Atomic Theory (AKA History)

November 10, 20158th grade Chapter 3.1

Meme Moment

Symbols for Molecules

HBrO4

Each atom has a symbolfrom the periodic table Symbols can be 1 or 2 letters.

First = capital, second = lower case or none

Numbers say how manyNo number = 1

Atomic Theory Changes

• Sometimes science changes very quickly

• 1500s alchemy• 1600s alchemy + chemistry• 1700s chemistry• 1803 atoms!• 1904 subatomic

particles!• 1911 it changes again!• 1913 it changes again!• 1926 quantum mechanics

Atoms• Atom: smallest unit of matter that can still be

considered an element

• Atomic theory uses different models of atoms to explain the trends we see

• Atomic theory changed as new experiments explained more and more about atoms

• Now we know that atoms aren’t the smallest unit of matter – we have subatomic particles too!

Dalton’s ModelDalton’s Experiments Showed:• Atoms are the smallest unit of matter

o You can’t get smaller according to his theoryo Hard spheres

• Atoms of the same element are exactly alike. Different elements have different atoms

• Atoms can never change• Compounds form when atoms combine

in set ratioso Table salt would always be 1 sodium + 1

chlorine

100 Years of Dalton• Democritus & Aristotle used the idea of atoms to

describe the universe – Dalton used science to prove it!

• For 100 years or so, this is unchanging fact

• Then Thomson comes along in1897 and tries a new experiment... In 1904, atomic theory changes to a more detailed formo Science fixes itself! We have to revise theories when

new data says we don’t have all of the facts

Thomson’s Experiment

Lights up when hit by beam

When electric charge was introduced,the beam bent. Opposite charges attract, so there must be a negative charge to this beam

Thomson’s Model• With his experiments, Thomson

discovered electrons• Electrons had a negative charge

if they were bent towards the positive charge

• Thomson’s electrons were smaller than atoms and found inside atoms

• Already knew atoms had a neutral charge, so there must be a positive part to balance it out

• “Seeds in a watermelon” or “chocolate chip cookie”

Rutherford’s Experiment

Rutherford’s Experiment

• Rutherford was trying to prove Thomson’s model

• Wanted to show that the slightly-positive gold foil could slightly bend the path of the mostly-positive atoms – he failed

• Some of the particles bounced• There must be something hard and

positive in the center of the atom in order to bounce like that – the nucleus!

• The nucleus contains protons• Thomson’s model only lasts for 7

years

Bohr’s Theory• Revisited an old experiment from 1800s, found new

interpretation

Background Info:• Different elements emit light in very specific places• The light is from electrons “jumping” and releasing

energy• Red = low energy, blue = high energy

Bohr’s Model• If you only got certain

colors, you could only be using certain energies, so electrons needed to be in certain places – orbits

• Was Rutherford’s student – changes his model in 2 years

• “Rings on a tree” or “planetary orbits”

Schrodinger & the Cloud Model

• This is very simplified in your books because the math looks like this:

• When the math stops using numbers, it’s a problem

Schrodinger/Cloud • 13 years later...• Electrons aren’t in set

places – they are probably in places

• Mapping where the electrons most likely are gives clouds around Bohr’s rings

• In bigger atoms, some of the clouds are more complicated than spheres

Chadwick’s Model• 8 years later...• James Chadwick also

worked for Rutherford – then he heard about an experiment from Irène and Frederic Joliot-Curieo Something was knocking

protons out of wax using radiation

• Chadwick proposes the neutron – equal mass to a proton, but uncharged

VocabularyNucleus: the small, heavy center of an atom where protons and neutrons are located

Proton: small, positively charged particle found in the nucleus. Each element has a unique number of protons

Neutron: small, neutral particle found in the nucleus.

Electron: small, negatively charged particle that moves around outside the nucleus

VocabularyEnergy level: roughly energy of an electron/where an electron is located

Atomic number: the number of protons.

Mass number: the number of protons + neutrons. This is often different from the atomic mass shown on the periodic table (which is an average of the mass numbers for different isotopes)

Isotope: atom with the same number of protons, but different number of neutrons compared to other atoms of the same element. E.g. Carbon-13 vs Carbon-12