Unit 4. the atom

A H I S T O R Y

The Atom

VOCABULARY

nucleus shell (to) set out (to) make up void lead sheet Foil deflection bounced back tiny (to) drop

The Atom: What Do You Already Know?

In the nucleus, there are ________ and _______.

In the outer shell, there are _____________.

Different kinds of atoms are categorized using the ___________ Table of Elements.

History of the Atom

When philosophers and physicists set out to study the fundamental units that make up matter, they were essentially all interested in the same question:

Where does all of the material in the world come from?

What is the most fundamental unit?

Timeline

B.C. Greeks – Fundamental elements make up all matter: air, fire, water,

and earth. Middle Ages

Alchemists – linked macrocosmic and microcosmic phenomena/workings in universe.

1803 Dalton – Basic Atomic Theory

1898 Thompson – Discovery of electron: Pudding Model

1909-11 Rutherford (1871–1937) – Discovery of nucleus: Planetary Model

1913 Bohr ( 1885-1962) – Discovery of electron orbitals: Quantic Model

Four Elements Theory

Circa 492-432 BC: Greek philosopher Empedocle theorized that all matter came from four basic elements: Earth, Air, Fire, and Water.

These elements generated

all material things.

Greek Atomists

420 BCE – The word "atom" comes from the Greek "a-tomos" and signifies "indivisible".

Leucippe of Milet invented this idea: there is a tiny, indivisible unit of matter, called the atom.

His disciple, Democrite of Abdere thought these atoms were always moving and:

Invisible (because of their extremely small size)

Indivisible and solid (no void inside)

Discovery of the Electron

1897 – J.J. Thompson discovered the

first component part of the atom:

the electron (e-), a particle with a

negative (-) electrical charge.

The Dalton theory was falling…the atom is divisible.

1904 – He proposed a model of the atom, called "Thompson’s pudding".

He imagined the atom as a sphere full of an electrically-positive

substance mixed with negative electrons, "like the raisins in a

cake".

J.J.Thompson (1856-1940)

English Pudding Thompson’s Pudding

Model

Traditional dessert. Studded with raisins.

Positive solid space, with little negative charges that resemble raisins.

Thompson Model: Two Kinds of Pudding

Ernest Rutherford (1871-1937)

A New Zealand-born British chemist

and physicist who became known as the

father of nuclear physics.

He discovered that the atom’s mass is

concentrated in one positive center, called the nucleus in

1911.

The Thomson model was passed over. The atoms have space inside.

(new idea!)

The negative electrons move around the center, a bit like

planets around the sun = a “Planetary Model”

E. Rutherford Thomson’s pupil

Gold Foil Experiment (1909)

1909 – Rutherford shot alpha particles (positive

particles) at a thin sheet of gold foil. He expected many

minor deflections. Instead, most of particles passed

through, and some were deflected at very large angles.

Only some particles even bounced directly back!

Planets Orbit Around Sun Electrons Around Nucleus

The force at work is gravity

The force at work is electrical attraction between (+) and (-)

Rutherford’s Planetary Model (1909-11) A comparision

In sumary… Rutherford: his atomic model

His new model of the atom showed that its positive electric

charge and the majority of its mass were concentrated

in a tiny center.

The electron in an atom moves around this nucleus like

planets around the Sun, the (-) charge of the electron

attracting the (+) charge of the nucleus: Atomic Planetary

Model

Note that Rutherford's atom is: Divisible Contains mostly empty space

Atomic Planetary Model fails…

Rutherford understood that the nucleus is itself composed of nucleons. These nucleons are of two types:

positively charged: proton. neutrally charged: neutron

The neutron was effectively discovered in 1932 by Chadwick.

The planetary model of the atom has a serious deficiency:

The electrons can emit light under certain conditions (in an

electric light bulb for example); in doing so, they lose energy and

should therefore get dangerously close to the nucleus right up

until they crash into it!

Such an atom would not therefore be stable.

Niel Bohr (1885–1962):

Danish physicist

Bohr won the Nobel Prize in 1922.

His theory: electrons travel in particular orbits

around the atom's nucleus.

Bohr also introduced the idea that an electron

could drop from a higher-energy orbit to a

lower one, in the process emitting a particle

of energy (light quantum, called photon) .

Scientist Model/Idea

1. Dalton (1803)

2. Thompson (1897)

3. Rutherford (1911)

4. Bohr (1913)

Discovery of nucleus: Planetary Model

Electron discovery: Pudding Model

Atomic Theory

Energy electron discovery: Quantum model

Review: Evolution of the Model of the Atom

Evolution of Atomic Models

Dalton’s solid Atom: No spaces

Thompson: Pudding Model

Rutherford: Nucleus and Planetary Model

Greeks: 4 elements

Bohr: Quantic Model

1898

1803

1913 B.C.

1911

Questions

What were the four elements that make up matter, according the Ancient Greeks?

Who discovered the electron?

Who realized that atoms have a concentrated, positive mass at their center, called a nucleus?

How did he discover it?

Who was the first person to discover that atoms have space inside?

According to Dalton’s atomic theory, are atoms divisible?

Which atomic model tastes the best?