Download By 1911 the components of the atom had been discovered

4º ESO. Physics and Chemistry
Unit 1. Thomson and Rutherford
J.J. Thomson and the discovery of the electron
Atoms were thought to be the smallest possible division of matter until 1897 when J.J. Thomson
discovered the electron through his work on cathode rays. These are created in a glass sealed tube in
which two electrodes are separated by a vacuum. When a voltage is applied across the electrodes,
cathode rays are generated, creating a glowing patch where they strike the glass at the opposite end of
the tube.
Through experimentation, Thomson discovered that the rays could be deflected by electrostatic
objects (and also by magnets). He concluded that these rays were composed of negatively charged
particles he called "corpuscles" (they would later be renamed electrons by other scientists) and that
they were much smaller than the lightest known atom: the hydrogen atom.
Thomson believed that the corpuscles emerged from the very atoms of the electrode. He thus
concluded that atoms were divisible, and that the corpuscles were their building blocks. To explain the
overall neutral charge of the atom, he proposed that the corpuscles were distributed in a uniform sea of
positive charge; this was the Plum Pudding Model as the electrons were scattered in the positive
charge like plums in a plum pudding.
E. Rutherford and the discovery of the nucleus
In 1911, E. Rutherford tested Thomson's model by devising
his "gold foil" experiment. Rutherford reasoned that if
Thomson's model was correct then the mass of the atom was
spread out throughout the atom. Then, if he shot high velocity
radioactive alpha particles (helium nuclei, positively charged)
at an atom then there would be very little to deflect the alpha
particles. He decided to test this with a thin film of gold
atoms. As expected, most alpha particles went right through
the gold foil but to his amazement a few alpha particles
rebounded almost directly backwards. Rutherford described
this in a lecture by saying: “it was almost as incredible as if
you had fired a 15-inch shell at a piece of tissue paper and it
came back and hit you.”
The actual results surprised Rutherford.
● If Thomson's Plum Pudding Model was correct, the positive charge of the atom is spread out over a
relatively large area.
○ this would mean that the effect of this positive charge would be very weak on other charges,
because it is so spread out.
○ the negative electrons are unimportant since they are all over the place, spaced out, and tiny.
○ this would result in very small (if any!) scattering angles for the alpha particles, since the
alpha particles would just about sail straight through.
● Rutherford found that the angles of deflection showed something different.
○ most particles did go off at small angles, but some went off at very large angles.
○ A small number of alpha particles even came bouncing right back in the direction they came
from.
The only model that could explain this was a concentration of positive charge, the protons, in the
center of the atom, a bunch of empty space, and electrons in orbit. A bunch of empty space and the
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4º ESO. Physics and Chemistry
Unit 1. Thomson and Rutherford
electrons far out in orbit meant that the alpha particles often went almost straight through. The
concentrated positive charge in the center would explain when the alpha particles deflected at big
angles, and if it hit the center straight on would result in the alpha particle bouncing straight back.
This is the basis of Rutherford's Planetary Model.
● The nucleus is where all the positive charge is found.
It is located at the center of the atom.
● The electrons orbit around the nucleus at pretty much
random positions. Nowadays, it is called the electronic
cloud.
● In a neutral atom the number of electrons equals the
number of protons.
● As a note, there is no mention of neutrons yet, since
no one had discovered them by this time. It was
Chadwick -an associate of Rutherford- who confirmed
the existence of neutrons in 1932.
The picture above greatly exaggerates the size of the nucleus relative to that of the atom. In fact, the
nucleus is about 100,000 times smaller than the atom. Nevertheless, the nucleus contains essentially
all of the mass of the atom.
This Planetary Model of atom had its limitations. According to classical physics when an electron (or
particle) is accelerating or decelerating, it emits electromagnetic radiation. In this case, electrons in
Rutherford model radiate all energy and finally collapse inwards into the nucleus (we know that this
does not happen, since atoms are actually very stable!)
Atomic masses
In order to discuss the mass of an atom we need to define a new unit of mass appropriate to that of an
atom. This new unit of mass is called the "atomic mass unit" or u1. The conversion between the u and
gram is: 1 u = 1.67x10-24 g
The mass and charge of the three particles is given in the table below:
symbol
mass (u)
Electrical
charge
proton
p+
1.0072
+1
neutron
no
1.0086
0
electron
e-
1/1840
-1
particle
The atomic number is the number of protons found in the nucleus of an atom. It is conventionally
represented by the symbol Z. The atomic number uniquely identifies a chemical element2. In an atom
of neutral charge, the atomic number is also equal to the number of electrons.
The mass number is the total number of protons and neutrons in an atomic nucleus. It is
conventionally represented by the symbol A. Isotopes are atoms with the same atomic number (atoms
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1
In fact, 1 u is equal to 1/12 the mass of an atom of
2
Increasing atomic number is the criteria used to arrange elements in the Periodic Table.
C.
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4º ESO. Physics and Chemistry
Unit 1. Thomson and Rutherford
of the same element) that have different mass numbers. They have the same number of protons in the
nucleus, but the number of neutrons will not be the same.
If you want to refer to a certain isotope, you write it like this: ZA X . Here X is the chemical symbol for
the element, Z is the atomic number, and A is the number of neutrons and protons combined, the mass
number. For instance, ordinary hydrogen is written 11 H , deuterium is 21 H , and tritium is 31 H .
Most elements exist in nature in two or more isotopic forms. Chlorine, Cl, is a typical example. All
samples of chlorine obtained from nature are composed of the same no changing mixture of two
isotopes, chlorine-35 (Cl-35) and chlorine-37 (Cl-37). The 35 and 37 are the mass numbers of the two
isotopes. Of course, both isotopes have the same atomic number: 17.
In nature, 75.7% of all chlorine atoms, by weight, are Cl-35, and 24.2% are the heavier isotope, Cl-37.
The percentage distribution of the isotopes of an element is called the percent natural abundance or
just natural abundance of the isotopes. In order to have an atomic mass that is accurate for an
element and accounts for the natural mixture of its isotopes, it is necessary to determine what is called
a “weighted average” atomic mass as follows:
Average mass natural chlorine = 0.757 x 34.97 + 0.242 x 36.96 = 35.45 u
And this is the “weighted-average” atomic mass of chlorine that appears on the periodic table
It is important not to confuse the mass number of an element with its atomic mass. Mass numbers are
whole numbers that equal the number of protons plus neutrons in an atom. Atomic masses compare
the relative masses of atoms (with C-12 as the standard). Atomic masses will always be decimal
numbers.
Ions
Atoms can gain or lose electrons. When the numbers of protons and electrons in an atom are equal,
scientists say the atom is balanced. It is possible, however, to unbalance an atom by brushing away
one or more electrons (and never protons or neutrons!). Unbalanced atoms occur when materials rub
together, as when people slide across the plastic seat of an automobile or walk across a wool rug.
Because of the rubbing, electrons are brushed away from some atoms. Unbalanced atoms are called
ions.
When atoms are unbalanced, there is a tendency for electrons to move until the atom's balance is
regained. If one electron is brushed away, there will be more positive charges than negative charges.
Because of this, this unbalanced atom will be said to have a positive charge and is called a cation.
When an extra electron is picked up by an atom, it becomes negatively unbalanced and is said to have
a negative charge. In this case is called an anion.
As an example, if you see: 146 C , that means that you have an atom with 6 protons and 8 neutrons in the
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Ca 2 , you have an atom with 20
nucleus and 6 electrons in the electronic cloud. But if you see 20
protons and 21 neutrons in the nucleus and only 18 electrons in the cloud (a cation). Finally if you
35
have 17
Cl - it means that the atom has 17 protons and 18 neutrons in the nucleus and 18 electrons
orbiting in the electronic cloud (an anion).
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4º ESO. Physics and Chemistry
Unit 1. Thomson and Rutherford
Questions
1. What did Thomson learn from the cathode ray tube experiment?
2. What are the key features of the Plum Pudding Model?
3. How is Dalton’s model simpler than Thomson’s model?
4. Why was Rutherford conducting the gold foil experiment?
5. What was Rutherford initial hypothesis?
6. How were Rutherford results different from his hypothesis?
7. Only 1 of 10000 alfa particles was bounced off of the gold foil. What does this tell you about the size of the
nucleus?
8. What are the key features of the Planetary Model?
9. How was Rutherford’s model different from Thomson’s model?
10. What are two ways an electron is different from a proton?
11. What two particles are in the nucleus? How are they different?
12. In what way do isotopes of an element differ from one another? In what way are they similar?
13. Carbon has an atomic mass of 12 and six protons. How many neutrons does it have? How can you tell?
14. Explain the structure of
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S2
15. Which of the following atoms has an equal number of protons and neutrons?
a) 27
b) 55
c) 39
d) 22
25 Mn
13 Al
19 K
11 Na
16. Which of the following would represent different isotopes of the same element?
a)
12
6
X and
12
7
X
b)
12
6
X and
13
7
X
c)
12
6
X and
12
6
X
d)
12
6
X and
14
6
X
17. Which of the following best describes a neutron?
a. positive charge; about the same mass as an electron c) no charge; about the same mass as a proton
b. negative charge; about the same mass as a proton d) no charge; about the same mass as an electron
18. Which of the following has the smallest size?
a) Electron b) Proton
c) Neutron
d) Nucleus
e) Atom
19. The volume of an atom is made up mostly of
a) Protons b) neutrons
c) electrons
d) empty space
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20. 52Te has
a) 126 neutrons, 52 protons, and 52 electrons
c) 74 neutrons, 52 protons, and 52 electrons.
b) 52 neutrons, 74 protons, and 74 electrons
d) 52 neutrons, 126 protons, and 126 electrons.
21. An atom has no net electrical charge because
a. its subatomic particles carry no electrical charges
b. the positively charged protons cancel out the negatively charged neutrons
c. the positively charged neutrons cancel out the negatively charged electrons
d. the positively charged protons cancel out the negatively charged electron
22. Which of the following statements is true?
a. Cations have negative charge
c. Anions have positive charge
b. Ions have no charge
d. An unbalanced atom is an ion.
23. The cathode ray tube revealed that
a. Atoms have a nucleus
c. Small negative particles exist within the atom
b. Neutrons have no charge
d. Atoms are made up of alpha particles
24. The element magnesium consists primarily of three isotopes: Mg-24 (23,985 u), Mg-25 (24.985 u), and
Mg-26 (25.982 u). The composition of naturally occurring magnesium is 78.79% Mg-24, 10.13% Mg-25,
and 11.17% Mg-26. The average atomic mass of Mg is..
a) 24.47 u
b) 24.98 u
c) 25 u
d) 24.66 u.
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