The Development of Atomic Theory

The Development of Atomic Theory

​What is the atomic theory?

Atomic theory is the ​theory of the nature of matter. It is the theory that all matter is made up of tiny indivisible particles.

​Origins of the Atomic Theory

• ​The first person to consider that atoms exist was Democritus.

• ​Democritus was a Greek philosopher who lived in the 4th century BCE.

• ​Democritus stated that everything in the universe was made of tiny, indivisible units called atoms.

• ​The word atom comes from the Greek word atomos which means unable to cut or divide.

• ​Although Democritus made many observations of how matter changes, he did not have any evidence to show that his theory was correct.

Multiple Choice

The word atom comes from a ____.

• atlas, an Italian word.

• atomos, Greek word.

• atman, a Spanish word.

Open Ended

What do you think is the reason that not everyone agreed with Democritus's theory?

​John Dalton and the Atomic Theory

• ​Like Democritus, Dalton proposed that atoms could not be divided into smaller parts.

• ​UNLIKE Democritus, Dalton performed scientific experiments to find data to support his theory.

• ​Dalton's experiments showed that atoms of different elements could combine in certain ways to form compounds.

Multiple Choice

How was Dalton's theory different from Democritus's?

• It did not change. It was the same theory.

• Dalton had evidence but Democritus did not.

• Dalton proposed that atoms could not be divided into smaller parts.

​J.J. Thomson and the Atomic Theory

• ​Thomson used cathode rays in vacuum tubes to help scientists better understand the structure of atoms.

• ​He used a vacuum tube that contained two electrodes, a cathode and an anode.

• When electricity was sent through a tube, a glowing beam came from the tube.

• ​Scientists were unable to determine what the beam was made of.

• ​Thomson did discover electrons. He stated that they were inside the nucleus and that they were negatively charged particles.

Multiple Choice

Which electrode in Thomson's experiment was positively charged?

• cathode

• anode

• denode

Multiple Choice

Subatomic particles with a negative charge

• Electrons

• Neutrons

• Protons

• Quarks

​Ernest Rutherford and the Atomic Theory

• ​Ernest Rutherford was a student of J.J. Thomson's.

• ​Rutherford and his student performed experiments based off of Thomson's findings.

• ​One of his students aimed a beam of positively charged particles at a very thin sheet of gold foil. Rutherford predicted that the positive charge in the gold atoms weak to affect the positively charged particles. The experiment showed different.

• ​Rutherford concluded that an atom's positive charge is concentrated at the center of the atom. The positively charged, dense core of the atom is called the nucleus.

Multiple Choice

In what location did Rutherford suggest the electrons were located?

• in the nucleus.

• outside the nucleus.

• only where the alpha particles were deflected.

• outside the atom after they collided with the alpha particle.

Multiple Choice

Ernest Rutherford discovered that atoms were mostly _________________. 

• negatively charged

• positively charged

• electrons

• empty space.