HISTORY OF ATOMIC THEORY

Chapter 14

2500 years agoEarly philosophers thought that matter

was composed of tiny particlesThey named the tiny particles atomsAtoms—”cannot be divided”They didn’t perform experiments, they

reasoned, debated, and discussed—not of evidence or proof

Democritus (400 BC) Greek philosopher Idea of atom No experiments to support idea No protons, electrons, or neutrons Solid, indestructible Hard substances have rough atoms that stick

together Smooth atoms are in liquids and they slide over

each other Sleep is caused by atoms escaping the brain

18th CenturyScientists in laboratories began debating

the existence of atoms once moreThey came to realize that all matter is

made up of elementsElement—matter made of atoms of only

one kind

Early 19th centuryJohn Dalton proposed the following ideas

about matter:1. Matter is made up of atoms2. Atoms cannot be divided into smaller

pieces3. All the atoms of an element are exactly

alike4. Different elements are made of

different kinds of atoms

Dalton (continued) 1803 proposed that elements consist of

individual particles called atoms He had the first atomic theory that had

evidence to support it Called elements “pure”

Dalton pictured an atom as a hard sphere that was the same throughout—like a tiny marble

2nd half of 19th centuryDalton’s theory was tested in 1870William Crookes did experiments with a

glass tube.Electrode—a piece of metal that can

conduct electricityAnode—an electrode with a positive

chargeCathode—an electrode with a negative

charge

Crookes discovered that cathode rays are negatively charged by studying deflection of cathode rays by magnetic fields

Crookes discovered that cathode rays:1. move in a straight line2. cause glass to glow3. carry negative charges4. are capable of being deflected by electric fields

1897J.J. Thomson placed a magnet beside the

tube from Crookes’s experiment.He concluded that cathode rays are

negatively charged particles of matter—he knew that opposite charges attract each other

The particles were traveling in a straight line toward the anode

He discovered the electron when he wasexperimenting with gas discharge tubes. He

noticed a movement in a tube. He called themovement cathode rays. The rays moved from

the negative end of the tube to the positiveend. He realized that the rays made of negatively

charged particles –electrons.

J.J. Thomson inferred that electrons are a part of every kind of atom because they are produced by every kind of cathode material.

The biggest surprise—particles smaller than the atom exist!

This contradicted Dalton’s theory and was not widely accepted by fellow physicists and chemists

If atoms contain one or more negatively charged particles, then all matter, which is made of atoms, should be negatively charged as well…..but…..all matter is not negatively charged.

Does matter contain a positive charge?

Thomson’s model of the atom was a sphere of positive charge with negatively charged electrons spread evenly among the positive charge

(Model looks like a chocolate chip cookie—also called the plum pudding model)

The positive charge of the sphere is equal to the negative charge of the electrons..therefore..the atom is neutral.

It was later discovered that all atoms are not neutral.

The number of elements within an element can vary.

1906Rutherford began an experiment to find

out if Thomson’s model of the atom was correct.

He fired alpha particles (positively charged) at a thin film of metal. Since alpha particles are positively charged, then they should be repelled by particles of matter which also have a positive charge

Rutherford reasoned that the thin, gold film did not contain enough matter to stop the speeding alpha particle or change its path.

There wasn’t enough charge in one place

Actually….the alpha particles were veering off at large angles Most particles passed straight through the foil like the foil

wasn’t even there…

Some particles went straight back or were deflected as if they had hit something…

The experiment shows: Atoms are made of a small, positive

nucleus; the positive nucleus repels (pushes away) positive alpha particles

Atoms are mostly empty space

The positively charged alpha particles were moving with such high speed that it would take a large positive charge to cause them to bounce back.

Rutherford’s new model of the atom showed that its positive electric charge and the majority of its mass were concentrated in an almost point sized nucleus.

1920Scientists identified the positive charges

in the nucleus as protons.The rest of each atom is empty space

occupied by the atom’s almost massless electrons

Finally, since all the mass wasn’t accounted for in Rutherford’s model, it was proposed that another particle must be in the nucleus…the neutron.

The model was revised again to include the neutron…now the model has a tiny nucleus tightly packed with +protons and neutral neutrons.

Negatively charged electrons occupy the space surrounding the nucleus

The number of electrons=the number of protons

Now we have an understanding that the nucleus is extremely small compared to the size of the atom.

Now we understand Rutherford’s experiment..most of the alpha particles went directly through the gold foil without interference from the gold atoms because there is plenty of open space.

20th centuryNeils Bohr—calculated exactly what

energy levels the orbit of the electrons would represent for the hydrogen atom.

However…scientists determined that it was impossible to know the precise location of an electron at any particular moment

Bohr’s calculations didn’t work for every element

New model of atom Small nucleus with electrons traveling in

an unpredictable pattern around it called the electron cloud.