unit 1b - atomic history scientists

When did Democritus live?

Around 460–370 BCE.

What did Democritus say about the atom that has been proven true?

Matter is made of tiny particles called atoms.

On what did Democritus base his beliefs?

Philosophical reasoning and observation rather than scientific experiments.

According to Democritus, what causes the different properties of matter?

Differences in the size, shape, arrangement, and movement of atoms.

Was Democritus able to prove his ideas? Why or why not?

No. He did not have the scientific equipment or experimental methods needed to test his ideas.

When did John Dalton publish his atomic theory?

He proposed his atomic theory in 1803 and published a detailed version in 1808.

What was John Dalton's occupation, and at what age did he begin working?

He was a teacher and began teaching at about age 12.

What did Dalton add to the atomic theory of Democritus?

He used scientific experiments and evidence to develop a testable atomic theory.

What is Dalton's atomic theory?

All matter is made of atoms; atoms of the same element are alike; atoms of different elements differ; atoms combine in simple whole

What parts of Dalton's atomic theory were proven wrong?

Atoms are not indivisible because they contain subatomic particles, and atoms of the same element are not always identical because isotopes have different masses.

What was Crookes actually studying when he contributed to atomic theory?

He was studying electricity passing through gases at very low pressure using discharge tubes.

Where did the term cathode ray come from, and why is it important?

The rays appeared to travel from the negatively charged electrode called the cathode. This was important because the rays were eventually shown to consist of negatively charged particles.

What household items use cathode ray tubes?

Older televisions and computer monitors used cathode ray tubes.

What do we call the negative particles found in cathode rays today?

Electrons.

What was Thomson looking for when he began experiments with the cathode ray tube?

He was trying to determine the nature of cathode rays and what they were made of.

What did Thomson use to deflect cathode rays?

Electric and magnetic fields.

What surprising fact did Thomson learn about the particles in cathode rays?

They were negatively charged particles much smaller than atoms and were present in all types of atoms.

How did Thomson's discovery change Dalton's atomic theory?

It showed that atoms were divisible and contained smaller subatomic particles.

What particle is Thomson credited with discovering?

The electron.

What is the name of Thomson's atomic model?

The plum pudding model.

Describe Thomson's plum pudding model.

The atom was pictured as a positively charged sphere with negatively charged electrons scattered throughout it.

What did Millikan discover about the electron, and when?

He measured the electron's electric charge in 1909.

What further information did Millikan learn about electrons?

His work helped determine the mass of the electron.

What is the approximate mass of an electron?

About 9.11 × 10⁻³¹ kg.

Describe Millikan's Oil Drop Experiment.

Millikan placed tiny charged oil droplets between two charged plates and adjusted the electric field until the electrical force balanced gravity, allowing him to calculate the electron's charge.

What did Millikan notice about the charges on different oil drops?

The charges were whole

How accurate were Millikan's measurements compared with today's value?

They were extremely close to today's accepted value.

When did Rutherford win the Nobel Prize?

1908.

What charge do alpha particles have?

Positive, with a charge of +2.

Describe Rutherford's Gold Foil Experiment.

Rutherford's team fired alpha particles at a thin sheet of gold foil and observed how the particles were deflected.

What occurred in the Gold Foil Experiment that surprised Rutherford?

Most particles passed straight through, but a few were strongly deflected or bounced backward.

What did the Gold Foil Experiment reveal about the atom?

The atom is mostly empty space with most of its mass and positive charge concentrated in a tiny nucleus.

What did Rutherford's discovery do to Thomson's plum pudding model?

It disproved the plum pudding model.

Describe Rutherford's atomic model.

A tiny, dense, positively charged nucleus is at the center of the atom, with electrons around it and mostly empty space.

What was still missing from Rutherford's model?

The neutron had not yet been discovered.

What subatomic particle did Rutherford discover in 1919?

The proton.

What subatomic particle is Chadwick credited with discovering?

The neutron.

When did Chadwick discover the neutron?

1932.

How did Chadwick discover the neutron?

He bombarded beryllium with alpha particles and discovered neutral particles with about the same mass as protons.

What scientist was Chadwick working with?

Ernest Rutherford.

What was Chadwick's important role during World War II?

He led the British scientific mission that worked with the Manhattan Project.

Describe today's current model of the atom.

Protons and neutrons are in the nucleus, while electrons occupy regions around the nucleus called the electron cloud.

What charge does a proton have?

Positive (+1).

What charge does a neutron have?

Neutral (0).

What charge does an electron have?

Negative (−1).