How do we know about atoms?

Atomic Theory

Democritus (460 BC- 370 BC)His mentor Leucippus

Everything is composed of “atoms” Atoms are physically indivisible

Between atoms is empty space Atoms are always in motionInfinite number of atoms and kinds of atoms

which differ in shape and size

John Dalton (1766-1844)Postulated the following:

that all matter was made of some type of particle- called atoms which could not be broken up or destroyed

Atoms could combine with different atoms to form compounds

All atoms of the same element are the same size (mass) and have the same properties

Atoms can be rearranged during a chemical reaction to form new substances

Discovery of ElectronJJ Thomson in 1897 Cathode Ray Tubes- A cathode ray forms when

high voltage is applied to a partially evacuated tube. The ray passes through a hole in the negative end of the tube and hits the coated end of the tube to produce a glow

Plum Pudding ModelKnow that there are electrons (negative)Must be a positive part as well in order to be

neutral

Discovery of the NucleusErnest Rutherford designed an experiment to

test the Plum Pudding Model. Expected all particles to go straight through

the foil

They didn’t all go straight through?

Why?

New Model = Nucleus

NucleusContains the mass of the atom.

20 years later, James Chadwick discovered the neutron because the entire mass of the nucleus could not be accounted for by just protons.

Re-examining Dalton’s Atomic TheoryWhich postulates had to be modified after JJ

Thomson and Ernest Rutherford’s experiments?

All atoms are the same size- isotopes

Atoms can’t be changed- nuclear reactions

All mater is made of atoms – a proton is still matter

Atomic Symbol -

IsotopesAll atoms of an element are identical in

atomic number, but not in atomic mass. Isotopes = different number of neutrons. Electrons determine chemical properties of

an element. Can determine neutrons from atomic mass

and atomic number

Using the info on the PTHow many protons, neutrons and electrons

does an atom of iron have?

Atomic Masses on the PTHow come they have decimals?

Does a single neutron or proton weight exactly 1 amu?

Atomic Mass Unit (amu)The mass of an atom is measured relative to

the mass of an atomic standard. The standard = carbon-12 atom. 1 carbon-12 atom has exactly 12 amu

Atomic MassThe mass represented on the periodic table is

the average of all of the naturally occurring isotopes weighted accordingly to their abundances.

PROBLEMS: find the abundances of isotopes based on the atomic mass. OR predict the atomic mass based on the abundances of certain isotopes

Finding Atomic MassMagnesium has 3 naturally occurring

isotopes, 24Mg (23.9850 amu, abundance 78.99%), 25Mg (24.9858 amu abundance 10.00%), and 26Mg (25.9826 amu, abundance 11.01%). Calculate the atomic mass of magnesium.

Sample ProblemBoron has 2 naturally occurring isotopes.

Find the percent abundance of Boron-10 and Boron-11 given the atomic mass = 10.81 amu. The isotopic mass of Boron-10 = 10.0129 amu, and the isotopic mass of Boron-11 = 11.093 amu.

Homework: Pg 69: # 1 (a,b,c), 2, 9, 20 (a,b,c) 21, 22, 23 (a,b), 29, 32,

91 Read about mass spectroscopy on page 48, and

summarize what it is