Atomic Theory. Contribution to Modern Atomic Theory

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1 Alief High School Chemistry STAAR Review Reporting Category 2: Atomic Structure and Nuclear Chemistry C.6.A Understand the experimental design and conclusions used in the development of modern atomic theory, including Dalton s Postulates, Thomson s discovery of electron properties, Rutherford s nuclear atom, and Bohr s nuclear atom. Scientist Dalton Thomson Rutherford Bohr Atomic Theory Contribution to Modern Atomic Theory Dalton s Postulates: Elements are made of very small, indivisible particles called atoms. All atoms of a given element are identical. Atoms of a given element are different from those of any other element and have different atomic masses. In a chemical reaction, atoms of one element can combine with atoms of another element in whole-number rations to form compounds. Chemical reactions change the arrangement of atoms, but do not change atoms of one element into atoms of another element. Used cathode rays to estimate electron s charge-to-mass ratio; model known as plum-pudding model ; described an atom as electrons submerged in a positively charged substance, similar to raisins stuck in dough. Gold foil experiment proved the existence of a dense, charged nucleus surrounded by electrons in mostly empty space; known as the nuclear atom model. Refined the nuclear atom model; electrons orbit the nucleus in distinct, definite energy levels The Modern Atomic Theory (also known as the Quantum Mechanical Model): proposes that electrons travel throughout an electron cloud, a region of space surrounding a nucleus that contains protons and neutrons.

2 C.6.B Understand the electromagnetic spectrum and the mathematical relationships between energy, frequency, and wavelength of light. Electromagnetic Spectrum A wave can be described by its frequency, wavelength, and energy. Vocabulary Words to Know Electromagnetic spectrum Definitions Show wave types from shortest wavelength to longest wavelength; some show longest wavelength to shortest wavelength depending of spectrum. Symbols/examples/SI units See diagram above Wavelength Distance between two adjacent crests or troughs λ is the symbol for wavelength. SI unit: meters

3 Frequency The number of waves that pass a given point per unit of time Waves/sec f is the symbol for frequency used on STAAR formula chart. SI unit: Hertz Speed of light Planck s constant All electromagnetic waves travel at the speed of light in a vacuum. The speed of light is a constant value of 3.00 x 10 8 m/s. German physicist Max Planck determined a relationship between energy and frequency. He determined that matter absorbs electromagnetic radiation in small, specific amounts, called quanta. Planck s constant has a value of 6.63 x J s c is the symbol for speed of light. SI unit: m/s h is the symbol for Planck s constant. SI unit: J s What are the mathematical relationships between wavelength and frequency? All electromagnetic waves travel at the speed of light in a vacuum. The speed of light (c) is a constant value of 3.00 x 10 8 m/s or 300,000,000 meters per second. The product of the wavelength and frequency of a wave equals the speed of light. c = λf Frequency and wavelength are inversely proportional to each other. This means that as one increases, the other decreases such that the product of the two is always the constant c. What are the mathematical relationships between energy and frequency? Planck showed that the amount of radiant energy (E) of a single quantum absorbed or emitted by a body is proportional to the frequency of radiation (f). E photon = hf The constant (h), which has a value of 6.63 x J s, is called Planck s constant. Energy and frequency are directly proportional to one another. This means that as one increases, the other also increases such that the ratio of the two is equal to the constant h.

5 C.6.C Calculate the wavelength, frequency, and energy using Planck s constant and the speed of light. Calculating Wavelength, Frequency, and Energy of Light How can you calculate the wavelength of light using its frequency and the speed of light? c = λf Because the product of wavelength and frequency is equal to a constant, you can always calculate one of these variables if you know the value of the other. For example, if you know the frequency of a wave, you can calculate its wavelength by dividing both sides of the equation by frequency. The result is: λ = c f EXAMPLE #1: Visible light has frequencies between 4.0 x hertz (Hz) and 7.9 x Hz. What are the wavelengths of the lowest frequencies of visible light? SOLUTION #1: λ = 3.00 x 10 8 m/s = 0.75 x 10-6 m or 7.5 x 10-7 m 4.0 x Hz How can you calculate the frequency of light using its wavelength and the speed of light? c = λf If you know the wavelength of a wave, you can calculate its frequency by dividing both sides of the speed of light equation by wavelength. The result is: EXAMPLE #2: If a particular green light has a wavelength of 4.9 x 10-7 m, what is its frequency? SOLUTION #2: f = f = 3.00 x 10 8 m/s = 0.61 x s -1 or 6.1 x s x 10-7 m c λ

6 How can you calculate the energy of light from its frequency using Planck s constant? E photon = hf The constant h is known as Planck s constant. Planck s constant is equal to 6.63 x J s, read jouleseconds. To determine the units of energy, multiply the unit for h, J s, by the unit for f, s -1, to obtain J, the quantity of joules. EXAMPLE #3: The human eye can see light with a frequency about as high as 7.9 x Hz or s -1, which appears violet. Calculate the energy that one photon of violet light carries. SOLUTION #3: E photon = hf E = (6.63 x J s) (7.9 x s -1 ) = 52.3 x J or 5.2 x J of energy. How can you calculate the energy of light from its wavelength using Planck s constant and the speed of light? E photon = hc λ EXAMPLE #4: Find the energy of violet light if λ = 4.0 x 10-7 m SOLUTION #4: E photon = (6.63 x J s)(3.00 x 10 8 m/s) = 5.0 x J 4.0 x 10-7 m

7 C.6.D Use isotopic composition to calculate average atomic mass of an element. Carbon Isotopes Carbon-11 Carbon-12 Carbon-13 Carbon-14 Number of protons Number of neutrons Mass number Calculating Average Atomic Mass What is an isotope? Atoms of the same element, so same # of protons, with different numbers of neutrons are called isotopes. The mass number of an atom is the sum of the atom s protons and neutrons. The mass number is used to identify an isotope and is written after the element name. For example, carbon-14 identifies an isotope of carbon with 6 protons and 8 neutrons. The different isotopes of carbon are shown below. Examples of carbon isotopes: How is isotopic compositions used to calculate average atomic mass of an element? Each element has several isotopes. Rather than listing the mass numbers for all isotopes, the periodic table lists the average atomic mass of each element. The atomic mass of an element is an average mass that is weighted based on the abundance of the isotopes, or isotopic composition, of the element. To calculate the average atomic mass of elements you will need to know the abundance of the isotopes for that particular element.

8 Let s use the isotopic composition of chlorine to calculate the average atomic mass for chlorine. See chart that follows: Isotopic Composition of Chlorine Isotope Atomic mass (amu) Approximate abundance (percent) Chlorine % Chlorine % Problem: Calculate the average atomic mass using the data shown above. To calculate the average atomic mass of elements, multiply the atomic mass (in amu) of each isotope given by its percentage abundance in decimal form. Then add the products together. So, to calculate the average atomic mass of chlorine you will have to do the following: See chart above. ( amu) (0.7578) + ( amu) (0.2422) = amu amu = amu The average atomic mass of chlorine is amu. C.6.E Express the arrangement of electrons in atoms through electron configurations and Lewis valence electron dot structures. Electron Configurations and Lewis Valence Electron Dot Structures How do quantum numbers describe atomic orbitals? The modern atomic theory states that the locations of electrons aren t exact. Instead, mathematical expressions called atomic orbitals describe the probability of finding an electron at various locations around the nucleus. Atomic orbitals differ by size, shape, and energy. The energy levels of electrons are labeled by principal quantum numbers (n). These numbers have positive integer values of 1, 2, 3, and so on. Electrons with the same principal quantum number are in the same principal energy level. Each level contains one or more sublevels. Each sublevel contains one or more atomic orbitals. The second quantum number describes the shape of the atomic orbitals in a sublevel. Each shape is denoted by a letter. The number of subleves is equal to the principal energy level number. For example, level n = 1 has one sublevel, the s sublevel. Level n = 2 contains two sublevels---the s and p sublevels. Level n = 3 has three sublevels---s, p, and d. The third quantum number describes the orientation of the orbital in space. It also describes the number of orbitals in a particular sublevel. The s sublevels contain one orbital, p sublevels contain three orbitals, d sublevels contain five orbitals, and f sublevels contain seven orbitals.

9 Maximum of 2 electrons can go one each orbital. Sublevel s p d f Number of Orbitals Maximum number of Electrons Each orbital can contain 0, 1, or 2 electrons; each with opposite spin direction. This is called the Pauli exclusion principle. Principal energy level Atomic Orbitals and Electrons in Principal Energy Levels Type of sublevel Number of orbitals in sublevels 1 s s, p = s, p, d = s, p, d, f = Maximum number of electrons How is the arrangement of electrons expressed by electron configurations? An electron configuration describes which atomic orbitals hold the atom s electrons. The arrangement of any atom s electrons can be determined by filling lowest energy orbitals first. One exception to pattern is 4s orbitals are lower energy than 3d orbitals. Element Electrons Configuration Alternate Format N 7 1s 2 2s 2 2p Ne 10 1s 2 2s 2 2p Mg 12 1s 2 2s 2 2p 6 3s 2 [Ne]3s 2 Br 35 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 5 [Ar]4s 2 3d 10 4p 5 How is the arrangement of valence electrons expressed by Lewis valence electron dot structures? Lewis valence dot structure: Uses dots to represent atom s valence electrons (highest energy electrons in outermost energy level). The first four dots are arranged individually on four sides of the symbol. Each additional dot is paired with one of the first four dots. 1

10 C.12.A Describe the characteristics of alpha, beta, and gamma radiation. Types of Radiation Unlike normal reactions, nuclear reactions affect the nucleus. They can convert one element to another, releasing particles and tremendous energy. Three main types of nuclear radiation are alpha radiation, beta radiation, and gamma radiation. Particle or Ray Symbols Charge Mass Speed Penetration Alpha (helium α= nucleus) +2 Large Slow Low Beta (electron from nucleus) Gamma ray (photon) β= = no electrical charge and no mass -1 Small Fast Low/medium 0 None Fastest Very high What are the characteristics of alpha radiation? An alpha particle is a helium nucleus emitted by a radioactive source. Another name for alpha particles is alpha radiation. The radioactive decay that results in alpha radiation is often called alpha decay. Each alpha particle consists of two protons and two neutrons and has a double positive charge. When the nucleus emits alpha radiation, the mass number decreases by 2 thereby changing the identity of the element. For example: The above nuclear equation shows alpha decay in a uranium isotope with atomic # = 92 and mass # is 238. When the nucleus emits alpha radiation, its atomic number decreases by 2, so the mass number becomes 90. Because the atomic number has changed, the remaining nucleus is no longer a uranium nucleus, but a thorium nucleus. The mass number decreases by 4, so the mass number is becomes 234. Since the number of protons has decreased, the overall charge of the nucleus has decreased by 2. What are the characteristics of beta radiation? An electron resulting from the changing of a neutron into a proton is called a beta particle. The neutron breaks apart a proton, which remains in the nucleus, and a fast-moving electron, which is released. neutron proton electron (beta particle)

11 Example 1 Example 2 The above nuclear equations show beta decay in thorium (example 1) and carbon (example 2). The mass number does not change. The number of protons increases by 1. Thus, the atomic number increases by 1 and the atom s identity changes. In example 1, Thorium becomes Protactinium. In example 2, Carbon becomes Nitrogen. A related radioactive decay process that is sometimes classified as beta plus radiation is positron emission. In this process, a proton in an unstable nucleus changes to a neutron and releases a positively charged electron. A positron is represented by the notation : Example 1 Example 2 What are the characteristics of gamma radiation? A third type of radioactive decay is called gamma radiation. A high-energy photon emitted by a radioisotope is called a gamma ray. The high-energy photons are a form of electromagnetic radiation. Nuclei often emit gamma rays along with alpha or beta particles. Gamma radiation differs from other types of electromagnetic radiation, such as visible light and radio waves, because it has much higher energy and because it is emitted by the nucleus. The symbol for a gamma ray is the Greek letter gamma, γ.

12 C.12.B Describe radioactive decay process in terms of balanced nuclear equations. Balanced Nuclear Equations Balanced nuclear equations can be used to describe radioactive decay. Nuclear reactions use a superscript on each term to show atomic mass and subscripts to show atomic number (protons); totals before and after reaction must equal (proton changes to neutron to make β). Example 1 : Alpha Decay-Why is this an example of alpha decay? Explain why Th is the new element formed? U Th + He Example 2: Beta Decay-Why is this an beta decay? Explain why N is the new element formed C N + e Positron Emission-How is this equation different from the one above? C B + e Example 3: Gamma Decay- Gamma rays usually are emitted along with other particles. They do not usually appear in nuclear equations since they do not affect the numbers. TRY THESE: Write a balanced nuclear equation for the beta decay of bismuth-210.

13 C.12.C Compare fission and fusion reactions. FISSION AND FUSION REACTIONS What is a fission reaction? When the nuclei of certain isotopes are bombarded with neutrons, they undergo fission, the breaking apart of a nucleus into smaller fragments (pieces). Fission can occur if an atom s nucleus is unstable. 3 neutrons also produced This is a simple diagram illustrating an example of nuclear fission. A U-235 nucleus captures and absorbs a neutron, turning the nucleus into a U-236 atom. The U-236 atom experiences fission into Ba-141, Kr-92, three neutrons, and energy. The ejection of neutrons as a result of the fission reaction is important. These neutrons can, in turn, strike more nuclei, causing them to undergo fission. The result is a chain reaction. Chain reactions release a tremendous amount of energy in a very short time. Fission must be controlled when it is used for the production of electricity and nuclear power plants. Why? Only 2 neutrons emitted in the above equation. What elements were formed? What is a fusion reaction? Fusion is a nuclear process in which two light nuclei combine to form a single heavier nucleus. An example of a fusion reaction important in thermonuclear weapons and in future nuclear reactors is the reaction between two different hydrogen isotopes to form an isotope of helium: 2 H + 3 H ----> 4 He + n

14 Fusion can release much more energy than fission, but fusion is not used for the production of electricity at power plants. An enormous amount of energy is required to start the reaction. Distinguishing between Fission and Fusion Both fission and fusion release enormous amounts of energy. Both fission and fusion reactions can occur in nuclear bombs. So, how can you tell fission and fusion apart? FISSION Heavy nucleus breaks into lighter nuclei Can emit neutrons that can cause other fission reactions Releases enormous amount of energy Radioactive byproducts Uses uranium, a nonrenewable resource FUSION Light nuclei combine to form a heavier nucleus (sun) Products do not result in a chain reaction Releases more energy than fission No radioactive by products Uses easily available hydrogen isotopes Much higher energy needed to initiate fusion than fission Extreme temperatures required make it unusable for production of electricity

Atomic Structure & Nuclear Chemistry Unit 3 Notes

Atomic Structure & Nuclear Chemistry Unit 3 Notes Academic Chemistry Name 52 24 Cr Mass Number Symbol Atomic Number Unit #3 Test Date You can never learn less, you can only learn more. R. Buckminster Fuller