Atomic Theory

Atomic History

Ancient Greeks John Dalton JJ Thomson Ernest Rutherford James Chadwick Neils Bohr Erwin Schrödinger

Ancient Greeks (~ 400 B.C.E)

Democritus (460 – 370 B.C.E.) All matter is made of tiny,

indestructible units called ATOMOS

Aristotle (384 – 322 B.C.E) & Plato (428 – 348 B.C.E) Completely disagreed with

Democritus. Public opinion sided with these guys that all matter was made of EARTH, AIR, FIRE, & WATER

John Dalton (1766 – 1844) & Dalton’s Atomic Theory John Dalton

Chemist & Physicist who made a living teaching

His theory is backed by many experiments

Theory Elements are made of

particles called atoms All atoms of an element are

identical Atoms of 1 element are diff.

from atoms of another element Atoms of elements can

combine to form compounds with simple, whole number ratios

Atoms can’t be destroyed or created, they’re just rearranged in a chemical reaction

JJ Thomson (1856 – 1940) & the Cathode Ray Experiment (1890s) Physicist who used a

cathode ray tube to show that atoms of any element can be made to give off tiny negative particles (ELECTRONS)

JJ’s Model- The Plum Pudding Model (Chocolate Chip Cookie anyone?)

Ernest Rutherford (1871 – 1937) & Gold Foil Experiment (1911) Ernest

Expected the positive α (alpha) particles to pass straight through to the back of the detector. He was surprised when they bounced off at strange angles.

He surmised that the alpha particles were bouncing off a small + charge in the Gold atoms. He called that part the NUCLEUS!

Gold Foil Experiment

Some More Stuff Ernie Did

In 1918 – He experimented by bombarding N2(g) with alpha particles. 1 of the results was that a whole lot of H2(g) was created. What’s going on? Ernie figured out that the H

atoms must have come from inside the N2. That means that ATOMS ARE DIVISIBLE!!!!!!!! He eventually isolated those H atoms and discovered they were actually PROTONS

Ernie found a friend- James Chadwick (1932). Ernie’s assistant, James Chadwick found the

NEUTRON in 1932. He put some Be in a chamber with Po (gives off alpha

particles). The alpha particles hit the Be which gave off some particles at high, high speeds. At first he thought they were gamma (γ) rays but they moved too fast for gamma rays. He worked his way thru some calculations and discovered they were not, but some neutral particles that he named (NEUTRONS)

Chadwick- Rutherford Model

Neils Bohr (1885 – 1962) and the Bohr Planetary Model

Developed a theory of the H atom, saying that electrons moved in specific ORBITS around the nucleus. Each orbit has a specific amount of energy to it.

Erwin Schrödinger, Albert Einstein, Neils Bohr, & Werner Heisenberg: Modern Model of the Atom (Wave Mechanical Model)

States that electrons exist in a state described by an ORBITAL An area where there is

a 90% probability of finding an electron

ATOMIC STRUCTURE

Physical structure Atomic Number Mass Number Isotopes Atomic Mass Calculating Atomic Mass

Atomic Structure

An atom is the defining part of what makes an element, an element. Cannot be broken down chemically.

Atoms are mostly empty space!

Atoms are very, very small! If an atom were 2 miles

wide, the nucleus would be the size of a baseball!

Atomic Structure

Particle Relative Mass (Actual Mass)

Relative Charge

Proton 1836(1.67262158 × 10-27 kg)

+1

Neutron 1839(1.67492729 × 10-27 kg)

0

Electron 1(9.10938188 × 10-31 kg)

-1

Atomic Number & Mass Number

Atomic Number = # of protons (p+) = # electrons (e-)

Mass Number

= # of p+ + # n0

How many p+, n0, and e-?

238 23

11 Na92 U

Isotopes

Atoms of an element that have different #’s of neutrons.

An element is composed of varying percentages (abundances) of isotopes

Isotopes

Examples1

1H 21H 3

1H

63Li 7

3Li

Atomic Mass

Weighted average of all the masses of each isotope of the element.

A.M. =

(% isotope 1)(Mass isotope 1) + (% isotope 2)(Mass isotope 2) + (% isotope 3)(Mass isotope 3) + ….

Examples

Calculate the average atomic mass of gold 50% of 197Au weighs 197 50% of 198Au weighs 198