matter and energy. unit 4: matter and energy chapter 10: the atom 10.1 atomic structure 10.2 quantum...
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Matter and Energy
Unit 4: Matter and Energy Chapter 10: The Atom
10.1 Atomic Structure
10.2 Quantum Theory of the Atom
10.3 Nuclear Reactions
10.1 Investigation: The Atom
Key Question:
How is an atom organized?
Objectives: Build atom models. Describe the relationship between the number of
protons, neutrons, and electrons in an atom to its atomic and mass numbers.
Infer that not all atoms of an element are identical.
Atomic structure English scientist John Dalton (1766–1844) started experimenting
with gases in the atmosphere in 1787.
In 1808, Dalton published a detailed atomic theory that contained the following four statements:
1. Matter is composed of tiny, indivisible, and indestructible particles called atoms.
2. An element is composed entirely of one type of atom. The properties of all atoms of one element are identical and are different from those of any other element.
3. A compound contains atoms of two or more different elements. The relative number of atoms of each element in a particular compound is always the same.
4. Atoms do not change their identities in chemical reactions. They are just rearranged into different substances.
Review: Elements and CompoundsAn element is composed of one type of atom.
A compound contains atoms of more than one element in specific ratios.
Thomson’s “plum pudding” model English physicist J. J.
Thomson (1856–1940) observed that streams of particles could be made to come from different gases placed in tubes carrying electricity.
Thomson identified a negatively-charged particle he called the electron.
A gold foil experiment and the nucleus In 1911, Ernest Rutherford
(1871–1937), Hans Geiger (1882–1945), and Ernest Marsden (1889–1970) launched fast, positively-charged helium ions at extremely thin pieces of gold foil.
They expected the helium ions would deflect a small amount as a result of hitting gold atoms.
Instead, most of the helium ions passed straight through the foil!
We now know that every atom has a tiny nucleus, that contains more than 99 percent of the atom’s mass.
Three subatomic particles
A proton is a particle with a positive charge.
An electron is a particle with a negative charge.
A neutron is a neutral particle and has a zero charge.
The charges on one proton and one electron are exactly equal and opposite.
Charge is an electrical property of particles that causes them to attract and repel each other.
Inside an atom The mass of the nucleus
determines the mass of an atom because protons and neutrons are much larger and more massive than electrons.
In fact, a proton is 1,836 times heavier than an electron.
Volume of an atom
The size of an atom is determined by how far the electrons are from the nucleus.
The electrons define a region of space called the electron cloud.
If an atom was the size of a football stadium, the nucleus would be the size of a pea, and the electrons would be like a few gnats flying around the stadium at high speed.
Fundamental forces inside atoms
Electrons are bound to the nucleus by electromagnetic force, the attractive force between electrons (-) and protons (+).
Because of Newton’s first law, the electrons do not fall into the nucleus, because they have inertia.
Fundamental forces inside atoms
What holds the nucleus together?
There is another force that is even stronger than the electric force.
We call it the strong nuclear force.
Historical development of atomic forceHenry Cavendish (1731–1810), a British scientist, was the first to measure the gravitational force between two masses using a torsion balance.
Cavendish detected a very small torque between the large and small spheres.
Historical development of atomic forceThe unit of electric charge is the coulomb (C) in honor of Charles-Augustin de Coulomb (1736–1806), a French physicist who measured the electromagnetic forces between charges in 1783.
Historical development of atomic forceHideki Yukawa (1907–1981), was the first Japanese to receive a Nobel Prize for his theory of the strong nuclear force.
This theory predicted the meson, an elementary particle that was discovered later.
Historical development of atomic forceA theory about the existence of the weak force was first proposed by Enrico Fermi (1901–1954), an Italian physicist who worked on the first nuclear reactor and its applications.
Fermi’s theory was based on his observations of beta decay.
How atoms of various elements differThe atoms of different
elements contain different numbers of protons in the nucleus.
Because the number of protons is so important, it is called the atomic number.
How atoms of various elements differ Isotopes are atoms of
the same element that have different numbers of neutrons.
The mass number of an isotope tells you the number of protons plus the number of neutrons.
How are these carbon isotopes different?
Average atomic mass
Elements in nature are usually a mixture of isotopes.
The element lithium has an atomic mass of 6.94.
On average, 94% of lithium atoms are lithium-7 and 6% are lithium-6.
How many neutrons are present in an aluminum atom that has an atomic number of 13 and a mass number of 27?
Calculating average atomic mass
1. Looking for: … number of neutrons in aluminum-27.
2. Given: … atomic no. = 13; mass no. = 27
3. Relationships: Periodic table says atomic no. = proton no. and mass no. = protons + neutrons
4. Solution: neutrons = mass no. – proton no.
no. neutrons = 27 – 13 = 14
Unit 4: Matter and Energy Chapter 10: The Atom
10.1 Atomic Structure
10.2 Quantum Theory of the Atom
10.3 Nuclear Reactions
10.2 Investigation: Energy and the Quantum
TheoryKey Question:
How does an atom absorb and emit light energy?
Objectives: Distinguish between atoms in the ground and excited
states.
Use the Photon and Lasers game to simulate the absorption and emission of light from an atom.
The spectrumEach different element has
its own characteristic pattern of colors called a spectrum.
The colors of clothes, paint, and everything else around you come from this property of elements to emit or absorb light of only certain colors.
Each element emits a characteristic color of light.
The spectrumEach individual color in a spectrum is called
a spectral line because each color appears as a line in a spectroscope.
A spectroscope is a device that spreads light into its different colors.
Quantum theory and the Bohr atom Danish physicist Neils Bohr
proposed the concept of energy levels to explain the spectrum of hydrogen.
When an electron moves from a higher energy level to a lower one, the atom gives up the energy difference between the two levels.
The energy comes out as different colors of light.
Quantum theory
A quanta is a quantity of something that cannot be divided any smaller.
One electron is a quanta of matter, because you can’t split an electron.
Quantum theory says that when a particle, such as an electron, is confined to a small space inside an atom, the energy, momentum, and other variables of the particle become quantized and can only have specific values.
The Bohr atom led to a new way of thinking about energy in atomic systems.
Quantum theory In 1925, Erwin Schrödinger (1887–
1961) proposed the quantum model of the atom we still use today.
Quantum theory says that when things get very small, like the size of an atom, matter and energy do not obey Newton’s laws or other laws of classical physics.
The electron is thought of as a “fuzzy” cloud of negative charge called a quantum state rather than as a particle moving around the nucleus.
Electrons and energy levels In the current model of the atom, we think of
the electrons as moving around the nucleus in an area called an electron cloud.
The energy levels occur because electrons in the cloud are at different average distances from the nucleus.
Pauli exclusion principle
According to the quantum model, two electrons can never be in the same quantum state at the same time.
This rule is known as the Pauli exclusion principle after Wolfgang Pauli (1900–1958).
Planck’s constantThe “smearing out” of particles into fuzzy
quantum states becomes important when size, momentum, energy or time become comparable in size to Planck’s constant.
If you measure the momentum of an electron in a hydrogen atom and multiply it by the size of the atom, the result is about 1 × 10–34 joule·seconds.
The uncertainty principle
The work of German physicist Werner Heisenberg (1901–1976) led to the uncertainty principle.
According to the uncertainty principle, a particle’s position, momentum, energy, and time can never be precisely known in a quantum system.
The uncertainty principle arises because the quantum world is so small.
The uncertainty principle
Probability and quantum theory
Because electrons are so tiny, this type of calculation is not possible. Instead, quantum theory uses probability to predict the behavior of large numbers of particles in a system.
Probability describes the chance for getting each possible outcome of a system.
Wave function In quantum theory, each quantum of matter or energy is described by its wave function.
The wave function mathematically describes how the probability for finding a particle is spread out in space.
Quantum theory can only make accurate predictions about the behavior of large systems with many particles.
Unit 4: Matter and Energy Chapter 10: The Atom
10.1 Atomic Structure
10.2 Quantum Theory of the Atom
10.3 Nuclear Reactions
10.3 Investigation: Nuclear Reactions and RadioactivityKey Question:
How do nuclear changes involve energy?
Objectives: Determine the fraction of a radioactive sample that remains
in its original isotope after an integer number of half lives.
Explain how probability and half life are related concepts.
Describe the three different types of radioactive decay (alpha, beta, and gamma decay).
Chemical vs. Nuclear ReactionsThe involvement of energy in
chemical reactions has to do with the breaking and forming of chemical bonds.
A nuclear reaction involves altering the number of protons and/or neutrons in an atom.
The total amount of mass and energy is conserved in nuclear reactions.
Nuclear reactions and energyProtons and neutrons
are attracted by the strong nuclear force and release energy as they come together.
Nuclear reactions often involve huge amounts of energy, as protons and neutrons are rearranged to form different nuclei.
Nuclear reactionsand energy
A nuclear reaction that changed 1 kg of uranium into 1 kg of iron would release 130 trillion J of energy.
Isotope notation In a nuclear reaction, each atom is represented
using isotope notation.
In this notation, the element symbol is given along with its mass number and atomic number.
How many protons, neutrons and electrons are found in this isotope?
Fusion reactions Fusion reactions (the combining of
atomic nuclei) only release energy if the final nucleus has lower energy than the initial nuclei.
The fusion reaction to make magnesium from carbon actually goes through a nuclear changes.
The end result is that 56 TJ are released as the protons and neutrons in 1 kg of carbon-12 are rearranged to make 1 kg of magnesium-24 nucleus.
FusionNuclear fusion occurs in the Sun and the
resulting energy released provides Earth with heat and light.
Fission reactions For elements heavier than iron,
breaking the nucleus up into smaller pieces (fission) releases nuclear energy
A fission reaction can be started when a neutron bombards a nucleus and makes it unstable
The fission of 1 kg of uranium into the isotopes molybdenum-99 and tin-135 releases 98 TJ.
Chain reactionsA chain reaction occurs
when the fission of one nucleus triggers fission of many other nuclei.
The increasing number of neutrons causes even more nuclei to have fission reactions and releases enormous amounts of energy.
FissionA nuclear reactor is a power plant that uses
fission to produce heat.
Radioactive Decay The products of fission
usually have too many neutrons to be stable and are radioactive.
Radioactive means the nucleus continues to change by ejecting protons, neutrons, or other particles.
A process of radioactive decay results in an unstable, radioactive isotope like carbon-14 becoming the more stable isotope nitrogen-14.
Radioactive Decay There are three types of
radioactive decay:
1. alpha decay,
2. beta decay, and
3. gamma decay.
Radioactive Decay In alpha decay, the nucleus ejects two protons and two neutrons. Beta decay occurs when a neutron in the nucleus splits into a
proton and an electron. Gamma decay is not truly a decay reaction in the sense that the
nucleus becomes something different.
Atomic NumberRemember, the atomic
number is the number of protons all atoms of that element have in their nuclei.
If the atom is neutral, it will have the same number of electrons as protons.
Half-lifeThe half-life of carbon-
14 is about 5,700 years.
If you start out with 200 grams of C-14, 5,700 years later only 100 grams will still be C-14.
The rest will have decayed to nitrogen-14.
Half-lifeMost radioactive
materials decay in a series of reactions.
Radon gas comes from the decay of uranium in the soil.
Uranium (U-238) decays to radon-222 (Ra-222).
Carbon dating
Living things contain a large amount of carbon.
When a living organism dies it stops exchanging carbon with the environment.
As the fixed amount of carbon-14 decays, the ratio of C-14 to C-12 slowly gets smaller with age.
Applications of radioactivityMany satellites use radioactive decay
from isotopes with long half-lives for power because energy can be produced for a long time without refueling.
Isotopes with a short half-life give off lots of energy in a short time and are useful in medical imaging, but can be extremely dangerous.
Smoke detectors contain a tiny amount of americium-241, a radioactive isotope.
Indirect Evidence and Archaeology
Using indirect evidence is transforming the field of archaeology.
Using remote sensing techniques, archaeologists can locate and describe features of ancient civilizations before a shovel ever touches the soil.
Centuries of foot travel and camel caravans packed down the desert floor so that 1,700 years later, they still reflected infrared radiation differently than the surrounding terrain.