The Nuclear Atom

Atomic Scientists’ Song 2:52

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

• 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)

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

J.J. Thompson (1856-1940)J.J. Thompson (1856-1940)(need to know this guy)(need to know this guy)

• 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)(need to know this guy)(need to know this guy)

• 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)(need to know this guy)(need to know this guy)

• 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)(need to know this guy)(need to know this guy)

• Heisenberg Uncertainty Principle – impossible to know the exact position and momentum 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

Particle Charge Mass

(atomic mass units)

Location

Proton + 1 1 nucleus

Neutron Ø 1 nucleus

Electron - 1 5.0 x 10-4

(considered negligible)

orbit, level, cloud

Carbon- 12 as a standard• carbon- 12

– ALL masses on the periodic table are based on their relationship to carbon-12

• the carbon- 12 isotope has been given the atomic weight of exactly 12.000000000 and is used as the basis upon which the atomic weight of other isotopes is determined

Even smaller particles• quarks

– make protons & neutronsmake protons & neutrons– 6 types6 types

He

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 in a neutral atom is1) 14 2) 6 3) 20

Isotopes

• same element but differ in their number of neutrons

• the atomic mass 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 for 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 SymbolAZ

Isotopes?

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

234 X 234

X235

X238

X

92 93 92 92

92 is the element uranium

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?

Dangerous, but worth the risk

Radioactive Isotopes (don’t need to know)

• 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

The Mass Spectrometer

• has many applications, but one of the simplest is to determine the natural abundances of the isotopes of a particular element – the relative atomic mass can be calculated from the

data from the mass spectrometer

Mass spectrometer video (2:26)http://www.youtube.com/watch?

v=_L4U6ImYSj0

• magnesium results from the mass spectrometer:– 80% 24Mg– 10% 25Mg– 10% 26Mg

Calculate the relative atomic mass of magnesium with the provided data.

• just a simple weighted mean.80(24) + .10(25) + .10(26) = 24.3 amu