Download Chapter 4 - The Structure of the Atom Atomic Models PIONEERS OF

Chapter 4 - The Structure of the Atom
Atomic Models
PIONEERS OF THE ATOM
Democritus
Lavoisier
Dalton
Thompson
Milliken
Rutherford
DEMOCRITUS
- Was a Greek philosopher that began wondering what the universe was made of.
- He proposed that all the matter in the universe is composed of tiny particles.
- These particles are indivisible.
- He called these particles atomos (means indivisible).
ARISTOTLE
- Aristotle was much more popular and he opposed the idea of atoms.
- He believed that matter was continuous.
So, chemistry died for about 1400 years, until the 1700’s.
FOUNDATIONS OF ATOMIC THEORY
- The predominate belief by the mid 1700’s was the modern definition of an element as a
substance that cannot be broken down by ordinary chemical means.
- It was also believed that elements combined to form compounds that have different
physical and chemical properties than those of the elements that formed them.
- However, there was controversy as to whether elements always combined in the same
ratio when forming a particular compound.
- In the 1790’s, the study of matter was revolutionized by a new emphasis on the
quantitative analysis.
- Scientists’ investigations were made more accurate by new improved balances.
Antoinne Lavoisier took advantage…
- He began to gather evidence about how matter behaved before and after a reaction.
- He discovered that the total mass of the ingredients of a chemical reaction was the same
as the total mass of the results.
Two other fundamental ideas were investigated…
Law of Definite Proportions
- States that a chemical compound is always composed of the same elements in the same
proportion by mass.
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Law of Multiple Proportions
- States if two different compounds are composed of the same two elements, then the ratio
of the elements is always a ratio of small whole numbers.
John Dalton’s thoughts…
- In 1808, John Dalton proposed an explanation for the Law of Conservation of Mass, the
Law of Definite Proportions, and formulated the Law of Multiple Proportions.
- He reasoned that elements were composed of atoms and that only whole numbers of
atoms can combine to form compounds.
- His theory can be summarized in five statements:
o All matter is composed of extremely small particles called atoms.
o Atoms of a given element are identical in size, mass, and other properties; atoms
of different elements differ in size, mass, and other properties.
o Atoms cannot be subdivided, created, or destroyed.
o Atoms of different elements combine in simple whole number ratios to form
chemical compounds.
o In chemical reactions, atoms are combined, separated, or rearranged.
Dalton and his contemporaries
- The earliest theorists thought the atom was hard and round, much like tiny marbles or ball
bearings.
Structure of the Atom
- Although Dalton thought atoms were indivisible, investigators in the late 1800’s proved
otherwise.
- It soon became clear that atoms are actually composed of several basic types of smaller
particles.
- And it’s the numbers and arrangements of these subatomic particles that determines the
identity of the atom.
- The first discovery of a subatomic particle resulted from the investigations into the
relationship between electricity and matters.
- In the late 1800’s, many experiments were performed in which electric current was
passed through various gases at low pressure. These were carried out in tubes called
cathode-ray tubes.
- Investigators noticed that when current was passed through a cathode ray tube, the
surface of the tube directly opposite the cathode would glow.
- The cathode ray traveled from the cathode to the anode when current was passed through
the tube.
- The cathode ray tube opened the door for another scientist, J.J. Thompson.
J.J. Thompson
- Thompson’s investigations supplied evidence that the cathode ray could be deflected by
applying a positively charged electric field.
- This helped Thompson discover that the cathode ray was negatively charged.
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Thompson was able to measure the ratio of the charge of cathode ray particles to their
mass.
He found that the ratio was the same regardless of the metal used as the cathode.
Thompson concluded that all cathode rays are composed of identical negatively charged
particles.
Which were later called electrons.
Thompson’s experiments revealed that the electron has a very large charge in relation to
it’s mass.
Robert Millikan
- In 1909, Robert Millikan, performend and ingenious experiment to calculate the mass of
an electron.
- He discovered that the mass of the electron is about 1/2000th the mass of the simplest
atom (hydrogen).
- Hil oil drop experiment showed that event hough the atoms of different elements are very
different, their electrons are identical.
- An electron is an electron
- The difference between two different atoms is their number of electrons not their type of
electrons.
- He also calculated that the electron’s mass is 9.109x10-31 kg.
What we know so far…
- Thompson’s and Millikan’s ideas:
o Atoms are in face divisible
o Electrons are present in atoms of all elements.
o One of the atom’s fundamental particles is negatively charged.
o Atoms are electrically neutral, so there must be a (+) charge to balance out the (-).
o Because electrons are basically massless there must be something else that
accounts for the atoms mass.
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Other significant discoveries…
- In 1886, when E. Goldstein observed a cathode ray tube and found rays travelling in the
opposite direction of that of the cathode rays.
o He called the rays canal rays and concluded that they were composed of positive
particles.
- It was soon discovered that the positive particles mass is about 2000 times that of the
electron
- In 1932, the English physicist James Chadwick confirmed the existence of yet another
subatomic particle.
o The neutron, which is a particle with no charge
o Has a mass nearly equal to that of the proton
- Therefore the subatomic particles are the electron, proton, and neutron.
The atomic nucleus
- When subatomic particles were discovered, scientists wondered how these particles were
put together in an atom.
- This was a difficult question to answer, given how tiny atoms are.
- Most scientists thought it likely that the electrons were evenly distributed throughout an
atom filled uniformly with positively charged material.
Ernest Rutherford
- In 1911, Rutherford performed on of the most brilliant investigations ever conceived.
o Rutherford set out to test the Thompson model of the atom.
o The test used relatively massive radioactive alpha particles.
o Alpha particles (α) are helium atoms that have lost their two (2) electrons and
have a double positive charge because of the two (2) remaining protons.
- In the experiment, Rutherford directed a narrow beam of alpha particles at a very thin
sheet of gold foil.
o According to the prevailing theory, the alpha particles should have passed easily
through the gold, with only a slight deflection due to the positive charge thought
to be spread out in the gold atoms.
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What happened…
- The majority of the α particles passed straight through the gold atoms, without any
deflection.
- Even more surprisingly, a small fraction of the α particles bounced off the gold foil atoms
at very large angles.
- Some even bounced back to the source.
- Based on the results, Rutherford suggested a new theory of the atom.
- He proposed that the atom is mostly empty space.
o This explained the lack of deflection of some of the particles.
- He concluded that all the positive charge and almost all the mass is concentrated in a
small core.
o He called this region the nucleus.
What do we know?
- There are three fundamental particles that together make up the atom.
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o Composed of electrons, protons, and neutrons
The protons and the neutrons make up the center of the atom.
o Center of the atom is called the nucleus.
The protons and neutrons occupy the mass of the atom.
The electrons surround the nucleus and occupy most of the atom’s volume
How, then, are atoms of hydrogen different from those of oxygen?
o A subatomic particle from one type of atom looks like particles from another
atom.
It’s the numbers of protons in an atom that makes atoms different.
o An oxygen atom has 8 protons in its nucleus.
o A hydrogen atom has 1 proton in its nucleus
The atomic number of an element is the number of protons in the nucleus of an atom of
that element.
o Since oxygen’s nucleus has 8 protons its atomic number is 8.
The protons identify the type of atom, the only atom with 8 protons is oxygen.
For each element listed, the number of protons equals the number of electrons.
o Remember atoms are electrically neutral.
In at atom, the number of electrons must equal the number of protons.
o A hydrogen atom has 1 electron
o An oxygen atom has 8 electrons
The mass of an atom is concentrated in its nucleus and depends on the number of protons
and neutrons.
The total number of protons and neutrons in an atom is called the mass number.
o A carbon atom, which has 6 protons and 6 neutrons, has a mass number of 12.
If you know the atomic number and mass number of an atom of any element, you can
determine the atom’s composition.
The composition of any atom can be represented in shorthand notation.
Isotopes
- Every chlorine atom has 17 protons, without exception.
o However, not every chlorine atom has 18 neutrons.
o Atoms with the same number of protons but contain different numbers of neutrons
are called isotopes.
- Because isotopes of an element have different numbers of neutrons they have different
mass numbers.
- Isotopes are chemically alike because they have identical number of protons and
eelectrons.
o It’s the electrons and protons that are responsible for chemical behavior.
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Ions
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An element’s atoms are not always netural in charge.
o When an atom loses or gains one or more of its electrons it becomes an ion.
An ion that has more electrons than protons has a negative electrical charge.
An ion that has fewer electrons than protons has a positive electrical charge.
Atomic mass
- The actual mass of a proton or a neutron is very small – 1.67 x 10-24 g
- The protons + neutrons is the mass of the atom.
- Therefore the mass of even the largest atom is incredibly small.
- Scientists have determined the tiny masses of atoms by using an instrument called a mass
spectrometer.
- Finding the mass of an individual atom can provide useful information, but in general,
these values are too small and impractical to work with.
- Instead scientists compare the relative masses of atoms to a reference atom.
o A standard has been set that defines the mass of all the other atoms.
- The isotope chosen was the carbon-12 atom.
- This isotope of carbon was assigned a mass of exactly 12 atomic mass units (amu).
- 1 atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom.
o Using that information we can say that a helium-4 atom has a mass 1/3 the mass
of C-12.
- A carbon-12 atom has 6 protons and 6 neutrons in its nucleus, and its mass is set as 12
amu.
- The 12 protons and neutrons account for nearly all of the carbon’s mass.
o Therefore, the mass of a single proton or a single neutron has a mass of 1 amu.
- Since the mass of an atom is based on individual particles of matter, the atomic mass of
an atom should be a whole number.
o Yet on the periodic table the masses aren’t whole numbers.
- The masses on the periodic table are weighted averages.
- In nature, most elements exist as a mixture of isotopes.
- Each isotope of an element has a fixed mass and a natural abundance.
o Natural abundance is an indication of how often we would encounter a particular
isotope.
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o One of the isotopes is almost always encountered more often so it gets more
weight.
Recall that the element Chlorine (Cl) has two (2) isotopes (Cl-35 and Cl-37).
75% of the chlorine atoms you’ll find in nature are Cl-35.
o Which has 17 protons and 18 neutrons in its nucleus.
25% of the chlorine atoms you’ll find in nature are Cl-37.
o Which has 17 protons and 20 neutrons in its nucleus.
Since we are more likely to find a Cl-35 atom the average mass of chlorine is closer to 35
than to the true average of 36.
Thereofore, their atomic mass of an atom is the weighted average mass of the atoms in a
naturally occurring sample of the element.
The mass number of an atom is the protons + neutrons of a particular atom.
o If we need the mass number from the periodic table we take the closest whole
number to the atomic mass found on the periodic table.
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