INVESTIGATING CHEMISTRY A FORENSIC SCIENCE PERSPECTIVE second

edition

CHAPTER 3:

ATOMIC CLUES

Copyright © 2009 by W. H. Freeman and Company

Matthew E. Johll

This presentation authored by Kenneth A. French, PhD, Blinn College

CASE STUDY: TO BURN OR NOT TO BURNA gruesome scene in Georgia

• In Georgia, in 2002, 334 bodies were found on a grisly landscape.

• Who could be responsible for all this?

• Families paid for cremations and did get urns with ashes.

• But did those urns all contain the remains of loved ones? Or what?

• Aristotle - believed that matter could be divided infinitely without changing its basic nature.

• Lavoisier founded modern chemistry by laying the foundation for the law of conservation of matter - reactant and product masses equal

• Joseph Louis Proust proposed the law of definite proportions – combine in set ratios

DALTON’S ATOMIC THEORYDALTON’S ATOMIC THEORY

1. Matter is composed of 1. Matter is composed of tiny, indivisible particles tiny, indivisible particles called atomscalled atoms..

2.2. Atoms can’t be created, destroyed, or transformed Atoms can’t be created, destroyed, or transformed into other atomsinto other atoms in a chemical reaction.in a chemical reaction.

3. All the atoms of a given element are identical. (Still true except for isotopes, which had not yet been discovered.)

4. Atoms combine in simple whole- number ratios to form compounds.

This reaction drives some fuel cells, but burning H2 also gives NOx when N2 and O2 in the air unite at high temperature.

2H2 + O2 2H2O

Atoms combine in simple whole- number ratios to form compounds.

THE LAW OF MULTIPLE PROPORTIONS

• Dalton’s theory led to the law of multiple proportions:– Any time two or more elements combine in different

ratios, different compounds are formed.

– An example is: H2O and H2O2 (water and hydrogen peroxide)

NO NO2 N2O N2O3 N2O4 N2O5

3.4 ATOMIC STRUCTURE: SUBATOMIC PARTICLES

• Atoms are composed of protons, neutrons, and electrons.• Electrons were discovered in the 1850’s as cathode rays in a cathode

ray tube at low pressure, high voltage.

In 1897, J. J. Thomson measured the charge to mass ratio of the electron.

CATHODE RAY TUBE:No Deflection Upward Deflection Downward

Deflection

3.4 ATOMIC STRUCTURE: SUBATOMIC PARTICLES

• In 1909, Ernest Rutherford bombarded thin gold foil with alpha particles and was very surprised to see them scattered or deflected; should not happen.

• In science a flawed theory is overturned when a new theory is found to be more correct.

• Rutherford reasoned that the very massive alpha particles, which have a positive charge, had encountered an even more massive, positively charged entity within the atom.

• He suggested that atoms have a nucleus where all the positive charge and most of the mass reside.

• Rutherford - The nuclear model of the atom.

A source of alpha particles (He nuclei) was allowed to impact gold foil. Instead of always passing right through as expected, a few particles were scattered at odd angles, and some even came backwards!

Notes• Dalton’s atomic theory – 4 postulates

• Law of Conservation of Matter (LCM) Lavoisier

• Law of Definite Proportions (LDP)

• Law of Multiple Proportions (LMP)

• Thompson – mass to charge,

• Rutherford – mostly empty space, dense nucleus

• Millikan – oil drop experiment, electronic charge

• Isotopes – radioactivity (spontaneous emission of particles)

SUBATOMIC PARTICLESSUBATOMIC PARTICLES

• Electrons and protons have the same size charges but very different masses.

• Neutrons and protons reside in the nucleus together.• Neutrons have no charge but are comparable in mass

to the protons.

Hydrogen is unique in having different names for its isotopes: Protium: Ordinary hydrogen, It has 1 proton, 1 electron, 0 neutrons. Deuterium: Heavy hydrogen, 1 proton, 1 electron, 1 neutronsTritium: 1 proton, 1 electron, 2 neutrons

3.5 ISOTOPES

Same #

Carbon-14 is represented 146C. Subtracting Z from A gives the

number of neutrons. In this case, 14 – 6 = 8 n.

Figure 3.9, pg. 66

Investigating Chemistry, 2nd Edition

© 2009 W.H. Freeman & Company

ISOTOPES: A SAMPLE PROBLEM

• How many neutrons are there in barium-136?• Solution: Barium, Ba, is element 56, that is, it

contains 56 protons. Its atomic number is therefore 56.

• Its mass number is 136 (the sum of its protons and its neutrons in this isotope).

• Answer: Subtracting 56 from 136 we get 80 neutrons.

Notes• 1. Fill in the blank spaces and write out all the

symbols in the left hand column in full, in the form   (i.e., include the appropriate values of Z and A as well as the correct symbol X).

• Symbol # protons # neutrons # electrons• … 17 18 …• Au … 118 …• … … 20 20

3.7 MATHEMATICS OF ISOTOPE ABUNDANCE AND ATOMIC MASS

• Each element is composed of isotopes.• There is a uniform ratio of these on Earth. It is called

the natural abundance.• Use weighted average

– The atomic mass is the sum of the masses of each of the The atomic mass is the sum of the masses of each of the isotopes multiplied by its fraction found in nature.isotopes multiplied by its fraction found in nature.

• For silver 51.8% of the atoms found have a mass no. of 107, and 48.2% have a mass no of 109.

• Here is the equation:– Atomic Mass = (mass of isotope A)(fraction of

isotope A) + (mass of isotope B)(fraction of isotope B) + …

• For Silicon, atomic mass 28.09 it is:– Atomic Mass of Si = (27.97693)x(0.9223) +

(28.976495)x(0.0467) = 28.09• 92.23% is Si-28 and 4.67% is Si-29.

3.7 MATHEMATICS OF ISOTOPE ABUNDANCE AND ATOMIC MASS

• Through a prism, white light produces all the colors of the rainbow, a continuous spectrum. [ROYGBIV]

• Atoms excited in a flame or by high voltage produce what seems to be one color, but when passed through a prism gives a set of bright lines called a bright line or emission spectrum, not a continuous spectrum.

• At room temperature, the electrons in atoms are in their lowest energy levels, the ground state. When atoms absorb energy, the electrons can enter an excited state.

• When the electrons move from the excited state back to their ground state, energy is released as a photon of light (one line).

• The energy states resemble not a ramp but are analogous to an irregular set of stairs.

3.8 ATOMIC STRUCTURE: ELECTRONS & EMISSION SPECTRA

• Since several excited states exist, each atom produces a unique set of bright lines, one line for each color of photon.

• So each element can be identified by the color it produces in the flame, but even better by the pattern of lines in its spectrum.

• Not all atoms produce visible spectra. The photons may correspond to ultraviolet light or other forms of electromagnetic radiation.

3.8 ATOMIC STRUCTURE: ELECTRONS AND EMISSION SPECTRA

- Passing the red-orange light from excited neon atoms through a prism gives a bright line spectrum (top). Passing sunlight or other white light through a prism gives instead a continuous spectrum of all the colors.- Since several excited states exist, each atom produces a unique set of bright lines, one line for each color of photon.- So each element can be identified by the color it produces in the flame, but even better by the pattern of lines in its spectrum.- Not all atoms produce visible spectra. The photons may correspond to ultraviolet light or other forms of electromagnetic radiation.

Emission spectra can be used to ID elements because each atom has its own distinct line spectrum.

Sodium, Na, looks intensely yellow in a flame.Potassium, K, is lilac or violet in a flame.Cesium, Cs, is pale violet in a flame.

3.9 MATHEMATICS OF LIGHT

• Nu (ν) is the frequency in cycles per second (1/s) or hertz (Hz). It corresponds to how many waves go past a fixed point each second.

• The shorter the wavelength, λ, the more waves that go by each second.

• c = wavelength times frequency

• Sample Problem: If the lead from gunshot residue is detected by light emitted at 220.4 nm, what frequency does this correlate with? 1 nm = 109 m

• Solution: c = lambda nu (or λ x ν)• So nu = c / lambda • Frequency = (3.00 108 m/s) / 220.4 nm• Answer: 1.36 1015 Hz (or s1) (cycles/second)

3.9 MATHEMATICS OF LIGHT

• The energy of a photon E = h nu

• The h represents Planck’s constant , The Energy = 6.626 1034 J . s 1.361 1015 (1/s)

• The dual nature of light (or any electromagnetic radiation) allows us to determine the photon’s energy, E.

• Frequency units: Hz = 1/s or cps = s1.

• Sample Problem: Find the energy of a single photon from a lead atom if the photon has a frequency of 1.361 1015 Hz.

• So E for this photon is 9.018 1019 J.

3.9 MATHEMATICS OF LIGHT

29. Boron has how many protons?  A) 2  B) 3  C) 4  D) 5

33. The isotope of cobalt with the symbol 59

27Co has how many electrons?  A) 33  B) 32  C) 59  D) 27

How many electrons are in atoms of Co?

  A) 27  B) 28  C) 58  D) 59