l2 chapt2 web

Chemistry 5

Chapter-2 Atoms & Atomic Theory

20 September 2002

Some Cool Chemistry Research Prof. S. Schreiber & Group

1. synchronize mating

2. harvest fertilized eggs

3. array in 96-well plates

4. Add small molecules & screen for phenotypes

Cool Chemistry Research, cont. Prof. S. Schreiber & Group

A V

2:1 atrium:ventricle contraction

cardia bifida (2 hearts)

Me

Cl O

N N

O

Me

NH2 N

(single defect seen at 6 nM-6 µM)

An Indication of Atoms ƒ A. Lavoisier (1743-1794) Key Observation/Finding: • Studied chemical reactions in closed systems and found that mass did not change. • conservation of mass: the total masses of substances present before and after a chemical reaction are the same!

ƒ J. Proust (1754-1826) Key Observation/Finding: • Investigated conversion of materials to different substances via different pathways and found same amount of starting material invariably produced same quantity of product independent of route. • constant composition: all samples of a compound have the same proportions by mass of the constituent elements!

ƒ J. Dalton (1766-1844)

Postulated the following: • Each element is composed of minute, indestructible particles called atoms. • All atoms of a given element are identical; atoms of different elements are different (e.g., different masses). • Atoms are not changed into different types of atoms during chemical reaction; atoms are neither created nor destroyed in a chemical reaction. • Compounds are formed from atoms in simple numerical ratios (e.g., AB and AB2)

Components of Atoms? ƒ Electrons (J.J. Thomson, 1856-1940) cathode ray tube:

• ‘cathode rays’ were independent of metal used as the cathode. • cathode rays deflected in manner consistent with negative particle • obtained mass/charge ratio from experiment

ƒ R. Millikan (1868-1953) Oil drop experiment: • Studied charged droplets under influence of gravity and electric field. • Observed that charge appeared to be quantized in integer values: q = ne • Assigning n=1 to charge of electron, yields mass of electron from m/e ratio!

More than just Electrons! ƒ A hint from radioactive materials: • Radioactive elements emit particle that can be distinguished by behavior in electric & magnetic fields. • Showed that elements consist of more than electrons Rutherford explained these and more elaborate experiments: • mass and positive charge centered in nucleus • positive charge is ca. ½ atomic weight of given element • electrons outside nucleus equal in number to positive charges

ƒ Protons, Neutrons & Electrons • Atoms consist of protons, neutrons and electrons • Each of these ‘fundamental’ particles has well defined charge and mass (see Table 2.1 of text) atomic number, Z: number of protons mass number, A: number of protons and neutrons

Chemical Elements ƒ Elements & Isotopes: • Each element can be defined by its atomic number, Z, or number of protons in nucleus. • The mass number of a given element can vary; elements with different mass numbers are called isotopes Hydrogen Carbon 1H 12C 2H 13C 3H 14C

ƒ Atomic Masses • atomic mass of an element is the average of the isotopic masses, weighted according to naturally occurring isotopic ratios • for example, atomic mass of carbon is 12.011 not 12; it consists of 1.108% carbon-13.

Do Atoms Really Exist? How do we know? ƒ Direct visualization:

Periodic Table

ƒ Some key points: • elements are arranged so that those with similar chemical and physical properties lie in vertical columns called groups or families main group elements transition group elements metals, nonmetals & metalloids • Horizontal rows of table are called periods.

Periodic Table Song There's antimony, arsenic, aluminum, selenium, And hydrogen and oxygen and nitrogen and rhenium, And nickel, neodymium, neptunium, germanium, And iron, americium, ruthenium, uranium, Europium, zirconium, lutetium, vanadium, And lanthanum and osmium and astatine and radium, And gold and protactinium and indium and gallium, And iodine and thorium and thulium and thallium. There's yttrium, ytterbium, actinium, rubidium, And boron, gadolinium, niobium, iridium, And strontium and silicon and silver and samarium, And bismuth, bromine, lithium, beryllium, and barium. There's holmium and helium and hafnium and erbium, And phosphorus and francium and fluorine and terbium, And manganese and mercury, molybdenum, magnesium, Dysprosium and scandium and cerium and cesium. And lead, praseodymium, and platinum, plutonium, Palladium, promethium, potassium, polonium, And tantalum, technetium, titanium, tellurium, And cadmium and calcium and chromium and curium. There's sulfur, californium, and fermium, berkelium, And also mendelevium, einsteinium, nobelium, And argon, krypton, neon, radon, xenon, zinc, and rhodium, And chlorine, carbon, cobalt, copper, tungsten, tin, and sodium. These are the only ones of which the news has come to Ha'vard, And there may be many others, but they haven't been discavard. Words by Tom Lehrer Music by Sir Arthur Sullivan

Moles & Avogadro ƒ What is a mole? • A mole (not an animal to us) is the amount of a substance that contains the same number of atoms or molecules as there are carbon-12 atoms in 12 grams of carbon-12. • That number is big– 6.02 x 1023/mol – and is defined as the Avogadro constant, NA.

ƒ Avogadro’s number– how big is it? • VERY! Consider a mole of gold (Au), and make an atomic chain– how long would that chain be? Au-Au distance = 0.3 nm x 1m/109nm = 3 x 10-10 m Chain length = (1-mole)x(3x10-10 m)x(6.0x1023/mole)x(1km/1000m) = 1.8 x 1011 km or 100 billion kilometers!

For comparison, the distance from the Earth to the Sun is 1.5x108 km…….10,000 times smaller than the Au-atom chain made from 1-mole.