Matter and Energy

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

Key Question: How is an atom organized? 10.1 Investigation: The Atom 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 theorythat contained the following four statements: • Matter is composed of tiny, indivisible, and indestructible particles called atoms. • 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. • 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. • Atoms do not change their identities in chemical reactions. They are just rearranged into different substances.

Review: Elements and Compounds • An 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 protonis a particle with a positive charge. • An electronis a particle with a negative charge. • A neutronis a neutral particle and has a zero charge. • The charges on one proton and one electron are exactly equal and opposite. • Chargeis 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 force • Henry 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 force • The 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 force • Hideki 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 force • A 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 differ • The 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.

Calculating average atomic mass How many neutrons are present in an aluminum atom that has an atomic number of 13 and a mass number of 27? • Looking for:… number of neutrons in aluminum-27. • Given:… atomic no. = 13; mass no. = 27 • Relationships:Periodic table says atomic no. = proton no. and mass no. = protons + neutrons • Solution:neutrons = mass no. – proton no. no. neutrons = 27 – 13 = 14

Key Question: How does an atom absorb and emit light energy? 10.2 Investigation: Energy and the Quantum Theory 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 spectrum • Each 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 spectrum • Each 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 • The Bohr atom led to a new way of thinking about energy in atomic systems. • A quantais 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 theorysays 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.

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 staterather 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 principleafter Wolfgang Pauli (1900–1958).

Planck’s constant • The “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 probabilityto 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.

Key Question: How do nuclear changes involve energy? 10.3 Investigation: Nuclear Reactions and Radioactivity 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 Reactions • The 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 energy • Protons 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.

Fusion • Nuclear 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 reactions • A chain reactionoccurs 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.

Fission • A nuclear reactor is a power plant that uses fission to produce heat.

Matter and Energy