Atoms & the Periodic Table The building blocks of matter Early Atomic History There have been many different theories, reflecting different times and cultures, to explain the composition of matter. There have been many different theories, reflecting different times and cultures, to explain the composition of matter. In addition, chemical reactions, refinements of ores, purification of salt, etc. have been carried out for thousands of years. In addition, chemical reactions, refinements of ores, purification of salt, etc. have been carried out for thousands of years. Early Atomic History The ancient Greek philosophers theorized that matter is discrete, rather than continuous. The ancient Greek philosophers theorized that matter is discrete, rather than continuous. Some, notably Demokritos, suggested that there is some small unit of matter that still retains the properties of the larger sample. It was thought that these smaller pieces of matter were indivisible, and were given the name atomos from which we get our modern word atoms. Some, notably Demokritos, suggested that there is some small unit of matter that still retains the properties of the larger sample. It was thought that these smaller pieces of matter were indivisible, and were given the name atomos from which we get our modern word atoms. Early Atomic Theory During the next 2000 years, a lot was learned about matter. Several elements were discovered, metals were refined, acids prepared, etc. During the next 2000 years, a lot was learned about matter. Several elements were discovered, metals were refined, acids prepared, etc. In the mid-1600s, the scientific (rather than the philosophical or applied) study of the nature matter began to take shape. In the mid-1600s, the scientific (rather than the philosophical or applied) study of the nature matter began to take shape. Early Atomic Theory Since most laboratories contained rudimentary equipment- burners and scales, many experiments involved the measurement of changes in volumes (for gases) and masses during chemical reactions. Since most laboratories contained rudimentary equipment- burners and scales, many experiments involved the measurement of changes in volumes (for gases) and masses during chemical reactions. Based on measurements and observations, several scientific laws were developed. These laws form the basis for our understanding of the composition of matter. Based on measurements and observations, several scientific laws were developed. These laws form the basis for our understanding of the composition of matter. Early Atomic Theory The fundamental laws based on mass and chemical reactions will be covered later. John Dalton examined the way matter behaves on the macro scale, and proposed his atomic theory of matter in Dalton proposed that the basis of all matter is tiny indivisible particles called atoms. Daltons Atomic Theory (1808) 1. Each element consists of tiny particles called atoms. 2. The atoms of a given element are identical, and differ from the atoms of other elements. 3. Compounds are formed when atoms of different elements combine chemically. A specific compound always has the same relative number and types of atoms. 4.Chemical reactions involve the reorganization of atoms, or changes in the way they are bound together. Sub-Atomic Particles The period from approximately involved the study of the nature of the atom, using two relatively new tools: electricity and radioactivity. The period from approximately involved the study of the nature of the atom, using two relatively new tools: electricity and radioactivity. Scientists knew that atoms of different elements had different relative atomic masses and different properties, and they wanted to find out the reasons for the differences. Scientists knew that atoms of different elements had different relative atomic masses and different properties, and they wanted to find out the reasons for the differences. Sub-Atomic Particles In the late 1880s, J.J. Thomson ( ) studied the properties of cathode rays. The rays are produced in partially evacuated tubes containing electrodes at either end. In the late 1880s, J.J. Thomson ( ) studied the properties of cathode rays. The rays are produced in partially evacuated tubes containing electrodes at either end. The rays are invisible, unless a phosphorescent screen is used. The rays are invisible, unless a phosphorescent screen is used. Sub-Atomic Particles (Cathode) (Anode) Cathode Rays Sub-Atomic Particles Thomson made the following observations: 1. The cathode rays had the same properties regardless of the metal used for the cathode. 2. The rays traveled from the cathode (- charged) to the anode (+ charged). 3. The rays were attracted to the positive plate of an external electrical field, and repelled by the negative plate. Sub-Atomic Particles Thomson concluded: 1. The cathode rays are a stream of negatively charged particles called electrons. 2. All atoms contain electrons, and the electrons from all elements are identical. 3. The atom must also contain matter with a positive charge, as atoms are neutral in charge. Sub-Atomic Particles Thomson also carried out deflection measurements, in which he applied a magnetic field to deflect the beam along with an external electrical field to straighten out the bent beam. Thomson also carried out deflection measurements, in which he applied a magnetic field to deflect the beam along with an external electrical field to straighten out the bent beam. Sub-Atomic Particles From his measurements, he was able to calculate the charge/mass ratio of the electron: From his measurements, he was able to calculate the charge/mass ratio of the electron: e/m = -1.76x10 8 coulombs/gram Sub-Atomic Particles Around the same time as Thomson (1886), Eugen Goldstein observed that if a cathode ray tube contained very small amounts of gas, a glowing substance travelled toward the cathode. Around the same time as Thomson (1886), Eugen Goldstein observed that if a cathode ray tube contained very small amounts of gas, a glowing substance travelled toward the cathode. Goldstein called the glowing substance canal rays, and observed that they were positive in charge, as they are attracted toward the negative cathode. Goldstein called the glowing substance canal rays, and observed that they were positive in charge, as they are attracted toward the negative cathode. Sub-Atomic Particles The apparatus contained a perforated cathode that contained many small holes. When an electric current is applied, the reddish glow forms in the stream of electrons, and travels toward the negative cathode and through the small holes. The apparatus contained a perforated cathode that contained many small holes. When an electric current is applied, the reddish glow forms in the stream of electrons, and travels toward the negative cathode and through the small holes.LMDgLMDgLMDgLMDg Sub-Atomic Particles It was several years before Goldstein could explain his observations. The rays were quite different from cathode rays: Unlike cathode rays, canal rays were barely deflected by a magnetic field or external electric field. The properties of the rays varied with the gas contained. Sub-Atomic Particles Later studies determined that hydrogen gas produced the ray with the largest charge to mass ratio (ie., the smallest mass). This particle, produced when a hydrogen atom loses its electron, was identified as the proton. Later studies determined that hydrogen gas produced the ray with the largest charge to mass ratio (ie., the smallest mass). This particle, produced when a hydrogen atom loses its electron, was identified as the proton. Sub-Atomic Particles Robert Millikan ( ) published the results of his Oil Drop Experiment in He designed an apparatus that could be used to determine the charge on an electron. Robert Millikan ( ) published the results of his Oil Drop Experiment in He designed an apparatus that could be used to determine the charge on an electron. The device used a fine mist of oil drops that had been exposed to ionizing radiation. The radiation caused some of the oil drops to take on one or more electrons. The device used a fine mist of oil drops that had been exposed to ionizing radiation. The radiation caused some of the oil drops to take on one or more electrons. Sub-Atomic Particles The Charge of the Electron Sub-Atomic Particles Millikin determined that the charge on the electron is x coulombs. Millikin determined that the charge on the electron is x coulombs. Using Thomsons value for the charge to mass ratio of the electron, the mass of the electron could be calculated. Using Thomsons value for the charge to mass ratio of the electron, the mass of the electron could be calculated. mass of e - = (-1.60 x coulombs) (-1.76 x 10 8 coulombs/gram) (-1.76 x 10 8 coulombs/gram) = 9.11 x grams = 9.11 x kilograms = 9.11 x kilograms Early Atomic Models J. J. Thomson had shown that all atoms contain negatively charged particles called electrons. Combined with the work of Millikan, they discovered that the electron has very little mass. J. J. Thomson had shown that all atoms contain negatively charged particles called electrons. Combined with the work of Millikan, they discovered that the electron has very little mass. Thomson proposed that the bulk of the atom is a positively charged gel or cloud, with most of the atomic mass and all of the positive charge uniformly distributed throughout the gel. Thomson proposed that the bulk of the atom is a positively charged gel or cloud, with most of the atomic mass and all of the positive charge uniformly distributed throughout the gel. Early Atomic Models The electrons were viewed as discrete, very small particles that were stuck into the positively charged gel or cloud like raisins in a pudding. This model is often called the plum or raisin pudding model of the atom. The electrons were viewed as discrete, very small particles that were stuck into the positively charged gel or cloud like raisins in a pudding. This model is often called the plum or raisin pudding model of the atom. The electrons could be knocked out of the gel if enough energy is applied, and this is the source of the cathode rays. The electrons could be knocked out of the gel if enough energy is applied, and this is the source of the cathode rays. Early Atomic Models One of the key features of Thomsons atomic model is that most of the atomic mass and all of the positive charge is uniformly distributed throughout the atom. Early Atomic Models Thomson had a graduate student, Ernest Rutherford. In 1911, Rutherford, Geiger and Marsden performed an experiment to confirm Thomsons atomic model. Thomson had a graduate student, Ernest Rutherford. In 1911, Rutherford, Geiger and Marsden performed an experiment to confirm Thomsons atomic model. They bombarded a thin gold foil with alpha () particles. The particles have twice the charge of an electron and are positive in charge, with a mass that is 7300 times greater than the mass of an electron. They bombarded a thin gold foil with alpha () particles. The particles have twice the charge of an electron and are positive in charge, with a mass that is 7300 times greater than the mass of an electron. Early Atomic Models The particles can best be thought of as a positively charged, fast traveling atomic sized bullet. They created a thin beam of particles and directed the beam at a very thin gold foil. The particles can best be thought of as a positively charged, fast traveling atomic sized bullet. They created a thin beam of particles and directed the beam at a very thin gold foil. Early Atomic Models If Thomsons model is correct, most of the particles should pass right through the gold atoms. Some slight deflection might occur if the positively charged particle travels near an electron. Gold Foil Experiment Early Atomic Models The film that lined the apparatus showed that most particles went through the foil with little or no deflection. However, some of the particles were deflected at great angles. The film that lined the apparatus showed that most particles went through the foil with little or no deflection. However, some of the particles were deflected at great angles. Early Atomic Models The deflection of the particles was consistent with a large concentration of positive charge and atomic mass. This very small extremely dense positively charged area is called the nucleus. Early Atomic Models The atom is mostly empty space, with the electrons found outside of the nucleus. If the nucleus was the size of a pea, it would have a mass of 250 million tons, and the electrons would occupy a volume approximately the size of a stadium. Atomic Nucleus Sub-Atomic Particles As the number of protons in the nucleus of elements became known, it became apparent that there was mass in the atom that wasnt accounted for. For example, hydrogen was known to contain 1 proton and 1 electron, and helium contained two protons and two electrons. Thus, helium should have twice the atomic mass of hydrogen. However, helium is four times heavier than hydrogen atoms. The Neutron Rutherford and other scientists postulated the existence of another sub-atomic particle, the neutron. Neutrons are neutral in charge, with a mass similar to that of a proton. In 1932, James Chadwick bombarded a thin sheet of beryllium with particles and detected a high energy radiation that was not deflected by electric or magnetic fields. Calculations involving the conservation of energy lead to the determination of the mass of the neutron. Sub-Atomic Particles We now know that the positive charge of an atom, contained in the nucleus, is due to particles called protons. We now know that the positive charge of an atom, contained in the nucleus, is due to particles called protons. Protons have a charge equal in magnitude to that of an electron, but positive in charge. Protons have a charge equal in magnitude to that of an electron, but positive in charge. The mass of a proton is roughly 1800 times greater than the mass of an electron. The mass of a proton is roughly 1800 times greater than the mass of an electron. Sub-Atomic Particles The nuclei of atoms also can contain neutrons. Neutrons are neutral in charge, with a mass similar to that of a proton. The nuclei of atoms also can contain neutrons. Neutrons are neutral in charge, with a mass similar to that of a proton. Neutrons are found in the nucleus of atoms, along with protons. Neutrons are found in the nucleus of atoms, along with protons. Sub-Atomic Particles During chemical reactions, atoms may lose or gain electrons to form charged particles called ions. During chemical reactions, atoms may lose or gain electrons to form charged particles called ions. Atoms of a given element may have differing numbers of neutrons. These forms of the same element are called isotopes. Atoms of a given element may have differing numbers of neutrons. These forms of the same element are called isotopes. It is the number of protons or the atomic number that defines the identity of the atom. It is the number of protons or the atomic number that defines the identity of the atom. Atomic Symbols The periodic table lists the elements in order of increasing atomic number (the number of protons). The periodic table lists the elements in order of increasing atomic number (the number of protons). The atomic number, represented by the letter Z, is linked with the atomic symbol. For example, oxygen is atomic number 8, and any atom containing 8 protons, regardless of the number of neutrons or electrons, is represented by the symbol O. The atomic number, represented by the letter Z, is linked with the atomic symbol. For example, oxygen is atomic number 8, and any atom containing 8 protons, regardless of the number of neutrons or electrons, is represented by the symbol O. Atomic Symbols To indicate a specific isotope, the atomic symbol must also contain the mass number. To indicate a specific isotope, the atomic symbol must also contain the mass number. The mass number is the number of neutrons plus protons for a particular isotope. The mass number is never found on the periodic table. The mass number is the number of neutrons plus protons for a particular isotope. The mass number is never found on the periodic table. Since the mass number is the number of particles (neutrons + protons) in the nucleus, it is always an integer. Since the mass number is the number of particles (neutrons + protons) in the nucleus, it is always an integer. Isotopes of Sodium Mass number Atomic number Atomic Symbols For example, there are three isotopes of carbon: For example, there are three isotopes of carbon: 12 C, 13 C and 14 C The mass number, if specified, appears in the upper left corner of an atomic symbol. Since all carbon atoms have 6 protons (carbon is atomic number 6 on the periodic table), atoms of carbon may have 6, 7 or 8 neutrons in the nucleus. The mass number, if specified, appears in the upper left corner of an atomic symbol. Since all carbon atoms have 6 protons (carbon is atomic number 6 on the periodic table), atoms of carbon may have 6, 7 or 8 neutrons in the nucleus. The isotopes are called carbon-12, carbon-13 and carbon-14. Atomic Symbols If the atom has lost or gained electrons, the charge is written in the upper right corner of the atomic symbol. If the atom has lost or gained electrons, the charge is written in the upper right corner of the atomic symbol. The atomic number, though optional, may be written in the lower left corner of the symbol. The atomic number, though optional, may be written in the lower left corner of the symbol. 37 Cl 1- This ion of chlorine contains 17 protons, 20 neutrons, and 18 electrons. Nuclear Stability Some nuclei are unstable, and will, over time, emit particles and/or electromagnetic radiation until they become stable. During a nuclear reaction, the products and reactants will contain different elements as the nuclei change. 47 What Causes Nuclei to Decompose? The particles in the nucleus are held together by a very strong attractive force only found in the nucleus called the strong force The particles in the nucleus are held together by a very strong attractive force only found in the nucleus called the strong force acts only over very short distances acts only over very short distances The neutrons play an important role in stabilizing the nucleus, as they add to the strong force, but dont repel each other like the protons do The neutrons play an important role in stabilizing the nucleus, as they add to the strong force, but dont repel each other like the protons do 48 What Causes Nuclei to Decompose? The principal factor in determining nuclear stability is the neutron to proton ratio. For elements with an atomic number 20, the n/p ratio is nearly 1.0. As the nucleus contains more protons Z>20, additional neutrons are needed to stabilize the nucleus. 49 Stability of Nuclei for Z = 1 20, stable N/Z 1 for Z = 20 40, stable N/Z approaches 1.25 for Z = 40 80, stable N/Z approaches 1.5 for Z > 83, there are no stable nuclei 50 Stability of Nuclei There are more stable nuclei (isotopes) of elements containing 2, 8, 20, 50, 82 or 126 protons. These are called magic numbers. There are many more stable nuclei with even numbers of protons and neutrons that with odd numbers of these particles. Relative Atomic Masses As the early chemists explored the nature of matter, they discovered that atoms of the elements had different masses. As the early chemists explored the nature of matter, they discovered that atoms of the elements had different masses. Avogadros Hypothesis which states that under constant temperature and pressure equal volumes of gases contain an equal number of particles could be used to determine relative atomic masses for gaseous elements. Avogadros Hypothesis which states that under constant temperature and pressure equal volumes of gases contain an equal number of particles could be used to determine relative atomic masses for gaseous elements. Relative Atomic Masses Equal volumes of gases contain an equal number of particles. Equal volumes of gases contain an equal number of particles. Although the number of particles (atoms or molecules) in a liter of gas (at a specific T and P) wasnt known, Avogadros Hypothesis said that a liter of any other gas under the same conditions would contain the same number of particles. Although the number of particles (atoms or molecules) in a liter of gas (at a specific T and P) wasnt known, Avogadros Hypothesis said that a liter of any other gas under the same conditions would contain the same number of particles. Relative Atomic Masses Equal volumes of gases contain an equal number of particles. Equal volumes of gases contain an equal number of particles. Since the masses of the gaseous samples could be determined, a comparative or relative scale of atomic and molecular masses could be derived. Since the masses of the gaseous samples could be determined, a comparative or relative scale of atomic and molecular masses could be derived. Relative Atomic Masses Equal volumes of gases contain an equal number of particles. Equal volumes of gases contain an equal number of particles. For example, if the masses of a liter of oxygen (O 2 ), chlorine (Cl 2 ) and hydrogen (H 2 ) were compared under identical conditions, the hydrogen sample has the smallest mass, and the chlorine sample has the largest mass. For example, if the masses of a liter of oxygen (O 2 ), chlorine (Cl 2 ) and hydrogen (H 2 ) were compared under identical conditions, the hydrogen sample has the smallest mass, and the chlorine sample has the largest mass. The ratio of the masses of the 1 liter samples is: The ratio of the masses of the 1 liter samples is:35.5/16.0/1.00 Cl 2 / O 2 / H 2 Relative Atomic Masses Equal volumes of gases contain an equal number of particles. Equal volumes of gases contain an equal number of particles. The ratio of the masses of the 1 liter samples is: The ratio of the masses of the 1 liter samples is:35.5/16.0/1.00 Cl 2 / O 2 / H 2 Since all three gases are diatomic, we can say that an oxygen atom is 16.0 times heavier than a hydrogen atom, and that a chorine atom is 35.5 times heavier than a hydrogen atom. Since all three gases are diatomic, we can say that an oxygen atom is 16.0 times heavier than a hydrogen atom, and that a chorine atom is 35.5 times heavier than a hydrogen atom. Relative Atomic Masses A scale of relative atomic mass was devised. Individual atoms are much too small to weigh, but the masses of large collections of atoms could easily be compared. A scale of relative atomic mass was devised. Individual atoms are much too small to weigh, but the masses of large collections of atoms could easily be compared. The relative masses of the atoms are listed on the periodic table. An arbitrary unit, the atomic mass unit (amu) is used for relative masses. The relative masses of the atoms are listed on the periodic table. An arbitrary unit, the atomic mass unit (amu) is used for relative masses. Relative Atomic Masses Eventually, the carbon-12 isotope ( 12 C) was assigned an atomic mass of exactly 12 atomic mass units, and all other atomic masses are expressed relative to this assignment. Eventually, the carbon-12 isotope ( 12 C) was assigned an atomic mass of exactly 12 atomic mass units, and all other atomic masses are expressed relative to this assignment. The atomic mass for carbon, found on the periodic table, is amu, and not amu. This is because the periodic table lists the average relative atomic mass for all isotopes of the element. The atomic mass for carbon, found on the periodic table, is amu, and not amu. This is because the periodic table lists the average relative atomic mass for all isotopes of the element. Relative Atomic Masses Carbon exists as three isotopes: Carbon exists as three isotopes: 12 C has a relative mass of exactly 12 amu 13 C has a relative atomic mass of amu 14 C has a relative atomic mass of 14.0 amu The value found on the periodic table, amu, reflects the relative abundance of the isotopes. The majority of carbon (98.89%) is The value found on the periodic table, amu, reflects the relative abundance of the isotopes. The majority of carbon (98.89%) is 12 C, with 1.11% 13 C, and a trace of 14 C. 12 C, with 1.11% 13 C, and a trace of 14 C. Relative Atomic Masses Chorine exists as two isotopes: Chorine exists as two isotopes: 35 Cl, with a relative atomic mass of 35.0 amu and 37 Cl, with a relative atomic mass of 37.0 amu. What does the atomic mass of chlorine on the periodic table tell you about the relative abundance of the two isotopes? Mass Spectroscopy The modern method for determining the atomic or molecular mass of a substance is called mass spectroscopy. A sample is vaporized and then ionized so that it loses 1 or more electrons. The positive ions are accelerated in a magnetic field, and their path is bent by the magnitude of the charge and the speed of the particle. Mass Spectroscopy The slowest particles (higher mass) are less deflected than the faster (lower mass) particles. A detector provides a spectrum which indicates the mass/charge of each particle, as well as its relative abundance. Mass Spectroscopy The Elements All matter is composed of approximately 100 elements, in various combinations, listed on the periodic table. The table groups elements with similar chemical and physical properties. The Periodic Table The modern periodic table was developed in 1872 by Dmitri Mendeleev ( ). A similar table was also developed independently by Julius Meyer ( ). The table groups elements with similar properties (both physical and chemical) in vertical columns. As a result, certain properties recur periodically. The Periodic Table Periodic tables group elements with similar properties in vertical groups or families. Periodic tables group elements with similar properties in vertical groups or families. Metals are on the left side of the table, and non- metals are on the right. Metals are on the left side of the table, and non- metals are on the right. A bold line resembling a flight of stairs usually separated metals from non-metals. A bold line resembling a flight of stairs usually separated metals from non-metals. The Periodic Table metal/non-metal line The Periodic Table Keep in mind: Elements along the metal/non-metal dividing line are called semi-metals or metalloids. These elements sometimes behave like metals, and sometimes exhibit non-metallic properties and behavior. Elements along the metal/non-metal dividing line are called semi-metals or metalloids. These elements sometimes behave like metals, and sometimes exhibit non-metallic properties and behavior. Hydrogen, though in group IA, is not a metal. It is sometimes also placed in group 7A. Hydrogen, though in group IA, is not a metal. It is sometimes also placed in group 7A. Moles Many chemical reactions are carried out using a few grams of each reactant. Such quantities contain huge numbers (on the order of ) of atoms or molecules. Many chemical reactions are carried out using a few grams of each reactant. Such quantities contain huge numbers (on the order of ) of atoms or molecules. A unit of quantity of matter, the mole, was established. A mole is defined as the number of carbon atoms in exactly 12 grams of 12 C. A unit of quantity of matter, the mole, was established. A mole is defined as the number of carbon atoms in exactly 12 grams of 12 C. Avogadro determined the number of particles (atoms or molecules) in a mole. Avogadro determined the number of particles (atoms or molecules) in a mole. Moles Avogadros number = x particles/mole Avogadros number = x particles/mole Atoms are so small, that a mole of most substances can be easily held in ones hand. Atoms are so small, that a mole of most substances can be easily held in ones hand. Cu Al Hg Fe I2I2 S Moles If we consider objects we can see, a mole of pennies would cover the entire planet and be 300 meters deep! However, the collection of atoms, called a mole, is very convenient in the laboratory (just like dozens are useful in buying eggs or pencils). If we consider objects we can see, a mole of pennies would cover the entire planet and be 300 meters deep! However, the collection of atoms, called a mole, is very convenient in the laboratory (just like dozens are useful in buying eggs or pencils). Moles A mole of any atom has a mass equal to the elements atomic mass expressed in grams. A mole of any atom has a mass equal to the elements atomic mass expressed in grams. A mole of iron atoms has a mass of grams; a mole of iodine molecules (I 2 ) has a mass of (126.9) (2) = grams. Moles The masses of each molar sample are provided below. The masses of each molar sample are provided below. Cu = 63.55g Al=26.98g Hg = g Fe=55.85 g I 2 =253.8 g S=32.07g Molar Mass For compounds, once the formula is known, the mass of a mole of the substance can be calculated by summing up the masses of all the atoms in the compound. For compounds, once the formula is known, the mass of a mole of the substance can be calculated by summing up the masses of all the atoms in the compound. For example, hydrogen peroxide has the formula H 2 O 2 : For example, hydrogen peroxide has the formula H 2 O 2 : 2H +2O = 2(1.008g) + 2(16.00g) = g/mol A mole of hydrogen peroxide has a mass of grams. A mole of hydrogen peroxide has a mass of grams. Problem Determine the number of silver atoms in a 10.0 gram sample of silver.