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Name: ________________
SNC1
Geissler/Crooks/Thompson: The Discovery of Electrons
A glass tube filled with air or another gas will
not conduct electricity. The tube has metal
electrodes at each end which are attached to
a high voltage source of electricity. An
ammeter measures the electric current.
If the air pressure inside the glass tube is
reduced, the gas will conduct electricity and
the gas inside the tube glows. The colour of
the light given off depends on the type of
gas. It does not matter what metal the
electrodes are made out of, the effect is the
same.
If the air pressure is reduced almost to zero,
the gas still conducts electricity, but only the
glass itself at the positive end of the tube
glows.
If a small paddle wheel is placed in the glass
tube, it rotates when the battery is
connected. In a separate experiment, an
object in the tube cast a shadow in the glow
at the positive end of the tube.
Conclusion
A particle is given off by the negative metal
electrode. It must be a particle because it makes
the paddle wheel rotate. It must be negative because it moves from the negative electrode to the positive
electrode (opposite charges attract and like charges repel). It is easy to remove from the atoms of the
electrode because only electric voltage is needed to remove it. Since the particles are given off by the
metal of the electrodes, they must be a component of the atoms of the metal.
Questions
1) How do we know that a particle is being given off by one of the electrodes?
2) How do we know what direction the particle is moving?
3) How do we know what charge the particle has and what is its charge?
4) What do we call these particles today?
5) Since atoms are neutrally charged, what other type of particle must atoms contain besides the ones
given off in this experiment?
6) Define the terms cathode and anode.
7) Why were the rays in the gas discharge tube called cathode rays?
8) What common household appliance still uses a cathode-ray tube?
The results of the gas discharge tube experiments were the discovery of a particle
that came to be called the electron. It has a negative charge, a very small mass, and is
removed easily from the atom. The electrons are the same in any atom, no matter what
the element.
Subsequent studies of gas discharge tubes found faint rays travelling away from the
anode in tubes with Hydrogen gas. These particles were much harder to bend with
magnetic and electrical fields, so they must have been much heavier. Since they travel
away from the anode, they must have a positive charge. These particles were called
protons.
9) Copy down the inferences about Protons and Electrons.
10) a) Write down Thompson’s model of the Atom.
b) How did Thompson’s model differ from Dalton’s?
c) Why is Thompson’s Atomic Model called the Raisin Bun Model?
d) In a neutral atom, how must the number of protons and electrons compare?
Rutherford: Discovery of the Nucleus
By the early 1900s, people knew that atoms were made of electrons (negative charge) and protons
(positive charge), but it was thought that the electrons and protons were mixed together into a single
mass.
Radioactivity also had been discovered by this time. Ernst Rutherford conducted exhaustive studies of
radioactivity. He found that there were three types of radiation given off by radioactive material:
alpha particles: heavy particles with a positive charge.
beta particles: light particles with a negative charge.
gamma rays: not a particle, but a type of light with a much higher frequency.
In 1909, Ernest Rutherford devised an experiment to probe the structure of the atom using a beam of
alpha particles given off by a radioactive source. The alpha particles were shot through a thin foil made
of Gold. An alpha particle detector was placed around the gold foil to determine what happened to the
alpha particles after they passed through the Gold foil.
Much to his
amazement, most
of the alpha
particles passed
through the Gold
(which is a very
dense material)
as if nothing was
there. However,
a few were deflected off to the side, and a smaller number even bounced back towards the source. This
was pretty impressive since alpha particles are very fast moving and the gold foil was very thin. It was
rather like firing a rifle at a sheet of paper and having the bullet bounce back off the paper.
Conclusions
The conclusions were that atoms must be mostly empty space with a small, very massive core. Since the
positive alpha particles were deflected or repelled by this small core, the core also must have a positive
charge.
Questions
1) How did Rutherford conclude that atoms were mostly empty space?
2) How did Rutherford conclude that atoms have a very dense core?
3) How did Rutherford conclude that the core of the atom has a positive charge?
4) What do we now call the core of the atom?
5) If the core of the atom has all of the positive charge, where must the electrons be in the atom?
6) If the core has most of the mass in an atom, how massive must electrons be compared to protons?
The Rutherford model differs from the Thompson Model in that:
-
the electrons are separate from the positive core of the atom
electrons orbit around the nucleus
almost all of the mass of the atom is concentrated in a very small nucleus
all of the positive charges are in the nucleus
atoms are mostly empty space with small electrons moving through this space
Many scientists began investigating atoms using methods similar to Rutherford’s as well
as other methods. One of the most important was the work of Henry Moseley.
Henry Moseley: The Significance of the Atomic Number
Henry Moseley, a student of Rutherford, bombarded samples of different elements with X-rays
and was able, through some pretty complicated math, that the atomic number of an element
must equal the number of protons in the nucleus. Remember, that the atomic number was only a
cataloguing number in the periodic table.
This conclusion was backed up by several other different experiments, such as the charges that
result when electrons are removed from atoms (if you can only remove so many electrons from a
neutral atom, must that number also equal the number of protons in the nucleus?)
The Neutron
If the number of protons in the nucleus increases by one as you from one element to the next in
the periodic table, why does the relative atomic mass not increase by one as you go from one
element to the next in the periodic table?
This was a very important question.
Since the relative atomic mass does not increase by one from one element to the next, there
must be another particle in the nucleus that has about the same mass as the proton, but does
not have a charge - in other words, a neutral particle. This particle was called the Neutron.
We now have independent evidence of the existence of
neutrons. They seem to help hold all of the positive protons in
the nucleus together. Stable nuclei (stable atoms) have about
the same number of neutrons and protons, although larger
nuclei seem to need proportionally more neutrons. Nuclei with
either too many neutrons or not enough neutrons emit
radiation as they either fall apart into several smaller nuclei
(nuclear fission) or eject a small number of particles to
become more stable.
Where do these different nuclei come from?
The Next Big Question: How are the Electrons Arranged in an Atom?
The evidence to answer this question came from an odd source: light.
Light comes in many colours.
The closer the colour is to the blue end of the spectrum, the more energy it has.
Normal white light (such as sunlight) consists of all colours mixed together. When we split the light into
its component colours with a prism, we see each colour.
The Discovery of Electron Shells
After 1910, it was known that electrons are negative and have little
mass. They orbit around a small nucleus made up of very heavy,
positive protons and neutral neutrons. It was not known, however, how
the electrons were arranged around the nucleus. Did they just orbit
around the nucleus any old way, or was there some organization to how
they were arranged?
When people look at the light from a gas discharge tube through a
prism (or diffraction grating) they found that only certain colours
from the spectrum were present, unlike sunlight or the light from incandescent light bulbs:
Each element gives off a different set of
colours in its spectrum.
Rutherford Atom:
a) electrons could exist at any distance from the nucleus.
b) electrons can lose potential energy as light and drop closer to the nucleus.
Problems:
a) what stops them from falling all the way into the nucleus? —> Thomson Atom
b) why do atoms only emit certain frequencies of light?
Niels Bohr (famous for theories on gun fighting) considered bright line spectra data and used these
spectra as evidence to work out how electrons are arranged around the nucleus.
Observation 1:
Conclusion:
Protons are positive and electrons are negative, thus they are attracted to each other.
You must add energy to pull electrons away from the nucleus (light, electricity, heat).
When electrons move closer to the nucleus, they must release energy.
the farther they fall towards the nucleus, the more energy they must release.
Ep = Cq1q2/d
Observation 2:
Electrons always release energy in the form of light.
The colour of light is directly related to its energy.
Observation 3:
The energy, and therefore colour of light, given off depends on the distance electrons
move towards the nucleus.
Observation 4: When you examine the light given off by energized gas, only certain colours of light.
Conclusion:
If electrons only give off certain colours, they must
only move certain distances.
Main Conclusion
If electrons only move certain distances, they must only exist at certain
distances from the nucleus.
In other words, electrons must exist in discrete shells around the nucleus.
From the line spectra colours, Bohr was able to work out the distances, or
differences in potential energy, between each electron shell. These
differences get smaller and smaller as the shells get farther from the
nucleus. Shell number “infinity” is the point at which the electron is
effectively removed from the atom. The potential energy (or distance
from the nucleus) of this shell is not much different from shell number 10.
The charge of the nucleus affects the distance (Ep) of each shell. The more e-, the lower is
each shell.
Questions
1) When you look at sunlight through a diffraction grating, what do you see?
2) When you look at light from an energized gas through a diffraction grating, what do you see?
3) What colours do you see in the light from:
a) Hydrogen gas:
b) Neon gas:
c) overhead light:
4) How did the character of light tell Bohr about the distances that electrons move?
5) How did Bohr decide that electrons only exist in certain shells?
SNC1
Name:_______________
Gas Discharge Emission Spectra
1) Draw the emission spectra for each of the gas sources and use them to identify the unknown gas sample.
Light Source
Sunlight
Unknown Gas
2) Describe the colour produced by burning each of the metal salts. Use this information to identify the
unknown metal salt.
metal salt
Colour of flame (describe the colour as completely as possible)
Unknown salt
3) What is causing the colours that are being given off by these tests?
4) How do the spectra (plural of spectrum) of the light given off by these tests compare to the spectrum
of sunlight?
SNC1
Name:________________
The Rutherford Model of the Atom
1) Complete the following chart: Read page 38 under the heading “Types of Atomic Particles”
Particle
Location in Atom
Charge
Relative Mass
electron
proton
neutron
The Bohr Model of the Atom
2) a) Where are electrons held in the Bohr model of the atom?
b) Can electrons be any distance they want from the centre of the atom?
c) Where are atoms the most stable?
3) Using some complex
first shell
second shell
third shell
forth shell
math, Bohr was able to work out how many electrons each shell could hold:
_______
_______
_______
_______
The number of electrons equals the number of protons in a neutral atom.
Electron shells are filled from the first shell outwards.
Each element has a different arrangement of electrons, which can be represented by a Bohr Diagram.
To make a Bohr diagram: i) draw the symbol of the element with the number of protons and neutrons in it.
ii) figure out how many electrons the atom has
iii) draw a circle for the first electron shell
iv) add dots to the circle to represent electrons,
add no more dots that the shell can hold
and add no more dots than the atom actually has electrons
v) subtract the electrons used from the total
vi) if you still have electrons left, add another shell and repeat steps iv to vi
Examples:
4) On a separate sheet, draw Bohr diagrams for all the elements from 1 to 20.
5) In a neutral atom, the number of protons must __________ the number of electrons.
Important Atomic Numbers
Elements: depend on the Atomic number; the atoms of each element have a specific number of protons.
Isotopes: - Not all atoms of an element are exactly alike.
- They all have the same number of protons, but may have different numbers of neutrons
- Atoms of the same element, but different numbers of neutrons are called isotopes.
- Different isotopes have the same chemical properties, but different masses.
- Thus, a heavier atom which behave the same as a lighter atom of the same element, but
substances made out of the heavier atom will be heavier and have slower evaporation rates.
Ions:
if an atom gains or loses electrons, it is no longer neutral, it is charged.
if it gains electrons, it will have a negative charge (more electrons than protons)
if it loses electrons, it will have a positive charge (more protons than electrons)
6) a) What is the charge of an atom that has 13 protons and 10 electrons?
b) What element is it?
7) a) What is the mass number of an element with 3 protons and 4 neutrons?
b) What element is it?
8) a) How many neutrons does an atom have if it has 6 protons and a mass number of 13?
b) What element is it?
9) How many protons does an atom have if it has a mass number of 14 and 8 neutrons?
10) Which of the following atoms are different isotopes of the same element?
a) 12 p+, 13 no, 10 eb) 11 p+, 12 no, 11 ec) 12 p+, 12 no, 12 e11) Which of the atoms in question 10 are ions?
12) Which of the atoms in question 10 has the largest mass number?
13) Which of the atoms in question 10 is not the same element as the other two?
14) How many protons, neutrons, and electrons do the following atoms have?
15) What are the atomic numbers and mass numbers and charges of each of the atoms in question 14?
SNC1
Name: ________________
Atomic Structure
Calculate the missing data and add it to the chart.
Type of Element
Name of Element
Symbol
Atomic #
Mass #
# of p+
1
metal
Ca
# of e-
0
Sodium
2+
13
14
23
1+
8
B
8
10
11
3+
Sulfur
15
8
Charge
0
40
14
metalloid
# of no
0
16
0
Helium
4
0
Helium
3
0
Al
27
13
15
17
non-metal
16
18
F
10
Li
6
1+
10
2
1+
Be
5
6
12
6
13
2+
0
6
7
Ar
10
4
24
H
1-
3
3
Magnesium
15
7
7
40
0
Ne
10
1
Cu
U
0
2
1
34
27
146
+
o
0
-
p = proton, n = neutron, e = electron