The Atom

Section 4.1-4.3 Notes

Some videos…

• http://www.youtube.com/watch?v=caYxrueCHiQ

• You might be giants - Elements

History of Atomic Models

• Democritus (~400BC) – atomos = small, solid, indestructible

particles of different shapes & sizes– These were just ideas, not truly

science…

• Dalton’s Atomic Theory– Billiard Ball Model - small solid

sphere– Developed notion of conservation of

mass and that atoms combine in specific ratios

History of Atomic Models

• J.J. Thomson– Plum Pudding Model - positive

and negative particles dispersed throughout the atom

– Used Cathode Ray tubes to discover the electron - first subatomic particle discovered!

http://www.youtube.com/watch?v=XU8nMKkzbT8

Cathode Ray Tube

• A tube that contains a stream of electrons going from a negative disk (cathode) to a positive disk.

• Deflected stream showed that electrons are negative.

Rutherford– Nuclear Model– Discovered dense positively charged

nucleus of the atom while working with alpha particles• Gold Foil Experiment

History of Atomic Models

• Neils Bohr - – Also Solar System Model– Electrons travel in specific, circular orbits

• Schrodinger & Heisenburg -– Quantum Mechanical Model– Dense nucleus w/ protons & neutrons– Electrons exist in ‘clouds’ called orbitals w/

specific energy levels– Mathematical predictions for probability of

finding electrons– Electrons have particle and wave properties

What do you already know about

atoms?• Brainstorm about what you already

know (or are pretty sure of…) about atoms.

• Write it down on your paper, then we’ll discuss.

How big is the nucleus?!?

Go to atoms videoclip

Comparing subatomic particles…

particle symbolrelative mass (amu)

electron e- 1/1840

proton p+ 1

neutron n0 1

sToP & tHinK

1. If an atom has 2 protons and 2 neutrons, what is its atomic mass in atomic mass units (amu)?

2. What is the charge on an atom that has 7 protons and 7 electrons?

Atomic Number

• # of protons• # of electrons in a NEUTRAL atom• Always a whole number

Mass Number• # of protons + neutrons• in atomic mass units (amu)• Isotopes - atoms of the same

element with different masses– differ in number of neutrons– Examples: Carbon-13 & Carbon-14,

Boron-10 & Boron-11• Element-mass#

Calculating # of neutrons

• Subtract atomic # from mass #• Example:

– Aluminum• 13 protons• 27-13 = 14 neutrons

sToP & tHinK

1. What element has the atomic number 8?2. How many protons, neutrons, and

electrons are in a neutral atom of Potassium (K)?

3. How many protons, neutrons, and electrons are in a neutral atom of Boron-11?

Average Atomic Mass & Molar Mass

• Average mass of all the atoms of a sample for an element

• In amu’s for individual atom• In grams for a mole of that

element = molar mass

Relative Abundance• The atomic mass on the PT is the average

of the isotopes for that element.• Example: Lithium has two isotopes, one

has a mass of 6 amu & one has a mass of 7 amu. Lithium-7 occurs 92.5% of the time…– Multiply the mass by the abundance– Add the two together to get the atomic mass

for the element– What is Lithium’s average atomic mass?

sToP & tHinK

• The published average atomic mass of phosphorus is 30.97. If it has two isotopes, phosphorus-30 and phosphorus-31, which one is more abundant. Explain.

sToP & tHinK

• An element has two isotopes. One has a mass of 24 amu and represents 70% of the atoms of the element, the other has a mass of 25 amu and represents 30% of the atoms of the element.– What is the average atomic mass?