Models of the AtomMs FedorArts and College Preparatory Academy

ChemCatalystThe model shown here is a tiny cube of gold. 1. What do you think a scientific model

is?2. The spheres in this model represent

atoms. What do you think atoms are?

Agenda

1. Collect Assignments2. Notes: Atomic Structure3. Activity: Atomic PuddingHW: Students who have completed quiz 2:

study and schedule Quiz 3Students who have completed Quiz 3:

work on organizing your notebook, finishing your preplanning for your element project

Objectives: describe the structure of an atom, understand how atomic theory developed over time

What I Need From YouSupplements for Quiz 2

◦Mole practice problems◦Chemical formulas/reactions

Activities: ◦A New Language ◦Copper Cycle Lab◦What Goes Around Comes Around

Earliest Ideas About Atoms

Democritus – 460 BCEPhilosopherAll matter is

made up of indivisible particles

Atoms means “indivisible”

No experimental evidence to support idea

Democritus

Greek banknote with Democritus and drawing of atom

Where did Democritus get his ideas forsmall, indestructible, indivisible

atoms?

Atoms are small…

new wedding rings old wedding ring

EVIDENCE: Old wedding rings get worn away slowly, and get thinner and thinner, but you never see the gold atoms on your finger, so gold atoms must be very small!

Atoms are indestructible…

Atoms are indivisible…

EVIDENCE: The Earth has been around a long time. Even though mountains get washed away, the rocks continue to exist. New plants grow where old plants die.

EVIDENCE: None.This was a HYPOTHESIS made by Democritus to explain nature as he saw it.

Aristotle had other ideas

ARISTOTLE: Famous Greek philosopher, born 384 BCE. He was a student of Plato (another famous philosopher) and the teacher of Alexander the Great, who later conquered the world.

Greek banknote and coin picturing Aristotle

Aristotle’s idea of matterAristotle did not believe that Democritus’s idea of atoms was correct.

He believed all matter was made from four elements:

EarthAirFireWater

Aristotle’s idea of matterAristotle was more famous than Democritus, so people believed him, even though he was wrong.

So Democritus’s idea of atomos (atoms) was lost for nearly 2000 years until John Dalton brought it back in 1803.

Then in 1803…

John Dalton, an English chemistry teacher, made several conclusions about atoms from his experiments: 1. All elements are composed of tiny particles

called atoms, and the atoms of different elements are different from one another.

2. Atoms of different elements combine to form new substances in whole number ratios.

3. Chemical reactions occur when atoms are rearranged so that they attach to different atoms.

Dalton’s Chemical Symbols

We use different symbols today.

Overarching QuestionThese ideas formed the basis of atomic theory.

Dalton was wrong about atoms being indivisible. So if atoms aren’t indivisible, then where does our modern idea of the atom come from?

HOW HAS ATOMIC THEORY CHANGED OVER TIME?

WHAT DO WE NOW KNOW ABOUT ATOMS?

Activity: Atomic Pudding

1. Examine and compare the different models

2. Use new evidence to determine the correct sequence of the models

3. Identify the parts of the atom

Objectives: • I can interpret the results of each major

experiment in the development of atomic theory

• I understand and can describe the structure of an atom, and am able to explain the differences between atoms of different elements

Atoms are the smallest units of an element

Sulfur (S)

Average Atomic Mass and Reactivity

MODEL: a simplified representation of something more complex, that facilitates understanding certain aspects of a real object or processATOMS: the smallest unit of an element that retains the chemical properties of that element.

ChemCatalystWhich model represents Dalton’s idea that atoms are indivisible?

Which model suggests that there are negative particles in an atom?

Atomic TheoryAll matter is made of atoms

Solid sphere model, 1803

Plum pudding model, 1897

nuclear model, 1911

Solar system model, 1913

cloud model, quantum model, 1927-1932

first proposal of the atom, 442 BC

How did Thomson know to run an electrical current through matter?

Michael Faraday, English chemist, born 1791.In 1834 he learned that matter would interact with electricity.He realized that electricity had to me made up of particles that could be counted, but he did not know what they were. (They were electrons.)

ChemCatalystWhat are the three particles that

make up an atom?What is the charge of each

particle?

J.J. Thomson1856-1940

Discovered electron 1897◦Cathode Ray Experiment:

electric charges make materials lose negatively charged particles.

◦Atoms contain negatively charged particles

Plum Pudding model 1904◦Electrons in a soup of

positive charges

More About Charge…Opposite charges attract, alike charges repel (push away).

Cathode Ray Experiment

Thomson’s “Plum pudding” atom

– raisin

RULE: Matter is NEUTRAL, so for every electron (one negative charge) they must also have a proton (one positive charge) to balance the math to zero charge (“neutral”).

ENGLISH PLUM PUDDING– electron

Ernest Rutherford1871-1937 Gold foil experiment:

◦Shot alpha particles (positively charged particles) at gold atoms

◦Most went through, but some bounced back

Nuclear model 1911◦An atom’s mass is

mostly in the nucleus◦The nucleus has a

positive charge◦Electrons in fixed orbit

Rutherford’s Gold Foil Experiment

Rutherford gold foil experiment

Rutherford expected the a particles to pass straight through, like this…

But what he got was most a particles went straight through, but a few were deflected, like this…

Problem with Rutherford’s Model

But there also was a problem with Rutherford’s nuclear atom!To prevent the negative (-) electrons from being attracted to the positive (+) nucleus, the electrons would have to orbit the nucleus like the Earth orbits the sun.

Why don’t the electrons crash into the nucleus?

Niels Bohr1885-1962

Each element emits specific set of wavelengths of light (colors)◦Wavelength = amount of

energy◦Electrons of those

elements have different energy levels

Solar system Model 1913◦Nucleus surrounded by

orbiting electrons at different energy levels

◦Electrons have definite orbits

Bohr’s Quantized Energy LevelsBohr’s new atomic model had quantized

energy levels, meaning the electrons could only move by jumping between levels (numbered n = 1, n = 2, n = 3, etc.). They absorb energy when they jump out, and emit (send out) energy when they jump in.

Ernest Rutherford1871-1937 Alpha particles can

knock positively charged particles out of the nucleus. ◦Nucleus contains

positively charged particles called protons

Proton model 1918◦Central nucleus made of

protons◦Tiny orbiting electrons

(negatively charged)◦Most atom is empty space

ChemCatalystWho discovered the electron?

What did Bohr’s research change about the atom?

Ernst Schrödinger 1887-1961

Quantum Mechanical Model 1926

◦Electrons are in probability zones called “orbitals”, not orbits and the location cannot be pinpointed (Heisenberg uncertainty principle)

Werner Heisenberg 1901-1976

Cloud/Quantum mechanical model

James Chadwick1891-1974

Discovered Neutron 1932

Neutrons hold positively charged nucleus together

400 BC 1803 1904 1910 1913 1926

Niels Bohr

J J Thomson

Democratus

John Dalton

Schroedinger / Heisenberg

Ernest Rutherford

Atomic Theory

The Complete Modern AtomAn atom is mostly empty space with a nucleus containing protons and neutrons which contain most of the mass of an atom.Electrons are in energy levels around the nucleus, and are very tiny.

Electrons jump between levels, emitting and absorbing energy as they jump. We often refer to these energy levels as “orbitals”, but real orbitals are much more complicated…

Simple atomic modelIncludes:1. Rutherford’s discovery of the

proton 2. Chadwick’s discovery of the

neutron3. Bohr’s discovery of electrons in

orbits

Regions of an atom

Nucleus:•center of the atom•Makes up most of its mass

Electron cloud:•Mostly empty space•Take up most of the radius of the atom

Electron Cloud

Nucleus

Subatomic Particles

Neutrons (Neutral)

electrons (-)

Protons (+)

Atomic ParticlesParticle Charge mass Relative

massproton +1 1.67 x 10-

24 g1 amu

neutron 0 1.67 x 10-24 g

1 amu

electron -1 9.11 x 10-28 g

1/1840 amu

* amu = atomic mass units

A neutral atom has no net charge: number of protons is equal to the number of electrons.

#P + #e = 0

Images of atoms Scanning tunneling electron microscope 1981Bombards

surface with electrical current (electrons)

Read changes in current, translate into image