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Table of Contents 17 Unit 4: The Nature of Matter Chapter 17: Properties of Atoms and the Periodic Table 17.1: Structure of the Atom 17.2: Masses of Atoms 17.3: The Periodic Table Structure of the Atom 17.1 Scientific Shorthand • Scientists have developed their own shorthand for dealing with long, complicated names. • Chemical symbols consist of one capital letter or a capital letter plus one or two smaller letters. Structure of the Atom 17.1 Scientific Shorthand • For some elements, the symbol is the first letter of the element's name. • For other elements, the symbol is the first letter of the name plus another letter from its name. • Because scientists worldwide use this system, everyone understands what the symbols mean. Structure of the Atom 17.1 Atomic Components • An element is matter that is composed of one type of atom, which is the smallest piece of matter that still retains the property of the element. • Atoms are composed of particles called protons, neutrons, and electrons. Click image to view movie Structure of the Atom 17.1 Atomic Components • Protons and neutrons are found in a small positively charged center of the atom called the nucleus that is surrounded by a cloud containing electrons. • Protons are particles with an electrical charge of 1+. Structure of the Atom 17.1 Atomic Components • Electrons are particles with an electrical charge of 1–. • Neutrons are neutral particles that do not have an electrical charge. Structure of the Atom 17.1 Quarks—Even Smaller Particles • Protons and neutrons are made up of smaller particles called quarks. • So far, scientists have confirmed the existence of six uniquely different quarks. Structure of the Atom 17.1 Quarks—Even Smaller Particles • Scientists theorize that an arrangement of three quarks held together with the strong nuclear force produces a proton. • Another arrangement of three quarks produces a neutron Structure of the Atom 17.1 Finding Quarks • To study quarks, scientists accelerate charge particles to tremendous speeds and then force them to collide with—or smash into— protons. This collision causes the proton to break apart. • The particles that result from the collision can be detected by various collection devises. Structure of the Atom 17.1 Models—Tools for Scientists • Scientists and engineers use models to represent things that are difficult to visualize—or picture in your mind. • Scaled-down models allow you to see either something too large to see all at once, or something that has not been built yet. • Scaled-up models are often used to visualize things that are too small to see. “Atomic Structure” Studying Atoms • Different Models of the atom – Greek Model – Dalton’s Atomic Theory – Thomson’s Model – Rutherford’s Atomic Theory – Bohr’s Model of the Atom – Electron Cloud Model Structure of the Atom 17.1 Models—Tools for Scientists • To study the atom, scientists have developed scaled-up models that they can use to visualize how the atom is constructed. • For the model to be useful, it must support all of the information that is known about matter and the behavior of atoms. Aristotle • Aristotle thought that all substances were built up from only 4 elements – Earth – Air – Fire – Water These elements were a combination of 4 qualities hot, cold, dry, and wet The Hellenic Market Fire ~ ~ Water Earth Air Aristotle Aristotle • Aristotle did not think there was a limit to the number of times matter could be divided. »For centuries people believed this model. Defining the Atom • The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”) ATOM – He believed that atoms were indivisible and indestructible – His ideas did agree with later scientific theory, but did not explain chemical behavior, and was not based on the scientific method– but just philosophy Greek Model “To understand the very large, we must understand the very small.” Democritus • Greek philosopher • Idea of ‘democracy’ • Idea of ‘atomos’ – Atomos = ‘indivisible’ – ‘Atom’ is derived • No experiments to support Democritus’s model of atom idea No protons, electrons, or neutrons • Continuous vs. discontinuous Solid and INDESTRUCTABLE theory of matter Who Was Right? • • • • • • • • California WEB Greek society was slave based Beneath famous to work with hands did not experiment Greeks settled disagreements by argument Aristotle was more famous He won! His ideas carried through middle ages. Alchemists change lead to gold John Dalton John Dalton • John Dalton (17661844) was known as the “Father of the modern atom.” • He was the first to actually test, previously the Greeks would just theorize. In his work he developed Dalton's Atomic Theory. Structure of the Atom 17.1 The Changing Atomic Model • In the 1800s, John Dalton, an English scientist, was able to offer proof that atoms exist. Because Dalton’s atomic theory was proven through many experiments His THEORY became widely accepted. Dalton’s Atomic Theory (experiment based) 1) All elements are composed of tiny indivisible particles called atoms 2) Atoms of the same element are identical. 3) Atoms Can’t be Created nor Destroyed! John Dalton (1766 – 1844) 4) Atoms of different elements combine in simple whole-number ratios to form chemical compounds 5) In a particular compound, atoms of different elements always combine in the same way 6) All atoms of the same element have the same mass, atoms of diff. elements have diff. masses Dalton’s Model • The Elements are pictured as solid spheres: • Each type of atom is represented by a tiny, solid sphere with a different mass. Dalton’s Elements John Dalton 1808 JJ Thomson JJ. Thomson • J.J. used the idea behind charges: -- Like charges repel -- Unlike charges attract With this idea in mind, J.J. used electric current to study the atom. Remember my ex: with the Magnets Sir William Crookes (1832 - 1919) was the British scientist who invented the cathode ray tube. His work paved the way to the discovery of the electron. - voltage source William Crookes + vacuum tube magnet metal disks Crooke’s Tube Discovery of the Electron Looking at what JJ saw, what do you think he determined was the charge on the particles in the beam? Thomson’s Experiment ON - OFF voltage source By adding an electric field… + + - he found that the moving pieces were negative. In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron The Electron was named by G. Johnstone Stoney ! ** Thomsons experiments were the 1st proof that atoms are made of smaller particles. Mass of the Electron Mass of the electron is 9.11 x 10-28 g The oil drop apparatus 1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge Conclusions from the Study of the Electron: a) Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. b) Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons c) Electrons have so little mass that atoms must contain other particles that account for most of the mass Some Modern Cathode Ray Tubes Cathode ray tubes pass electricity through a gas that is contained at a very low pressure. J.J. Thomson • J.J. reasoned that if the atom had negative little particles inside it, but was considered to be NEUTRAL, then there must also be some positive particles too. Thomson’s Atomic Model J. J. Thomson Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model. Conclusions from the Study of the Electron: Eugen Goldstein in 1886 observed what is now called the “proton” particles with a positive charge, and a relative mass of 1 (or 1840 times that of an electron) 1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton Rutherford’s Atomic Theory Ernest Rutherford (1871-1937) • Learned physics in J.J. Thomson’ lab. • Noticed that ‘alpha’ particles were sometime deflected by something in the air. • Gold-foil experiment Ernest Rutherford’s Gold Foil Experiment - 1911 Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil Particle that hit on the detecting screen (film) are recorded Rutherford ‘Scattering’ 1. 2. 3. In 1909 Rutherford undertook a series of experiments He fired a (alpha) particles at a very thin sample of gold foil According to the Thomson model the a particles should only be slightly deflected 4. Rutherford discovered that they were deflected through large angles and could even be reflected straight back to the source Lead collimator Gold foil a particle source q Rutherford’s problem: In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target? Target #1 Target #2 The Answers: Target #1 Target #2 Rutherford’s Findings Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected “Like howitzer shells bouncing off of tissue paper!” Conclusions: a) The nucleus is small b) The nucleus is dense c) The nucleus is positively charged 152 mm howitzer-gun M1937 The Rutherford Atomic Model Based on his experimental evidence: The atom is mostly empty space All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus” The Rutherford Atomic Model • The nucleus is composed of protons and neutrons –The electrons distributed around the nucleus, and occupy most of the volume –His model was called a “nuclear model” Bohr’s Model of the Atom Niels Bohr • In the Bohr Model (1913) the neutrons and protons occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun. • They are not confined to a planar orbit like the planets are. Bohr Model Planetary model After Rutherford’s discovery, Bohr proposed that electrons travel in definite orbits at constant speeds around the nucleus like planets around the sun. Staircase Idea • To understand energy levels, picture them as steps in a staircase. • Just as you can’t stand on 2 steps at one time, Electrons can’t occupy more than 1 energy level at a time. • Bottom step = Lowest Energy Level • Top Step = Highest Energy Level • Cont… Staircase Model • An electron in an ATOM can move from one energy level to another if enough energy is provided. • The size of the jump ( 1 level or 2) determines the amt. of energy gained or lost. Humor • Two atoms are walking down the street. • One atom says to the other, “Hey! I think I lost an electron!” • The other says, “Are you sure??” • “Yes, I’m positive!” • • • • A neutron walks into a restaurant and orders a couple of drinks. As she is about to leave, she asks the waiter how much she owes. The waiter replies, “For you, No Charge!!!” Structure of the Atom 17.1 The Electron Cloud Model • By 1926, scientists had developed the electron cloud model of the atom that is in use today. • An electron cloud is the area around the nucleus of an atom where its electrons are most likely found. Structure of the Atom 17.1 The Electron Cloud Model • The electron cloud is 100,000 times larger than the diameter of the nucleus. • In contrast, each electron in the cloud is much smaller than a single proton. • Because an electron's mass is small and the electron is moving so quickly around the nucleus, it is impossible to describe its exact location in an atom. • EX: Propeller of an Airplane’s wings ! Section Check 17.1 Question 1 Which is the smallest piece of matter that still retains the property of the element? A. atom B. quark C. neutron D. proton Section Check 17.1 Question 2 What particles are found in the nucleus of an atom? A. protons and electrons B. protons and neutrons C. neutrons and electrons D. quarks and electrons Section Check 17.1 Question 3 What is the name of the small particles that make up protons and neutrons? Question 4 • The Greek philosopher Democritus coined what word for a tiny piece of matter that cannot be divided? • a. element c. electron • b. atom d. molecule Question 5 Which of the following is NOT part of John Dalton’s atomic theory? • a. All elements are composed of atoms. • b. All atoms of the same element have the same mass. • c. Atoms contain subatomic particles. • d. A compound contains atoms of more than one element. Question 6 Which of the following most accurately represents John Dalton’s model of the atom? • a. a tiny, solid sphere with an unpredictable mass for a given element • b. a hollow sphere with a dense nucleus • c. a tiny, solid sphere with predictable mass for a given element • d. a sphere that is hollow throughout Question 7 • JJ Thomson’s experiments provided evidence that an atom • a. is the smallest particle of matter. • b. contains negatively charged particles. • c. has a negative charge. • d. has a positive charge. Question 8 Rutherford’s gold foil experiment provided evidence for which of the following statements? • a. Negative and positive charges are spread evenly throughout an atom. • b. Alpha particles have a positive charge. • c. Gold is not as dense as previously thought. • d. There is a dense, positively charged mass in the center of an atom. Question 9 • Who provided evidence for the existence of a nucleus in an atom? • a. John Dalton c. Democritus • b. J.J. Thomson d. Ernest Rutherford Question 10 In an atomic model that includes a nucleus, positive charge is • a. concentrated in the center of an atom • b. spread evenly throughout an atom. • c. concentrated at multiple sites in an atom. • d. located in the space outside the nucleus. Part 2 Masses of Atoms 17.2 Atomic Mass • The nucleus contains most of the mass of the atom because protons and neutrons are far more massive than electrons. • The mass of a proton is about the same as that of a neutron— approximately Masses of Atoms 17.2 Atomic Mass • The mass of each is approximately 1,836 times greater than the mass of the electron. Masses of Atoms 17.2 Atomic Mass • The unit of measurement used for atomic particles is the ______________________. atomic mass unit (amu) • The mass of a proton or a neutron is almost equal to 1 amu. • The atomic mass unit is defined as onetwelfth the mass of a carbon atom containing six protons and six neutrons. Masses of Atoms 17.2 Protons Identify the Element • The number of protons tells you what type of atom you have and vice versa. For example, every carbon atom has six protons. Also, all atoms with six protons are carbon atoms. • The number of protons in an atom is equal to a number called the atomic number. Masses of Atoms 17.2 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 an atom. Masses of Atoms 17.2 Mass Number • If you know the mass number and the atomic number of an atom, you can calculate the number of neutrons. number of neutrons = mass number – atomic number Masses of Atoms 17.2 Isotopes • Not all the atoms of an element have the same number of neutrons. • Atoms of the same element that have different numbers of neutrons are called isotopes. • Remember – Protons Never Change Masses of Atoms 17.2 Identifying Isotopes • Models of two isotopes of boron are shown. Because the numbers of neutrons in the isotopes are different, the mass numbers are also different. • You use the name of the element followed by the mass number of the isotope to identify each isotope: boron10 and boron-11. Masses of Atoms 17.2 Identifying Isotopes • The average atomic mass of an element is the weighted-average mass of the mixture of its isotopes. • For example, four out of five atoms of boron are boron-11, and one out of five is boron-10. • To find the weighted-average or the average atomic mass of boron, you would solve the following equation: Plug the following in on a calculator Order: 1. 4 / 5 X 11 = ? 2. 1 / 5 X 10 = ? 3. Add the 2 numbers = ? Section Check 17.2 Question 1 How is the atomic number of an element determined? Section Check 17.2 Question 2 The element helium has a mass number of 4 and atomic number of 2. How many neutrons are in the nucleus of a helium atom? Section Check 17.2 Question 3 How much of the mass of an atom is contained in an electron and what is the charge of an electron? Part 3 Masses of Atoms 17.2 Atomic Mass The Periodic Table 17.3 Organizing the Elements • Periodic means "repeated in a pattern." • In the late 1800s, Dmitri Mendeleev, a Russian chemist, searched for a way to organize the elements. • When he arranged all the elements known at that time in order of increasing atomic masses, he discovered a pattern. The Periodic Table 17.3 Organizing the Elements • Because the pattern repeated, it was considered to be periodic. Today, this arrangement is called a periodic table of elements. • In the periodic table, the elements are arranged by increasing atomic number and by changes in physical and chemical properties. The Periodic Table 17.3 Mendeleev's Predictions • Mendeleev had to leave blank spaces in his periodic table to keep the elements properly lined up according to their chemical properties. • He looked at the properties and atomic masses of the elements surrounding these blank spaces. The Periodic Table 17.3 Mendeleev's Predictions • From this information, he was able to predict the properties and the mass numbers of new elements that had not yet been discovered. The Periodic Table 17.3 Mendeleev's Predictions • This table shows Mendeleev's predicted properties for germanium, which he called ekasilicon. His predictions proved to be accurate. The Periodic Table 17.3 Improving the Periodic Table • On Mendeleev's table, the atomic mass gradually increased from left to right. If you look at the modern periodic table, you will see several examples, such as cobalt and nickel, where the mass decreases from left to right. The Periodic Table 17.3 Improving the Periodic Table • In 1913, the work of Henry G.J. Moseley, a young English scientist, led to the arrangement of elements based on their increasing atomic numbers instead of an arrangement based on atomic masses. • The current periodic table uses Moseley's arrangement of the elements. The Periodic Table 17.3 The Atom and the Periodic Table • The vertical columns in the periodic table are called groups, or families, and are numbered 1 through 18. • Elements in each group have similar properties. The Periodic Table 17.3 Electron Cloud Structure • In a neutral atom, the number of electrons is equal to the number of protons. • Therefore, a carbon atom, with an atomic number of six, has six protons and six electrons. Bohr Model Practice • In atomic physics, the Bohr model, devised by Niels Bohr, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity Examples of Bohr Models What Element am I ? The Periodic Table 17.3 Electron Cloud Structure • Scientists have found that electrons within the electron cloud have different amounts of energy. The Periodic Table 17.3 Electron Cloud Structure • Scientists model the energy differences of the electrons by placing the electrons in energy levels. The Periodic Table 17.3 Electron Cloud Structure • Energy levels nearer the nucleus have lower energy than those levels that are farther away. • Electrons fill these energy levels from the inner levels (closer to the nucleus) to the outer levels (farther from the nucleus). The Periodic Table 17.3 Electron Cloud Structure • Elements that are in the same group have the same number of electrons in their outer energy level. • It is the Number of electrons in the outer energy level that determines the chemical properties of the element. The Periodic Table 17.3 Energy Levels • The maximum number of electrons that can be contained in each of the first four levels is shown. The Periodic Table 17.3 Energy Levels • For example, energy level one can contain a maximum of two electrons. • A complete and stable outer energy level will contain eight electrons. The Periodic Table 17.3 Rows on the Table • Remember that the atomic number found on the periodic table is equal to the number of electrons in an atom. The Periodic Table 17.3 Rows on the Table • The first row has hydrogen with one electron and helium with two electrons both in energy level one. • Energy level one can hold only two electrons. Therefore, helium has a full or complete outer energy level. The Periodic Table 17.3 Rows on the Table • The second row begins with lithium, which has three electrons—two in energy level one and one in energy level two. • Lithium is followed by beryllium with two outer electrons, boron with three, and so on until you reach neon with eight outer electrons. The Periodic Table 17.3 Rows on the Table • Do you notice how the row in the periodic table ends when an outer level is filled? • In the third row of elements, the electrons begin filling energy level three. • The row ends with argon, which has a full outer energy level of eight electrons. The Periodic Table 17.3 Electron Dot Diagrams • Elements that are in the same group have the same number of electrons in their outer energy level. • These outer electrons are so important in determining the chemical properties of an element that a special way to represent them has been developed. The Periodic Table 17.3 Electron Dot Diagrams • An electron dot diagram uses the symbol of the element and dots to represent the electrons in the outer energy level. • Electron dot diagrams are used also to show how the electrons in the outer energy level are bonded when elements combine to form compounds. The Periodic Table 17.3 Same Group—Similar Properties • The elements in Group 17, the halogens, have electron dot diagrams similar to chlorine. • All halogens have seven electrons in their outer energy levels. The Periodic Table 17.3 Same Group—Similar Properties • A common property of the halogens is the ability to form compounds readily with elements in Group 1. • The Group 1 element, sodium, reacts easily with the Group 17 element, chlorine. • The result is the compound sodium chloride, or NaCl—ordinary table salt. The Periodic Table 17.3 Same Group—Similar Properties • Not all elements will combine readily with other elements. • The elements in Group 18 have complete outer energy levels. • This special configuration makes Group 18 elements relatively unreactive. The Periodic Table 17.3 Regions on the Periodic Table • The periodic table has several regions with specific names. • The horizontal rows of elements on the periodic table are called periods. • The elements increase by one proton and one electron as you go from left to right in a period. The Periodic Table 17.3 Regions on the Periodic Table • All of the elements in the blue squares are metals. The Periodic Table 17.3 Regions on the Periodic Table • Those elements on the right side of the periodic table, in yellow, are classified as nonmetals. The Periodic Table 17.3 Regions on the Periodic Table • The elements in green are metalloids or semimetals. The Periodic Table 17.3 A Growing Family • In 1994, scientists at the Heavy-Ion Research Laboratory in Darmstadt, Germany, discovered element 111. • Element 112 was discovered at the same laboratory. • Both of these elements are produced in the laboratory by joining smaller atoms into a single atom. The Periodic Table 17.3 Elements in the Universe • Using the technology that is available today, scientists are finding the same elements throughout the universe. • Many scientists believe that hydrogen and helium are the building blocks of other elements. The Periodic Table 17.3 Elements in the Universe • Exploding stars, or supernovas, give scientists evidence to support this theory. • Many scientists believe that supernovas have spread the elements that are found throughout the universe. Section Check 17.3 Question 1 How are the elements arranged in the periodic table? Section Check 17.3 Question 2 What do the elements in a vertical column of the periodic table have in common? Section Check 17.3 Question 3 What do the dots in this electron dot diagram represent?