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 %