Ch2: The Atom Flashcards
What is diffusion?
Evidence for the existence of small particles
e.g. spreading out of gases
What is the law of conservation of mass?
The mass of a closed system will remain constant regardless of the processes acting inside the system.
(Mass of reactants must equal mass of the products)
Name the order of the people who contributed to the history of the atom
-Greek philosophers(Democritus)
-John Dalton
-William Crookes
-J.J. Thomson
-George Stoney
-Robert Millikan
-Ernest Rutherford
-James Chadwick
How did Greek Philosophers contribute to the history of the atom?
-They were the first to propose that matter was composed of small, indivisible particles
-Democritus
How did John Dalton contribute to the history of the atom? / What is Dalton’s Atomic Theory?
-All matter is made of small particles called atoms
-Atoms are indivisible
-Atoms cannot be created or destroyed
How did William Crookes contribute to the history of the atom?
-He discovered negatively charged particles in rays he called Cathode rays
-He conducted experiments with the Crooke’s Tube
Explain the experiments William Crookes conducted
-He used a vacuum tube
-Connected a cathode and an anode to a battery source (CNAP: Cathode -, Anode +)
-As the current passed through, the cathode beamed rays of radiation (Cathode rays) that caused a shadow, from the Maltese Cross, at the other end
-Then, Crookes added a paddle wheel
-He found that the cathode rays were made of particles because they could turn the paddle wheel
What are cathode rays?
Streams of negatively charged particles called electrons.
-They travel in straight lines from the cathode to the anode
-They are deflected by electric and magnetic fields
-They have sufficient energy to move a small object like a paddle wheel
-Cause glass to fluoresce
How did J.J. Thomson contribute to the history of the atom?
He discovered that cathode rays are made of negative particles and he calculated the ratio of charge to mass for electrons
Explain the experiment Thomson conducted
-A hole was placed in the anode to allow a beam of rays
-2 parallel metal plates, fluorescent screen at the end
-No charge on the plates = beam passed straight through
-Positive charge on top plate = beam was deflected
-This showed that cathode rays are made of negative particles because they are attracted to the positive plate
How was Thomson able to calculate the ratio of charge to mass for electrons?
In a 2nd experiment, he used a magnetic field from an electromagnet to deflect the electrons
How did George Stoney contribute to the history of the atom?
He named the negative particles founded by Crookes - Electrons
How did Robert Millikan contribute to the history of the atom?
He conducted the famous oil-drop experiment which allowed him to measure the charge on an electron
Explain the oil-drop experiment
-Sprayed tiny droplets of oil between two charged metal plates
-X-rays were used to ionise the air by stripping electrons of their atoms (ions)
-As the oil drops fell through the air, they picked up electrons and became negatively charged
-He focused on one droplet and increased the positive charge until the drop hovered
Explain Thomson’s Plum Pudding Model
He believed that the atom was a sphere of positive charges, with negative electrons embedded
How did Ernest Rutherford contribute to the history of the atom?
-He conducted the gold foil experiment, that allowed him to disprove Thomson’s Plum Pudding model
-Discovered the nucleus
-Discovered protons were located in the nucleus
-Proposed a new structure of the atom (Nucleus with surrounding ‘electron cloud’)
Explain Rutherford’s gold foil experiment
-He fired alpha particles at a thin piece of gold foil
-The plum pudding model predicted that the particles would pass through with little deflection (this did not happen)
-Instead, most alpha particles passed straight through, some were deflected at large angles and some even rebounded
What are alpha particles?
Positively charged particles produced by certain radioactive substances
What happened when and why the alpha particles
1. collide head on with the nucleus
2. passed near
3. passed straight through
- They reflect straight back due to the strong repulsion like charges. (because positively charged alpha particles and positively charged nucleus repel each other)
- They deflect at large angles
- Some pass straight through because the atom is mostly empty space
How did Rutherford discover the proton?
-Rutherford conducted other experiments, where he fired the alpha particles at different elements such as nitrogen and oxygen
-Small positive particles were given off and Rutherford called them protons
How did James Chadwick contribute to the history of the atom?
He discovered neutrons
How did Chadwick discover neutrons?
-Conducted experiments where he bombarded a sample of beryllium with alpha particles
-Small particles were given off, which were neutral, and had the same mass as the proton
-He named them neutrons because they were neutral
