Atomic Theory and Electron

Dalton's atomic theory, william crookes, paddle wheel, plum pudding

What is a key tenet of Dalton's Atomic Theory regarding elements?

Each type of atom makes up a specific substance we call an element.

According to Dalton's Atomic Theory, what happens to atoms in chemical reactions?

Atoms are never created or destroyed in chemical reactions.

How do atoms combine to form compounds, according to Dalton's Atomic Theory?

Atoms combine in simple whole number ratios to form compounds which are constant and define a particular substance.

What does measuring mass tell us about particles according to Dalton's ideas?

Measuring mass indicates the amount or number of particles, because particles (atoms) have mass.

What did William Crookes invent in the 1870s that contributed to understanding atoms?

He invented the Crookes tube.

What observation made within the Crookes tube suggested that cathode rays had a charge?

Magnets could change the polarity and move the rays.

What was J.J. Thomson's conclusion about the green glow in his Maltese cross experiment?

The green glow flows from the cathode to the anode and are negatively charged cathode rays.

What did the paddle wheel experiment demonstrate about cathode rays?

Cathode rays have mass and are made of particles because they can push the paddle wheel away from the cathode.

What did J.J. Thomson discover through quantitative experiments with cathode rays?

Cathode rays are much lighter than hydrogen atoms, and discovered electrons as the first subatomic particles.

Why did scientists believe there had to be positive charges within an atom?

Since electrons are negatively charged and matter is usually neutral, there has to be positively charged subatomic particles to balance the negative charge.

Describe the Plum Pudding Model of the atom.

JJ Thomposon: A ‘dough’ of positive charge with negatively charged electrons dispersed throughout it, with uniform mass and charge density, no concentrated areas

summary of crookes tube experiment

evacuated tube with a cathode (-) and anode (+) connected to a power supply emitted a green glow that could be deflected by magnets → it must consist of charge

summary of maltese cross experiment

a different shape of evacuated tube had a cathode at the tope and anode at the bottom, and a maltese cross at one end, when connected to a power supply emitted a green glow that created a shadow, and if polarity were switched no glow would appear → the glow travels from the cathode to anode, so they are negatively charged cathode rays

summary of paddle wheel experiment

(thompson or crookes) a paddle wheel inserted in an evacuated tube and the cathode rays caused the wheel to move → cathode rays have mass