The Atom Topic 2.1

History• this is NOT IB material

• it is very interesting from a geeky-science stand point

• it will help you understand and appreciate the structure of the atom

• you are not responsible for knowing the information from the following scientists

• however, it would be “embarrassing” if you went through this course and never heard of these guys

Aristotle Aristotle (460 B.C. – 370 B.C.)(460 B.C. – 370 B.C.)

• emphasized that nature consisted of four elements: air, earth, fire, and water

• did not believe in discontinuous or separate atoms, but felt that matter was continuous

Democritus (460 B.C. – 370 B.C.)Democritus (460 B.C. – 370 B.C.)

• first to suggest the existence of “atoms” (Greek word “atomos” = indivisible)

• atoms are indivisible and indestructible

• no experimental support

http://www.stenudd.com/myth/Greek/images/democritus_1628_Brugghen.jpg

John Dalton (1766-1844)John Dalton (1766-1844)

• used scientific method to test Democritus’s ideas

• Dalton’s atomic theory1. elements composed of atoms2. atoms of the same element are alike3. different atoms can combine in ratios to form

compounds4. chemical reactions can occur when atoms are

separated, joined, or rearranged (but atoms are not created nor destroyed)

J.J. Thompson (1856-1940)J.J. Thompson (1856-1940)

• discovered the electron

• thought atom was negative charges stuck in a positive charged lump– referred to as the

“plumb pudding model”

Robert A. Millikan (1868-1953)Robert A. Millikan (1868-1953)

• found the quantity of charge carried by an electron (one unit of negative charge)

• calculated the mass of an electron (1/1840th the mass of a hydrogen atom)

Ernest Rutherford (1871-1937)Ernest Rutherford (1871-1937)

• proposed that the atom is mostly empty space

• positive charges and almost of the mass are in a small, centralized region called the nucleus

“Like howitzer shells bouncing off of tissue paper!”

Rutherford Flash Animation

Try it Yourself!Try it Yourself!In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

The AnswersThe Answers

Target #1 Target #2

Niels Bohr (1855-1962)Niels Bohr (1855-1962)

• electrons found only in specific circular paths (orbits) around the nucleus

• based on information about how the energy of an atom changes when it absorbs and emits light

• called these fixed energies “energy levels”

Erwin Schrodinger (1926)Erwin Schrodinger (1926)

• quantum mechanical model – probability of

electron locations around the nucleus

– not an exact orbit• eventually became

the electron cloud model

Schrödinger's Cat video 1:41

Werner Heisenberg (1927)Werner Heisenberg (1927)

• Heisenberg Uncertainty Principle – impossible to know the exact position and velocity of an electron at the same time

• “the observer affects the observed”

http://www.deutsches-museum-bonn.de/ausstellungen/heisenberg/bilder/heisenberg_2.jpg

Structure of the Atom Topic 2.1 really starts here

Particle Charge Mass

(atomic mass units)

Location

Proton + 1 1 nucleus

Neutron Ø 1 nucleus

Electron - 1 5.0 x 10-4 orbit, level, cloud

Learning Check

An atom has 14 protons and 20 neutrons.

A. Its atomic number is1) 14 2) 16 3) 34

B. Its mass number is1) 14 2) 16 3) 34

C. The element is1) Si 2) Ca 3) Se

D. The number of electrons is1) 14 2) 6 3) 20

Isotopes

• same element but differ in their number of neutrons

• the mass number on periodic table is the WEIGHTED AVERAGE MASS of all the isotopes of that element– this is based on an isotope’s natural abundance

• the percentage of each isotope of an element that occurs in nature

• have the same chemical properties (reactivity) but different physical properties (density, melting/boiling point…)

2.3

Chemical symbols for isotopes• two different ways to write isotopes

– example sodium• sodium- 23

– only shows mass number (23) of the sodium isotope

• 23 Na– shows the mass number (23) and the atomic # (11)

of the sodium isotope

11

XMass Number Atomic Number Element Symbol

A

Z

Isotopes?

Which of the following represent isotopes of the same element? Which element?

234 X 234

X235

X238

X

92 93 92 92

How many protons, neutrons, and electrons are in-- C14

6

How many protons, neutrons, and electrons are in--

C116

6 protons, 8 (14 - 6) neutrons, 6 electrons

6 protons, 5 (11 - 6) neutrons, 6 electrons

Do You Understand Isotopes?

Radioactive Isotopes• unstable isotopes that break down over time

• uses:

– cobalt 60• radiation treatment for cancer

– carbon 14• used to date objects up to 60,000 years old

– iodine 125 and iodine 131• ingested and used for medical imaging

Dangerous, but worth the risk