So...... a neutron walks into Starbucks and orders a drink.
The barista promptly serves him his drink.
"How much will that be?" asks the neutron.
"For you?" replies the barista, "no charge!"
(get it?)
Thomson - Electrons
• In 1897, English scientist J.J. Thomson discovered electrons while doing an experiment to see how electric currents affected cathode rays.
Thomson - Electrons• A cathode ray is a stream of particles that
can be seen when an electric current passes through a vacuum tube.
• The ray travels from a negatively charged disk at one end to a positively charged disk at the other end.
Thomson - Electrons
• Thomson discovered the cathode rays did not travel in a straight line, but bent towards the positively charged plate.
Thomson - Electrons
• Opposite charges attract, so cathode ray particles must be negative if they are attracted to something positive.
• He named the particles electrons.
Thomson - Electrons
• He also said (like Dalton), that an atom was a solid sphere
• He added that an atom had a positive charge spread evenly throughout, with electrons mixed in to balance the charge.
Rutherford – Nucleus (Protons & Neutrons) (1899)
• Students of Ernest Rutherford shot alpha particles (protons + neutrons) through gold foil.
Rutherford – Nucleus (Protons & Neutrons) (1899)
2 protons bound to 2 neutrons
(a Helium nucleus)
• Rutherford thought the path of the alpha particles would bend only slightly because there was not anything big enough to strongly repel them.
Rutherford – Nucleus (Protons & Neutrons)
• Most particles did pass straight through.
• Some particles were strongly bounced to the side.
• One in about 8000 bounced completely backwards.
Rutherford – Nucleus (Protons & Neutrons)
• Rutherford believed the alpha particles were hitting something with a positive charge and relatively large mass.
Rutherford – Nucleus (Protons & Neutrons)
• He revised the atomic model: The atom is mostly empty space with the nucleus in the middle.
• He discovered the proton, and predicted the existence of the neutron.
• His model did not accurately explain how electrons are arranged in the atom.
Rutherford – Nucleus (Protons & Neutrons)
Bohr - Hydrogen Atom (1913)
• A Danish scientist, Niels Bohr, studied the hydrogen atom and then proposed a new model for the arrangement of electrons in an atom.
Bohr studied under bothThomson and Rutherford.
Bohr - Hydrogen Atom
• Bohr thought electrons traveled in circles with a certain diameter.
• Bohr studied hydrogen because it has only one electron.
• When atoms are excited, they absorb and release energy in the form of light.
Bohr - Hydrogen Atom
• A spectral line is a single wavelength of light that can be seen when the light from an excited element is passed through a prism.
• When excited, hydrogen and neon give off unique narrow bands of light on the spectrum.
Bohr - Hydrogen Atom
• An energy level is a region in space corresponding to a certain energy through which an electron moves (similar to a planet’s orbit).
Bohr - Hydrogen Atom
• He said that electrons move in specific orbits that are a set distance outside the nucleus.
• Each energy level can hold a certain number of electrons according to these rules:
Bohr’s Model
Bohr - Hydrogen Atom
– The first level holds 2 electrons, the second level holds 8 electrons.
Neon’s atomic number = 10It has 10 protons.It has 10 electrons.
10NeNeon20.18
Bohr - Hydrogen Atom
– The second level has no electrons until the first level is full.
10NeNeon20.18
Ne
Bohr - Hydrogen Atom
– The second level has no electrons until the first level is full.
10NeNeon20.18
Ne
Bohr - Hydrogen Atom
– The second level has no electrons until the first level is full.
10NeNeon20.18
Ne
Bohr - Hydrogen Atom
– The second level has no electrons until the first level is full.
10NeNeon20.18
Ne
Bohr - Hydrogen Atom
– The second level has no electrons until the first level is full.
10NeNeon20.18
Ne
Bohr - Hydrogen Atom
– The second level has no electrons until the first level is full.
10NeNeon20.18
Ne
Bohr - Hydrogen Atom
– The second level has no electrons until the first level is full.
10NeNeon20.18
Ne
Bohr - Hydrogen Atom
– The second level has no electrons until the first level is full.
10NeNeon20.18
Ne
Bohr - Hydrogen Atom
– The second level has no electrons until the first level is full.
10NeNeon20.18
Ne
Bohr - Hydrogen Atom
– The last energy level may or may not be filled to the max.
Ne
10NeNeon20.18
3Li
Lithium6.94
Li
Bohr - Hydrogen Atom
• An element will react with other elements to try to receive a full outer energy level. (also known as “completing the octet”)
Bohr - Hydrogen Atom
• This diagram shows how electrons are placed in the elements with atomic numbers 1–10 (according to Bohr’s model)
Bohr - Hydrogen Atom
• Different numbers of electrons in outer energy level results in different elements.
• Unreactive elements have a full outer energy level.
• Elements with incomplete outer energy levels are likely to form compounds.
Bohr - Hydrogen Atom
Atomic Model
http://glencoe.mcgraw-hill.com/sites/0078741858/student_view0/brainpop_movies.html#
Modern - Electron Cloud Model
• Today scientists think of an electron in an atom as being in an electron cloud, which is a region surrounding an atomic nucleus where an electron is most likely to be found.
Electron Cloud Model
Modern - Electron Cloud Model
• Electrons are more likely to be near the nucleus because they are attracted to the positive charge of the protons.
An excited hydrogen atom emits narrow bands of light called ____.
A energy lines
B wave lines
C spectral lines
D wavelengths
4.2 Discovering Parts of the Atom
1. A
2. B
3. C
4. D
Elements that do not react with other elements must have ____.
A completely filled energy levels
B excited electrons
C empty energy levels
D the same number of protons and neutrons
4.2 Discovering Parts of the Atom
1. A
2. B
3. C
4. D
In the gold foil experiment, why did some particles pass straight through the foil?
A because they were repelled by the protons in the foil
B because they were attracted by the protons in the foil
C because atoms have no effect on charged particles
D because atoms are mostly empty spaces
4.2 Discovering Parts of the Atom
1. A
2. B
3. C
4. D
Atoms of the same element always have the same number of ____.
A electrons
B neutrons
C ions
D protons
An electron moves into a higher energy level. What must have happened?
A The electron released energy.
B The electron absorbed energy.
C The atom gained a negative charge.
D The atom gained a positive charge.
A spectral line is caused by ____.
A an excited electron releasing energy and falling to a lower energy level
B an excited electron absorbing electricity and jumping to a higher energy level
C an atom gaining an ionic charge
D an atom losing a proton