Atomic Theories Chapter 4.1

How do we know about atoms when no one has

ever seen inside an atom?

Greek Philosopher - Democritus

• Believed it is IMPOSSIBLE to divide matter ad infinitum.

• Theory: “Atomos” – smallest particle of matter.

Lived 460-370 BCE

Greek Philosopher: Aristotle

• Theory: All matter consists of 4 elements. – Air– Water– Fire– Earth

• Theory: Matter can be divided indefinitely.

Lived 384-322 BCE

John Dalton• Math & Chemistry teacher• Member of the Royal Society• Received many awards and honors.• Theory (1803): All matter is

composed of tiny, solid, indestructible particles – called atoms.– Atoms of same elements are alike

in size and mass.– Atoms of different elements have

different sizes and masses.– Nickname: “Billiard Ball Theory”

Lived 1766-1844

(Two thousand years after the Greek philosophers.)

Country: England

John Dalton (continued)

• Example: Electrolysis of water – electro – refers to electricity– lysis – means “to cut”

Experiments: He measured amounts of elements and masses of compounds.

John Dalton (continued)This experiment shows: 2 times the VOLUME of hydrogen gas compared to the volume of oxygen.

So, there must be twice the proportion of hydrogen atoms as oxygen in each water molecule. Therefore the formula for water is H2O.

John Dalton (continued)Also, when he measured the MASS of hydrogen and oxygen, he found out that the ratio of oxygen to hydrogen 8 :1 in every experiment.

(Don’t forget there are 2 hydrogen and 1 oxygen atom in every water molecule)Oxygen has a molar mass of 16 g/moleHydrogen has a molar mass of 1 g/mole

John DaltonTheory: Law of Definite and Multiple Proportions

1. Chemical compounds are formed by the union of 2 or more atoms of different elements.

2. Atoms combine to form compounds in simple, whole number ratios.

Examples: • CO – carbon monoxide is made of 1 carbon and 1 oxygen atoms• CO2 – carbon dioxide is made of 1 carbon and 2 oxygen atoms• H2O – water is made of 2 hydrogen and 1 oxygen atoms• H2O2 – hydrogen peroxide made of 2 hydrogen and 2 oxygen

atoms

JJ Thomson

• Won numerous awards and prizes for his work.

• Member of the Royal Society.

• Experimented with gases and electricity in sealed glass tubes (called cathode ray tubes) showing a glowing stream of particles.

Lived 1856-1940Country: England

JJ Thomson

• Noticed that when he held a magnet up to the tube, the gas was deflected.

• A positive magnet attracted the stream of particles.• A negative magnet repelled the stream of particles. • He discovered electrons!

JJ Thomson

Thomson’s Theory (1897): • Atoms consist of negatively charged

particles (we now call electrons) embedded in positive matter.

• Nicknames: “raisin bun theory” or “plum pudding theory” – electrons are like the raisins or plums in the bread or pudding.

Ernest Rutherford

• He had many honors, including being President of the Royal Society.

• Won many awards, including becoming an English knight and baron.

• Famous Experiment (1911): Gold Foil Experiment

Lived 1871-1937

Country: England and New Zealand

Rutherford’s Famous Gold Foil Experiment

Theories about Atoms:1. Atoms have a small, dense core (we now call the nucleus)2. The nucleus is positively charged.3. The nucleus contains most of the mass of an atom.4. An atom’s volume is mostly empty space!

Rutherford discovered the NUCLEUS of the atom!

Niels Bohr

• Studied under both JJ Thomson and Ernest Rutherford.

• President of the Royal Danish Academy of Sciences

• Foreign member of the Royal Society.

• Won Nobel Prize in Physics in 1922.• Worked with leading scientists on

the Atomic Energy Project.

Lived 1885-1962

Country: Denmark; worked in England and USA

Niels Bohr

Experiment: • Bohr studied the bright-line spectra of

hydrogen. • He measured the energy released by

hydrogen’s energized electrons whenthey return to ground state.

• He noticed that the released energy was in certain packets of light energy (called photons).

Niels Bohr

Theory (1913): Electrons travel in particular orbits around the

atom’s nucleus like planets around the sun.

Also, the chemical properties of each element is determined by the number of electrons in the outer orbit of its atoms.

Modern Theory of the Atom

De Broglie & Shrodinger’s work led to the develop of the most current model of the atom

called the Electron Cloud Model

• There is a nucleus containing positively charged protons and neutrons which have no charge

• There is a space surrounding the nucleus called the “Electron Cloud” which is a probable location of where the electrons are found.