average atomic mass of natural abundance carbon = (0.9889 x 12.0000) + (0.0111 x 13.0034) = 12.01 amu
The Mole: Weighing and Counting Molecules Since we cannot count atoms or molecules individually, we determine how many are present by weighing the sample and using a conversion factor: Avogadro's number (N A ). Avogadro's number is defined as the number of atoms in exactly 12 g of 12 C. N A = 6.022 x 10 23 The mass (in grams) of Avogadro's number of any element is numerically equal to the relative atomic mass of that element. The same concept applies to molecules: the mass of N A molecules is equal to the relative molecular weight.

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Sample Question What is the mass of 6 atoms of americium (Am, 243 amu)? 1 mole of Am has a mass of 243 g 6.02 x 10 23 atoms of Am have a mass of 243 g 1 atom of Am has a mass of (243/6.02 x 10 23 ) g 6 atoms of Am have a mass of 6 x (243/6.02 x 10 23 ) = 2.42 x 10 –21 g
Quantum Mechanics and Atomic Theory (Chapter 12) Rutherford demonstrated that most of an atom contains empty space and that most of the mass resides in a positive nucleus which is surrounded by electrons. Rutherford proposed a "planetary" model for the atom in which the motion of the electrons around the atom prevented the electrons from collapsing onto the nucleus. .. but this simple model was to face some problems. Rutherford's atom positively charged nucleus consists of positive protons and neutral neutrons negatively charged electrons

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Unlike a planet, an electron has a charge. Acording to classical mechanics, an electric charge that undergoes acceleration (changes velocity and direction) will emit electromagnetic radiation and lose energy. Therefore, an electron moving in a circular orbit must lose its energy and spiral into the nucleus. Death Spiral of Electron
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## This note was uploaded on 11/03/2011 for the course MATH 1090 taught by Professor Greenwood during the Spring '08 term at MIT.

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Lecture 3 - Fractions of different isotopes not universal...

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