The Atom - Chp 2

Diffusion

the spreading of particles from a region of high concentration to a region of low concentration. The movement of these particles provides evidence for the existence of atoms

Describe an experiment which demonstrates that matter is made from particles

• one piece of cotton wool soaked in ammonia (NH₃) and one piece of cotton wool soaked in hydrochloric acid (HCl), both put into a glass pipe at opposite ends, at the same time

• a white cloud of ammonium chloride (NH₄Cl) forms closer to the HCl end

• the HCl is denser than NH₃, and moves more slowly which is why the white cloud is closer to that end

• since the substances did not come into direct contact with eachother but a new substance was formed, this proves that the particles must have diffused through the glass pipe and then reacted with each other

What did the Greek Philosophers do?

around 400BC, they were the first to propose that matter was composed of small particles, they believed that they were indivisible, the greek word atomos gave us the English word atom

What did Dalton do?

he was the first to put forward idea of an atom

Dalton's Atomic Theory

• All matter is made up of very small particles
• All atoms are indivisible, they cannot be broken down into simpler particles

What did Crookes do?

he discovered cathode rays

What did Thompson do?

• discovered electron

• discovered charge/mass ratio of an electron

• put forward the plum pudding model of an atom

What did Rutherford do?

• discovered the nucleus

• discovered the proton

• proposed new structure of the atom, consisting of a nucleus and electron cloud

Millikan

• discovered the charge and mass of an electron
• disproved the plum pudding model

Describe Crookes's cathode ray experiment - diagram

• Crookes passed electric current through air at low pressure

• the glass tube used is called a vacuum tube

• rays travelled from the negative electrode and to the positive electrode

• the rays caused the glass to fluoresce and a shadow of a Maltese cross was formed at the other end of the tube

Conclusion of Crookes's cathode ray experiment

• found that rays were produced from the cathode, which he called cathode rays

• shadow proves that they travel in straight lines

• did not realise that these rays were streams of negatively charged particles called electrons

How did Crookes investigate the properties of cathode rays?

• he mounted a light paddle wheel on rails in front of the cathode

• the current was switched on, the paddle wheel rotated away from the cathode and travelled down the tube, and concluded that the vanes were being struck by particles coming from the cathode

What are the properties of cathode rays?

• they travel in straight lines

• causes glass to fluoresce

• they possess enough energy to move a paddle wheel

Describe how Thomson modified Crookes's experiment - diagram

• passed the cathode rays through a doughnut shaped anode, to get a focused beam

• the cathode beam passed between two parallel metal plates, and struck a fluorescent screen at the end of the tube

• when the plates were charged, the cathode beam was deflected towards the positive plate

• Thomson concluded that cathode rays were made up of negative particles called electrons (George Stoney)

Conclusion of Thomson's modified experiment

• focused cathode rays were attracted towards the positively charged plate, meaning that the cathode rays were negatively charged

How did Thomson work out the charge/mass ratio?

• placed a large electromagnet outside the glass tube and adjusted the strength of the magnetic field
• electrons were also deflected by this and adjusted the magnet until it was back down to its original position
• from this he calculated the e/m ratio

What is charge measured in?

Coulombs

Describe + draw Thompsons plum pudding model

• described as a sphere of positive charge with negative electrons embedded in it at random

Describe Millikan's oil drop experiment

• sprayed oil droplets between two charged plates

• the air was ionised using x-ray

• electrons lost in the air picked up by oil droplets

• adjusted charge until the droplets were suspended in mid air

• the electrical force pulling the droplet up was balanced by the weight of the droplet due to gravity, using this he was able to calculate the charge of an electron

• using Thomson's charge/mass ratio he was then able to calculate the mass of an electron

Millikan's oil drop experiment - diagram

Describe Rutherford's experiment which proved Thomsons model of the atom wrong - diagram

• he bombarded a thin piece of gold foil with alpha particles

• used a phosphorescent screen made of zinc sulfide to detect the alpha particles

What 3 things were observed in Rutherford's experiment + what are the implications of these observations

• most particles go through the gold, atom is mainly empty space

• some particles are deflected through wide angles when it gets too close to the nucleus of a gold atom, nucleus has a positive charge

• very rarely a few particles will bounce straight back, meaning that an atoms mass is located in a small space

How would Rutherford’s experiment have been different if the plum pudding model was accurate?

• the alpha particles would only have been slightly deflected and most would have gone straight through

Why did Rutherford’s experiment only work for heavier atoms?

• the alpha particles were breaking up the nuclei of the lighter particles
• the alpha particles were repelled by the heavier atoms before they reached their nucleus

What was the new model for an atom proposed by Rutherford?

• consisted of a nucleus containing protons and an electron cloud surrounding nucleus

What did Chadwick do?

• discovered the neutron

Describe Chadwick's experiment to discover the neutron

• he fired alpha particles at beryllium

• neutrons were knocked out of the nucleus of the beryllium atoms

• the neutrons were difficult to detect due to their neutral charge

• the neutrons knocked protons out of the paraffin wax, which was easy to detect the positive charge

• he found that these particles had about the same mass as the proton, and named them neutrons as they were neutral

What unit measures the mass of sub-atomic particles?

atomic mass units, A.M.U.

Electron: mass + charge

charge: -1
mass: 1/1840 amu

Proton: mass + charge

charge: +1
mass: 1

Neutron: charge + mass

charge: 0
mass: 1

What makes neutrons such a significant discovery?

• brought about the development of nuclear fission
• if there were no neutrons in the nucleus it would collapse because the repulsive forces of the protons repel eachother

Cathode rays

• streams of negatively charge particles called electrons
• they travel in straight lines from the cathode to the anode,
• are deflected by electric and magnetic fields
• and have sufficient energy to move a small object such as a paddle wheel