Atoms and ElementsWhat is chemistry?
“A branch of science which deals with the elementary substances or forms of matter, of which all bodies are composed, the laws that regulate the combination of these elements in the formation of compound bodies, and the phenomena that accompany their exposure to diverse physical conditions”.
How does it interact with others or react with its surroundings ?
Composition
Preparation
Reaction
What is it made of ?
How is it made?
Ex) COFFEE
Composition:
i) Organic Compounds: ii) Inorganic Compounds:
Proteins
AcidsEsters
SugarsCaffeine Pesticides
Dissolved SaltsDissolved minerals
Water
Preparation
Grown
• Biochemical processes make the organic & biological compounds
Roasted
• Heat combined with air burns off undesired compounds &
converts some to those that give flavor• Caffeine is burned off if roasted too long
• Decaffeination
• Hot water poured over powder, where all water soluble compounds dissolve. The liquid is separated from the bean residue by a filtration.
Preparation
• Pulverization of the bean to increase the surface are to aid extraction process
Ground
Extraction
• Makes it more vulnerable to oxidation affecting taste & shelf life
• When it sits the element exposed to the air the organic compounds oxidizes causing a bitter taste.
Reaction with Surroundings
• Stimulant – increases heart rate by promoting adrenaline production
Caffeine
Burns
• Diuretic – stimulates urine production
Atomic Theory
Greeks
Aristotle
Plato
Atom ( A – not, tomos – to cut)
- Revelation of truth through logic
- Cosmic order - Hierarchy of being
Atomic Theory
GreeksFive perfect shapes
TetrahedronCubeOctahedronDodecahedronIcosahedron
Five elements
FireWaterWindEarthEther Technology
Steam Engines Organs Jewelry Reinforced Concrete
EnlightenmentScientific Method
Determinism
Times of Change/Discovery
“ Mechanistic Understanding of the Universe”
- French and American Revolution. - Industrial Revolution
Mechanistic ThinkingMaterialism Earth Centered
Individualism Career Scientist
- Rapid exploration of chemistry began: New Elements Natural Products Synthetic Methods
Lavoisier 1785 “Conservation of mass”
Joseph Proust 1794 “Law of Definite Proportions”
John Dalton 1808 “Atomic Theory of Matter”
1. All matter consists of solid and indivisible atoms.
2. All of the atoms of a given chemical element are identical in mass and in all other properties.
3. Different elements have different kinds of atoms; these atoms differ in mass from element to element.
4. Atoms are indestructible & retain their identity in all chemical reactions.
5. The formation of a compound from its elements occurs through the combination of atoms of unlike elements in small whole-number ratios.
Modifications Required to Daltons Theory1. Atoms can be further divided into subatomic particles.
2. Different isotopes of an element have different masses
3. Valid, However some have very similar masses.
4. In nuclear reactions, atoms do not retain their identity.
5. Valid, however, Dalton was unaware that not all elements are made up of single atoms.
Ex) Radium-226 → Radon-222 + a-4
Ex) Protons, neutrons, electrons
Ex) Carbon-12 12.000 u Carbon-13 13.003 u Carbon-14 14.003 u
Ex) Nitrogen-14 14.003 u. Carbon-14 14.003 u.
Modern Atomic Theory
In the late 19-th and early 20-th century the basic principles of modern atomic theory were laid down
Radioactivity
Electron
Proton/Nucleus
J.J. Thomson 1896
R. A. Millikan 1909
Henri Becquerel 1996
Marie and Paul Currie 1899
Ernest Rutherford 1919
Neutron J. Chadwick 1932
Electrons
Anode: positive electrode
Cathode Ray Tube
Current flows when tube is evacuated
Hole drilled in tube. Gass entering tube glows
Cathode: negative electrode
Cathode Rays
Electron charge-to-mass ratioJ.J. Thomson – 1897 - cathode rays are negatively
charged particles
CRT with electric and magnetic fields applied at right angles
Beam deflects to positively charged plate
Magnetic field applied to deflected beam
Changes in the deflection behaviour allowed the mass to charge ratio of the electron to be determined at 1.7588202 C/kg
Oil Drop ExperimentR Millikan and H A Fletcher (1909)
Accurate measurement of the electron charge.
Balanced the force of gravity with an opposing electric force
The balancing force between droplets had common factor
He surmised that the charge of a single electrone = 1.60217646 10-19 C Applying the charge/mass ratio,
mass of e = 9.1093819 10-31 kg
“Canal Rays” and Protons
+Anode
-Cathodee-
e-
e-
++
+
E Goldstein (1850-1930) discovered canal rays in 1886 using a “reverse cathode ray” tube
Electrons emitted from the cathode hit gas molecules causing ionization into (more) electrons and leaving positively charged “ions” which travel to the cathodeThose that pass through
the hole (“canal”) can be analyzed for charge-mass ratio, which are much smaller than electron, but largest for hydrogen
E. Rutherford determined that the hydrogen cation is a fundamental particle, and named it the proton
Radioactivity
Three fundamental types of nuclear radiation were identified by how they respond to electric fields by E. Rutherford.
Paul and Marie Currie isolated the radioactive elements Radium and Polonium. They postulated that their spontaneously emitted radiation was the result of nuclear disintegration.
Three types of radiation:
alpha, a , beta, b, and gamma, g.
Radioactivity: propertiesFrom their charge-mass ratios and other experiments of these rays were characterized and identifiedAlpha particles: He2+ nuclei m = 4 amu q =+2)Beta particles: electron (e-) (identical to cathode rays)Gamma rays: high-energy light, with wavelengths shorter
than X-rays
Rutherford experimentUsing alpha particles, he bombarded a very thin foil of gold and observed deflections using a circular fluorescent screen
The nuclear atom
Rutherford said of the alpha particles deflected almost straight back.
He tried to prove the plum pudding model of the atom propose by Thomson, which is composed of electrons imbedded in a sphere of uniform positive charge.
Deflection angle and frequency were carefully measured, which led to the conclusions:
1. Most of gold foil is empty space2. There are small centers of highly-positive charge3. Centers have high mass to resist displacement4. Size of atom estimated from distance between centers to be ~10-10 m diameter.5. Size of centers estimated to be ~10-15 m diameterCenters were called the nucleus.
Electrons occupy the volume of the atom outside the nucleus
Constituents of the atomIn 1920 Rutherford predicted the existence of the neutralparticle with mass equal to that of a proton and electron.
In 1932 Chadwick verified experimentally the existence of the neutron
Relative mass of carbon defined t be 12 u
The mass spectrometerMass spectrometer is a variation on the CRT, developed by J.J. Thomson, which allows the determination of m/z ratios of cations.
Cations of differing m/z ratio’s can be selected by adjusting the magnetic field strength
Average atomic mass
35Cl has 17 protons and 18 neutrons
37Cl has 17 protons and 20 neutrons
Isotopes are atoms of the same element that differ in mass due to differences in the number of neutrons
The atomic mass of Chlorine is a weighted average between the two isotopes as:
Atomic Mass = Mass(Cl-35) *frac.(Cl-35) + Mass(Cl-37) *frac.(Cl-37) = (34.968)*(0.7537) + (36.956)*(0.2463) = 35.46 u
Defining an Element The atomic mass unit (u) is defined as one twelfth of the mass of a carbon atom containing six protons, six neutrons and six electrons: 1 u = 1.661 × 10-24 g
The mass of an atom in u will be approximately equal to the combined number of protons and neutrons it contains.
C12
6
mass numbersymbol
atomic numberAtomic number (Z) = # protons
Mass number (A) = # protons + # neutrons
The atomic # determines the identity of the element (optional).
If # p’s = #e’s neutral
If # p’s > # e’s cation
If # p’s < # e’s anion
e.g. Gallium has two naturally occurring isotopes and an average atomic mass of 69.723 u:
Calculate the percent abundance of each isotope of gallium.
69G 71G68.926 u 70.925 u
Exercise
At. Mass = M(69G)*frac(69G) + M(71G)*frac(71G)
frac(69G) + frac(71G) =1 frac(69G) =1- frac(71G) =1-x
At. Mass = M(69G)*(1-x) + M(71G)*x
69.723 = (68.926)*(1-x) + (70.925)*x= 68.926+1.999*x
x =(69.723-68.926)/1.999 = 0.3987 = 39.87 %