Figure 3-4: Cathode Ray Tube

Figure 3.5: Cathode Ray Experiment Found… Found… 1. An object in the tube casts a shadow 2. The wheel rolled from cathode to anode 3. Rays were deflected by a magnetic field like an electric current, known to be negative 4. Rays were deflected from a negative charge. Conclusions: Conclusions: the cathode ray is made of electrons! the cathode ray is made of electrons! Electrons have a negative charge Electrons have a negative charge Electrons are present in atoms of all elements Electrons are present in atoms of all elements

Millikan Oil Drop Experiment

Millikan was able to determine the exact charge of an electron by suspending oil droplets in an electric field Millikan was able to determine the exact charge of an electron by suspending oil droplets in an electric field By determining the mass of the oil droplet he could see the charge required to suspend the drops were discreet multiples of a specific charge. By determining the mass of the oil droplet he could see the charge required to suspend the drops were discreet multiples of a specific charge. Confirmed electrons are negative Confirmed electrons are negative Showed they have a tiny mass compared to their large charge Showed they have a tiny mass compared to their large charge

J. J. Thomson (cathode ray) 1904 – Model of the atom…figured since the atom is neutral there must be something positive to balance out the negative 1904 – Model of the atom…figured since the atom is neutral there must be something positive to balance out the negative Called the “Plum Pudding Model” Called the “Plum Pudding Model” A positive cloud with negative electrons within it. A positive cloud with negative electrons within it.

Figure 3-6: Rutherford’s Gold Foil Experiment

Bombarded thin gold foil with alpha particles and expected the particles to pass through. Bombarded thin gold foil with alpha particles and expected the particles to pass through.BUT….

Figure 3-6: Rutherford’s Gold Foil Experiment

Particles were deflected greatly and some went straight back to the source Particles were deflected greatly and some went straight back to the source Conclusion: the volume of the nucleus is very small compared to the volume of the atom Conclusion: the volume of the nucleus is very small compared to the volume of the atom

Figure 3-7: Rutherford’s Gold Foil Experiment

Ernest Rutherford 1911 – Model of atom includes a nucleus surrounded by electrons 1911 – Model of atom includes a nucleus surrounded by electrons Rutherford determined there was a small, dense nucleus in the atom because some of the particles were deflected. Rutherford determined there was a small, dense nucleus in the atom because some of the particles were deflected.

Section 3.2 Composition of the Atomic Nucleus Atomic nuclei are made of protons and neutrons Atomic nuclei are made of protons and neutrons Rutherford is typically credited with the discovery of the proton in Rutherford is typically credited with the discovery of the proton in James Chadwick is credited with the discovery of the neutron in 1932 James Chadwick is credited with the discovery of the neutron in 1932 Proton has a positive charge equal to the magnitude of the negative charge of the electron. Proton has a positive charge equal to the magnitude of the negative charge of the electron.

Section 3.2 Simplest hydrogen atom is simply a proton with an electron orbiting. Simplest hydrogen atom is simply a proton with an electron orbiting. Proton mass is 1836 times greater than the mass of the electron. Proton mass is 1836 times greater than the mass of the electron. The number of protons in the nucleus determines the identity of the element and is the atomic number. The number of protons in the nucleus determines the identity of the element and is the atomic number.

Section 3.2 Forces in the Nucleus Generally, particles with the same electric charge repel each other Generally, particles with the same electric charge repel each other However, when two protons are extremely close to each other, there is a strong attraction between them. However, when two protons are extremely close to each other, there is a strong attraction between them. Nuclear forces: these short-range proton-neutron, proton-proton, and neutron-neutron forces hold the nucleus together. Nuclear forces: these short-range proton-neutron, proton-proton, and neutron-neutron forces hold the nucleus together.

Section 3.2 Forces in the Nucleus

Section 3.2 The Sizes of Atoms The region occupied by the electrons is an electron cloud. The region occupied by the electrons is an electron cloud. Atomic radius is the distance from the nucleus to the outer portion of the electron cloud. Atomic radius is the distance from the nucleus to the outer portion of the electron cloud. Atomic radii are expressed in picometers ( 1 pm = m) Atomic radii are expressed in picometers ( 1 pm = m) Atomic range is 40 to 270 pm. Atomic range is 40 to 270 pm.

Robert Frost American Poet ( ) American Poet ( ) Typically wrote pastoral poems about rural life Typically wrote pastoral poems about rural life

Fireflies in the Garden Here come real stars to fill the upper skies, And here on earth come emulating flies, That though they never equal stars in size, (And they were never really stars at heart) Achieve at times a very star-like start. Only, of course, they can’t sustain the part

Robert Frost American Poet ( ) American Poet ( ) Typically wrote pastoral poems about rural life Typically wrote pastoral poems about rural life Very interested in science Very interested in science Curious about the structure of the atom Curious about the structure of the atom Read Scientific American Read Scientific American Attended lectures atomic structure and quantum physics by the chemist Niels Bohr Attended lectures atomic structure and quantum physics by the chemist Niels Bohr