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Atomic Structure and Nuclear Radiation CHAPTER 19.1, 25 Structure of the Atom CHAPTER 19, SECTION 1 Chemical Symbols • Chemical symbol: a letter or combination of letters to represent an element • First letter is always capitalized • For some elements, the symbol is the first letter of the element’s name • Some symbols are derived from Latin • Ex: the symbol for silver, Ag, comes from the Latin word argentum Chemical Symbols Atomic Components • Atom: the smallest piece of matter that retains the property of an element • Atoms are made up of subatomic particles: • Protons: subatomic particles with a charge of +1 • Electrons: subatomic particles with a charge of -1 • Neutrons: subatomic particles with no charge • Atoms of different elements have a different number of protons Location of Subatomic Particles electrons protons nucleus neutrons 6 Quarks • Protons and neutrons are made up of smaller particles called quarks • Scientists have found evidence that supports the existence of 6 quarks • They theorize that a certain arrangement of 3 quarks produce a proton • Other arrangements of 3 quarks produce neutrons Atomic Mass • The nucleus contains most of the mass of an atom because protons and neutrons are far more massive than electrons • The mass of the proton and neutron are the same • Proton: 1.6726 X 10-24 • Neutron:1.6726 X 10-24 • Electron: 9.1093 X 10-28 Protons Identify an Element • Atoms of different elements are different because they have different numbers of protons • The protons tells you what type of atom you have • The number of protons in an atom is equal to a number called the atomic number • Atomic number of Carbon is 6 • So carbon has 6 protons Mass Number • The mass number of an atom is the sum of the number of protons and the number of neutrons in the nucleus of a tom • Atomic Mass = Number of neutrons + Atomic Number (Number of Protons) • Number of neutrons = Mass number – atomic number How to write this… Example Mass Number 23 11 Atomic Number Element Symbol Na Another Way to Write Elements… Mass Number 23 11 Na Mass Number Sodium – 23 or Na - 23 Practice! • How many protons, electrons, and neutrons are in each atom? 1. 94Be 2. 147N 3. 6530Zn 4. 8035Br Bell Work 2/25/16 1. What are the rules for writing a Chemical Symbol for an element? (you should have two of these) 2. How do you find the number of neutrons present in an atom? 3. Complete the following table Particle Proton Neutron Electron Location in Atom Charge Mass Let make sure we got this before we move on Substance Sodium Silver Zinc Chlorine Chemical Symbol Atomic Number (Z) Atomic Mass # of Protons (A) # of Neutrons # of Electrons Atomic Mass & Number CHAPTER 19, SECTION 2 Atomic Mass • The unit of measurement for atomic particles is the atomic mass unit (amu) • The mass of a proton is about the same as that of a neutron (~1 amu) • Atomic mass unit (amu): one-twelfth the mass of a carbon atom containing six protons and six neutrons Atomic Number • The number of protons tells you what type of atom you have • Atomic number: the number of protons in an atom • The atomic number is different for each of the elements • Ex: State the number of protons for atoms of each of the following: • Nitrogen (N) • Sulfur (S) • Barium (Ba) Atomic Number • If an atom is neutral (it has NO charge), then there must be the same number of electrons as protons in that atom Neutral atom: # of protons = # of electrons Mass Number • Mass number: the sum of the protons and neutrons in the nucleus of an atom Mass Number = # protons + # neutrons • So, if you want to find # neutrons… # of neutrons = mass # - atomic # How to write this… Example Mass Number 23 11 Atomic Number Element Symbol Na Another Way to Write Elements… Mass Number 23 11 Na Mass Number Sodium – 23 or Na - 23 Practice! • How many protons, electrons, and neutrons are in each atom? 1. 94Be 2. 147N 3. 6530Zn 4. 8035Br Isotopes • Isotopes: • Atoms with the same number of protons, but different numbers of neutrons • Atoms of the same element (same atomic number) with different mass numbers Isotopes of chlorine: 35 Cl 17 37 Cl 17 chlorine – 35 chlorine - 37 Isotopes Isotopes of carbon include: carbon-12, carbon-13, and carbon-14 Write the symbol for each isotope using superscripts and subscripts to represent the mass and atomic numbers Learning Check Write the atomic symbols for atoms with the following: A. 8 p+, 8 n0, 8 e- ___________ B. 17p+, 19n0, 17e- ___________ C. 47p+, 60 n0, 47 e- ___________ Ions • Ion: an atom that has gained or lost electrons • In non-nuclear chemistry, an atom never gains or loses a proton; only the number of electrons is affected during chemical reactions Ions • To determine the charge of an ion, compare the # of protons and # of electrons • If there are more protons, the ion has a positive charge equal to the extra protons • If there are more electrons, the ion has a negative charge equal to the extra electrons Cations • Cations • Positive charge • Formed by losing electrons • Metals form cations Ca Ca2+ + 2e- Anions • Anions • Negative charge • Formed by gaining electrons • Non-metals form anions O + 2e- O2- Writing Chemical Symbols for Ions • Oxygen (O) has 8 protons, 8 neutrons, but has a tendency to gain two electrons. • This is written as follows: 16 O28 • Write the chemical symbol for the ion with 13 protons and 10 electrons. 27 Al3+ 13 Average Atomic Mass • Average atomic mass: the weighted-average mass of all the atomic masses of the isotopes of that atom • Ex: Four of five atoms of boron are boron-11 and one out of five atoms of boron are boron-10 4/5 (11 amu) + 1/5 (10 amu) = 10.8 amu *Note that the average atomic mass of boron is closer to boron-11 as it is more abundant in nature Homework Quiz • Part I • Complete the Chart • Part II • #1-10 on the backside (Matching) Complete the Following Table Atomic # Mass # # p+ # e- 1) 17 2) 180 3) 4) 92 5) 40 238 # n0 charge 19 0 71 109 38 46 Symbol 86 206 82 Pb4+ Complete the Following Table Atomic # Mass # # p+ # e- 1) 14 2) 16 75 3) 4) 99 5) # n0 51 252 36 42 54 79 charge Symbol +4 96 208 82 Pb2+ Bell Work 2/29 • Please write the following questions and answers in your composition notebooks 1. What is an ion? 2. What is an Isotope? 3. What is the number of neutrons that Carbon-14 has? 4. How many electrons does Ni+2 have Radioactivity C H AP TER 2 5 , SE C T I ON 1 Elements and Protons • Recall that every element has a different number of protons • For an atom of one element to change into a different element, the number of protons in its nucleus must change • The nucleus of an atom contains almost all the mass, but it occupies only a tiny fraction of space in the atom • The size of a nucleus in an atom can be compared to a marble sitting in the middle of an empty football stadium The Strong Force • Within the nucleus, the positive electric forces of the protons repel each other • So why don’t the protons push each other away? • The strong force causes protons and neutrons to be attracted to one another • This force is 100 times stronger than electric force • In order for the strong force to be effective, the particles must be close to one another The Strong Force • The fewer protons and neutrons in a nucleus of an atom, the stronger the strong force that holds them together • The more protons and neutrons in a nucleus of an atom, the weaker the strong force that holds them together Radioactivity • Many nuclei are held together permanently and are stable • Some types of nuclei are unstable • These nuclei break apart, or decay, by emitting particles and energy • Radioactivity: the ability of an atom to emit, or give off, charged particles and energy from its nucleus Radioactivity • Nuclei that contain a large number of protons and neutrons tend to be unstable • All nuclei that contain more than 83 protons are radioactive • Almost all elements with more than 92 protons don’t exist naturally on Earth Converting Mass into Energy • According to the law of the conservation of mass, mass cannot be created or destroyed • However, in nuclear reactions, mass can be converted into energy • A large amount of energy is produced by the conversion of only a small amount of mass Isotopes & Nuclear Numbers • Isotopes of the same element contain the same number of protons, but a different number of neutrons • The total number of protons and neutrons in an atom is called the mass number • When writing the symbol for an atom, use the AZX format • You can also indicate the mass of an isotope by writing it after the name of the element • Ex: carbon-14 has a mass of 14 amu Nuclear Decay C H AP TER 2 5 , SE C T I ON 2 Nuclear Radiation • There are three types of nuclear radiation: • Alpha • Beta • Gamma • Alpha and beta radiation are particles, while gamma radiation is an electromagnetic wave Alpha Particles • Alpha particle: a particle made of two protons and two neutrons that is emitted from a decaying nucleus • An alpha particle is the same as the nucleus of a helium atom and has the symbol: 4 He 2 • Alpha particles have the most mass and electric charge of all types of nuclear radiation Alpha Particles • Alpha particles exert an electric force on electrons in atoms along their path, leaving charged ions behind • Alpha particles lose energy quickly • Alpha particles are the least likely to travel through matter and can be stopped by a sheet of paper Alpha Particles • Alpha particles can be dangerous if they are released by radioactive atoms inside the human body • Damage from alpha particles can cause cells not to function properly, leading to illness and disease • Some smoke detectors give off alpha particles to ionize the surrounding air • If smoke particles enter the ionized air, they will absorb the ions and electrons so the circuit is broken and the alarm goes off Alpha Particles • Transmutation: the process of changing one element to another through nuclear decay • In alpha decay, two protons and two neutrons are lost from the nucleus • Thus, alpha decay forms a new element that has an atomic number two less than that of the original element • In addition, the mass number of the new element is four less than the original element Alpha Particles Beta Particles • Beta particle: the electron emitted from an unstable nucleus when a neutron decays into a proton • Beta particles are symbolized like this: 0 e -1 or 0 β -1 • Beta decay is caused by another basic force called the weak force • The new atom formed in beta decay has one more proton, but the mass of the atom is unchanged from the original element Beta Particles Beta Particles • Beta particles are much faster and can travel through matter much better than alpha particles • They can pass through paper, but are stopped by a sheet of aluminum foil • Beta particles can damage cells when they are emitted by radioactive nuclei inside the human body Beta Particles Beta Particles • Positron emission: a subtype of beta decay in which a proton inside a radioactive nucleus is converted to a neutron while releasing a positron (positively charged particle) • Symbolized as follows: 0 β +1 Gamma Rays • Gamma rays: electromagnetic waves with the highest frequencies and the shortest wavelengths in the electromagnetic spectrum • Gamma rays have no mass and no charge and travel at the speed of light • Gamma rays are symbolized as follows: 0 γ 0 • Gamma rays are usually emitted from a nucleus when alpha decay or beta decay occurs Gamma Rays Gamma Rays • Gamma rays can be stopped by blocks of dense materials, such as lead and concrete • However, gamma rays cause less damage to biological molecules as they pass through living tissue because it has no mass or electric charge Radioactive Half-Life • Half-life: a measure of the time required by the nuclei of an isotope to decay • The nucleus left after the isotope decays is called the daughter nucleus • Half-lives vary widely among radioactive isotopes • Ex: Polonium-214 has a half-life of less than a thousandth of a second • Ex: Uranium-238 has a half-life of 4.5 billion years Calculating Half-Life • There are two ways to calculate half-life: 1. Use the formula: t1/2 = (t x log2) / log (N0/Nt) 2. Manually calculate the half-lives of the substance until the desired amount is reached Half-Life Example • Suppose the initial number of atoms of a radioactive isotope is 2016 and after 35 days it decays to 63. Calculate the half-life of the substance. Initial amount = 2016 Final amount = 63 Time = 35 days • Simply divide the initial amount by 2 until it is reduced to 63 Half-Life Example • First half-life: 2016/2 = 1008 • Second half-life: 1008/2 = 504 • Third half-life: 504/2 = 252 • Fourth half-life: 252/2 = 126 • Fifth half-life: 126/2 = 63 • Therefore, it takes 5 half-lives to undergo decay from 2016 to 63 atoms. In other words, it takes 35 days to complete 5 half-lives. • The half-life of the isotope is therefore, 35 days / 5 half-lives = 7 days Carbon Dating • The radioactive isotope carbon-14 is often used to estimate the ages of plant and animal remains • Carbon-14 has a half-life of 5,730 years and is found in molecules such as carbon dioxide • The decaying carbon-14 in a plant or animal is replaced when an animal eats or when a plant makes food • As a result, the number of carbon-14 atoms compared to the number of carbon-12 atoms remains nearly constant Carbon Dating • But when an organism dies, its carbon-14 atoms decay without being replaced • The carbon-14 to carbon-12 ratio decreases with time allowing the age of the organism’s remains to be estimated • However, only material from plants and animals that lived within the past 50,000 years contains enough carbon-14 to be measured Uranium Dating • Radioactive dating also can estimate the ages of rocks • Some rocks contain uranium, which has two radioactive isotopes with long half-lives • Each uranium isotope decays into a different isotope of lead • From the ratios of the uranium isotopes to their daughter nuclei, the number of half-lives since the rock was formed can be calculated •What is the half-life of a 100.0 g sample of nitrogen-16 that decays to 12.5 grams in 21.6 seconds? •A 208 g sample of sodium-24 decays to 13.0 g of sodium-24 within 60.0 hours. What is the half-life of this radioactive isotope? Nuclear Reactions C H AP TER 2 5 , SE C T I ON 4 Nuclear Fission • Nuclear fission: the process of splitting a nucleus into several smaller nuclei • Only large nuclei, such as uranium and plutonium undergo nuclear fission • A fission reaction usually produces several individual neutrons in addition to the smaller nuclei Nuclear Fission • The total mass of the products is slightly less than the mass of the original nucleus and the neutron • This small amount of missing mass is converted to a tremendous amount of energy during nuclear fission Nuclear Fission Nuclear Fission • Albert Einstein proposed that mass and energy were related in his special theory of relativity: E = mc2 • A small amount of mass can be converted into an enormous amount of energy • Ex: if 1 g of mass is converted to energy, about 100 trillion joules of energy are released Nuclear Fission • When a nuclear reaction occurs, the neutrons emitted can strike other nuclei in the sample and cause them to split • These reactions release more neutrons, causing additional nuclei to split • Chain reaction: the series of repeated fission reactions caused by the release of neutrons in each reaction Nuclear Fission • Chain reactions can be controlled by adding materials that absorb neutrons • In order for a chain reaction to occur, a critical mass of material that can undergo fission must be present • Critical mass: the amount of material required so that each fission reaction produces approximately one more fission reaction • If less than the critical mass of material is present, a chain reaction will not occur Nuclear Fusion • Nuclear fusion: two nuclei with small masses combine to form a nucleus of larger mass • Nuclear fusion reactions can release even more energy than nuclear fission reactions • Nuclear reactions release much more energy than chemical reactions because the strong force is much stronger than the electric force Nuclear Fusion • For nuclear fusion to occur, positively charged nuclei must get close to each other • If they are moving fast, their kinetic energy overcomes the repulsive electrical force between them • As the temperature increases, the kinetic energy of atoms and molecules increases • Only at temperatures of millions degrees Celsius are nuclei moving so fast they can get close enough for fusion to occur • Ex: the Sun Nuclear Fusion • The Sun is made of mostly hydrogen • Most of the energy given off by the Sun is produced by the fusion of hydrogen nuclei • The net result is that four hydrogen nuclei are converted into one helium nucleus Nuclear Fusion Nuclear Fusion • Earth receives a small amount of energy as thermal energy and light • As the Sun ages, the hydrogen nuclei are used up as they are converted into helium • It is estimated that the Sun has enough hydrogen to keep reacting for another 5 billion years The Periodic Table C H AP TER 1 9 , SE C T I ON 3 Mendeleev’s Periodic Table • In the 1800s, Dmitri Mendeleev searched for a way to organize the elements • He arranged them in order of increasing atomic mass Today’s Periodic Table • Mendeleev found that certain chemical properties were repeated in specific patterns • However, masses of the elements do not follow a strict pattern • Today, we arrange the periodic table in order of increasing atomic number Modern Periodic Table Groups & Valence Electrons • Groups(aka families): the vertical columns that are numbered 1 through 18 • Elements in the same group have similar properties • According to the Electron Cloud Model, electrons within a certain cloud have different amounts of energy Groups & Electrons • Electrons are placed in specific energy levels according to the amount of energy they have • Energy levels nearer the nucleus have lower energy than those farther away Groups & Electrons • Electrons fill energy levels of lowest energy first • Elements in the same group have the same number of electrons in their outer energy levels (valence electrons) • It is the number of valence electrons that determine the chemical properties of an element Groups & Electrons • For Groups 1 and 2, the number of valence electrons is equal to the group number • For Groups 13 (3A) through 18 (8A), the number of valence electrons is equal to the group number minus 10 … or equal to groupA # Periods & Energy Levels • Periods: the horizontal rows on the periodic table • Energy levels are numbered 1-7 and correspond to the horizontal rows or periods, on the periodic table Metals, Nonmetals, and Metalloids • Metals: generally good conductors of heat and electric current • Most elements (80%) • Nonmetals: poor conductors of heat and electric current • Upper right-hand corner of periodic table except for hydrogen • Metalloids: (aka semiconductors) have similar properties to both metals and nonmetals • Border the stair-step line that separates metals from nonmetals Classifying Elements further • Elements can be classified as metals, nonmetals, and metalloids • Elements are further classified into five families • The elements in a family have the same number of valence electrons Group Number # of valence electrons Name of Family 1 1 Alkali Metals 2 2 Alkaline-earth Metals 3-12 Varied Transition Metals 17 7 Halogens 18 8 (except He) Noble Gases • Soft and shiny • Reacts violently with water • Alkali metals are often store in oil to prevent them from reacting with moisture in the air • Alkali metals are very reactive because they have just one valence electron that can be easily removed to forma a positive ion • Only found in nature as a compound because of its reactivity Alkali Metals Alkaline- Earth Metals • In general, alkalineearth metals are harder, denser, stronger, and have higher melting points than alkali metals • Two Valence Electrons • Less reactive than alkali metals, but still react to form ions with +2 charge • Groups 3-12 • Much less reactive than alkali and alkaline metals • Some transition metals can form as many as four differently charged cations because of their complex arrangement of electrons • Transition metals are harder, more dense and have higher melting points (except Mercury) Transition Metals Halogens • Most reactive non-metals • 7 valence electrons Nobel Gases • Unreactive because its s and p orbitals are filled • Do not gain or lose electrons Bell Work 3/9 • Please write the questions and the answer for the following questions in your composition notebook 1. What is the group number for the following elements? • • • • 2. Sodium Fluorine Sulfur Germanium What is the period number for the following elements? • • • • 3. Arsenic Potassium Chlorine Bismuth What is the family name of the following elements? • • • • 4. Mercury Cesium Radon Chlorine How many valence electrons do the following elements have? • • • • Calcium Carbon Fluorine Argon Bohr Diagrams • Steps to drawing a Bohr Diagram: 1. Find your element on the periodic table 2. Determine the number of electrons (note: it will be the same as your protons for a neutral atom) • This is how many electrons you will draw Bohr Diagrams 3. Find out which period your element is on • Elements in the 1st period have one energy level • Elements in the 2nd period have two energy levels • And so on… 4. Draw a nucleus and indicate the number of protons (p) and neutrons (n) Bohr Diagrams 5. Add the electrons • One at a time • Start on the top and go clockwise • Fill the lowest energy levels first • Use the following table to help Electron Shell # of Electrons 1 2 2 8 3 8 4 18 5 18 6 32 7 32 Bohr Diagrams • Note: The electron shells are a “model” for locating valence electrons • In actuality, electrons are found in orbitals (clouds) according to energy levels • The third energy level can contain a maximum of 18 electrons because of an overlapping in orbitals • The fourth energy level can contain a maximum of 32 electrons because of an overlapping in orbitals • BUT… we will only deal with the first few periods Examples Electron Dot Diagrams • Recall that elements in the same group have the same number of electrons in their outer energy levels (i.e. they have the same number of valence electrons!) • Electron dot diagram: uses the symbol of the element and dots to represent valence electrons • The electron configuration of an atom determines how it reacts with other atoms Electron Dot Diagrams • Steps to drawing electron dot diagrams: 1. Find your element on the periodic table and write its chemical symbol 2. Determine the number of valence electrons (the number of electrons in the outermost shell) • Determine what group (column) your element is in • This will tell you the number of valence electrons to draw Electron Dot Diagrams 3. Draw your valence electrons • One at a time • Starting on the right moving clockwise • Only place one electron per “side” before coupling up • You may only have a maximum of 8 valence electrons around the symbol Examples Complete a Electron Dot Models for the following elements in the space on your worksheet 1. Calcium 2. Potassium 3. Argon 4. Aluminum 5. Bromine 6. Carbon 7. Helium 8. Oxygen 9. Phosphorus 10. Hydrogen