2. 1 Chapter 4 Atomic Structure

3. 2 Section 4.1 – Defining the Atom Objectives Objectives Describe Democritus’s ideas about atoms Describe Democritus’s ideas about atoms Explain Dalton’s atomic theory Explain Dalton’s atomic theory Identify what instrument is used to observe individual atoms Identify what instrument is used to observe individual atoms

4. 3 Trust? I am going to ask you to believe in something that you can not see with your unaided eye… I am going to ask you to believe in something that you can not see with your unaided eye… We can not see the tiny fundamental particles that make up matter – yet all matter is composed of such particles called atoms We can not see the tiny fundamental particles that make up matter – yet all matter is composed of such particles called atoms

5. 4 Atom Atom Atom The smallest particle of an element that retains its identity in a chemical reaction The smallest particle of an element that retains its identity in a chemical reaction Think of a coin the size of a penny and composed of pure copper (Cu) Think of a coin the size of a penny and composed of pure copper (Cu) Imagine grinding the coin into fine dust … each speck still has the properties of Cu Imagine grinding the coin into fine dust … each speck still has the properties of Cu If you could continue to grind the dust into smaller and smaller Cu dust particles you would eventually come upon a particle of Cu that could no longer be divided and still have the chemical properties of Cu … this final particle is an atom If you could continue to grind the dust into smaller and smaller Cu dust particles you would eventually come upon a particle of Cu that could no longer be divided and still have the chemical properties of Cu … this final particle is an atom

6. 5 Early Models of the Atom Greek Philosopher Democritus (460 BC – 370 BC) Greek Philosopher Democritus (460 BC – 370 BC) Among the first to suggest the existence of atoms Among the first to suggest the existence of atoms atoms He believed that atoms were indivisible and indestructible Although his ideas agree with later scientific theory, they did not explain chemical behavior and lacked experimental support – not based on the scientific method – just philosophy

7. 6 Early Models of the Atom Who was next? Who was next? John Dalton (1766 – 1844) John Dalton (1766 – 1844) Experiment based – transformed Democritus’ ideas into scientific theory Experiment based – transformed Democritus’ ideas into scientific theory Results of his experimentation and observations are summarized in Dalton’s Atomic Theory Results of his experimentation and observations are summarized in Dalton’s Atomic Theory Dalton studied the ratios in which elements combine in chemical reactions Dalton studied the ratios in which elements combine in chemical reactions

8. 7 Early Models of the Atom Dalton’s Atomic Theory Dalton’s Atomic Theory 1. All elements are composed of tiny indivisible particles called atoms 2. Atoms of the same element are identical. The atoms of any other elements are different from those of any other element 3. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds 4. In chemical reactions, atoms are combined, separated or rearranged – but never changed into atoms of another element

9. 8 Sizing up the atom Remember our copper (Cu) penny…We ground it into smaller and smaller particles until we came upon a Cu atom Remember our copper (Cu) penny…We ground it into smaller and smaller particles until we came upon a Cu atom How big (or small) would that Cu atom be? How big (or small) would that Cu atom be? Here’s a little perspective… a pure Cu coin the size of a penny contains 2.4 x 10 22 atoms! a pure Cu coin the size of a penny contains 2.4 x 10 22 atoms!

10. 9 Sizing up the atom Earths population is only 6 x 10 9 people! Earths population is only 6 x 10 9 people! If you could line up 1 x 10 8 Cu atoms side by side, they would produce a line only 1 cm long! If you could line up 1 x 10 8 Cu atoms side by side, they would produce a line only 1 cm long! The radii of most atoms is The radii of most atoms is 5 x 10 -11 to 2 x 10 -10 m

11. 10 Sizing up the atom Despite their small size individual atoms are observable with instruments such as scanning tunneling microscope The image below shows an image of iron atoms generated by a scanning tunneling microscope

12. 11 Section 4.2 – Structure of the Nuclear Atom Objectives Objectives Identify three types of subatomic particles Identify three types of subatomic particles Describe the structure of atoms according to the Rutherford atomic model Describe the structure of atoms according to the Rutherford atomic model

13. 12 Subatomic Particles One important change to Dalton’s Atomic Theory… One important change to Dalton’s Atomic Theory… Atoms are known to be divisible Atoms are known to be divisible Atoms can be broken down into even smaller, more fundamental particles called subatomic particles Atoms can be broken down into even smaller, more fundamental particles called subatomic particles Three kinds of subatomic particles are electrons, protons and neutrons Three kinds of subatomic particles are electrons, protons and neutrons How was this discovered? How was this discovered?

14. 13 Subatomic Particles Electrons Electrons Discovered in 1897 by J.J. Thomson Discovered in 1897 by J.J. Thomson Electrons are negatively charged subatomic particles Electrons are negatively charged subatomic particles Thomson performed experiments using a cathode ray tube to deduce the presence of negatively charged particles Thomson performed experiments using a cathode ray tube to deduce the presence of negatively charged particles Cathode ray tubes pass electricity through a gas that is contained at a very low pressure Cathode ray tubes pass electricity through a gas that is contained at a very low pressure

15. 14 Thomson’s Experiment Voltage source +- Metal Disks

16. 15 n Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source +-

17. 16 Voltage source Thomson’s Experiment By adding an electric field he found that the moving pieces were negative By adding an electric field he found that the moving pieces were negative + -

18. 17 Thomson’s Experiment During experiment he used many different metals During experiment he used many different metals By the amount the beam bent he could find the ratio of charge to mass By the amount the beam bent he could find the ratio of charge to mass This ratio was the same for every material This ratio was the same for every material Thomson concluded it must be the same type of piece in every kind of atom…the electron Thomson concluded it must be the same type of piece in every kind of atom…the electron

19. 18 Mass of the Electron In 1961 Robert Millikan determined the mass of the electron to be 1/1840 the mass of a hydrogen atom In 1961 Robert Millikan determined the mass of the electron to be 1/1840 the mass of a hydrogen atom The mass of an electron is 9.11 x 10 -28 g The mass of an electron is 9.11 x 10 -28 g

20. 19 Conclusions from the study of the Electron 1. Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. 2. Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons 3. Electrons have so little mass that atoms must contain other particles that account for most of the mass

21. 20 Conclusions from the study of the Electron 4. 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) 5. 1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton

22. 21 Subatomic particles Electron Proton Neutron NameSymbolCharge Relative mass Actual mass (g) e-e- p+p+ n0n0 +1 0 1/1840 1 1 9.11 x 10 -28 1.67 x 10 -24

23. 22 The Atomic Nucleus Once subatomic particles were discovered scientists wondered how they were put together in as atom Once subatomic particles were discovered scientists wondered how they were put together in as atom We will look at two models: We will look at two models: Thomson’s Atomic Model (Plum Pudding Model) Thomson’s Atomic Model (Plum Pudding Model) Rutherford’s Atomic Model Rutherford’s Atomic Model

24. 23 The Atomic Nucleus He believed the electrons were struck into a lump of positive charge He believed the electrons were struck into a lump of positive charge Thomson’s model turned out to be short-lived due to groundbreaking work of Ernest Rutherford a former student of Thomson Thomson’s model turned out to be short-lived due to groundbreaking work of Ernest Rutherford a former student of Thomson

25. 24 The Atomic Nucleus Ernest Rutherford’s Gold Foil Experiment Ernest Rutherford’s Gold Foil Experiment Ernest Rutherford decided to test the plum pudding model Ernest Rutherford decided to test the plum pudding model Wanted to determine how large the atoms were Wanted to determine how large the atoms were

26. 25 The Atomic Nucleus Used Alpha particles Used Alpha particles Helium atoms that have lost their two electrons leaving a double positive charge Helium atoms that have lost their two electrons leaving a double positive charge Helium atom Alpha particle

27. 26 The Atomic Nucleus Ernest Rutherford’s Gold Foil Experiment Ernest Rutherford’s Gold Foil Experiment The alpha particles were fired at a thin sheet of gold surrounded by a detecting screen The alpha particles were fired at a thin sheet of gold surrounded by a detecting screen Particles that are hit on the detecting screen are recorded Particles that are hit on the detecting screen are recorded

28. 27 What Rutherford expected… The Atomic Nucleus Because evenly distributed + charges of the plum pudding model were not enough to stop the ++ of the alpha particles to pass through without

29. 28 What he found …

30. 29 The Atomic Nucleus What Rutherford’s results meant: What Rutherford’s results meant: The atom is mostly empty space. The atom is mostly empty space. The electrons are distributed around the nucleus, and occupy most of the volume The electrons are distributed around the nucleus, and occupy most of the volume This is why most alpha particles passed through without deflection (no + charge to deflect the ++ alpha particle) All the positive charge, and almost all the mass is concentrated in a small area in the center. All the positive charge, and almost all the mass is concentrated in a small area in the center. This is why some alpha particles were deflected. The + charge and mass of the center was enough to deflect the ++alpha particle. The center is composed of protons and neutrons. He called this a “nucleus” The center is composed of protons and neutrons. He called this a “nucleus” His model was called the nuclear atom His model was called the nuclear atom

31. 30 The Atomic Nucleus

32. 31 Section 4.3 – Distinguishing Among Atoms Objectives Objectives Explain what makes elements and isotopes different from each other Explain what makes elements and isotopes different from each other Calculate the number of neutrons in an atom Calculate the number of neutrons in an atom Calculate the atomic mass of an element Calculate the atomic mass of an element Explain why chemists use the periodic table Explain why chemists use the periodic table

33. 32 Atomic Number All atoms are composed of protons, neutrons and electrons All atoms are composed of protons, neutrons and electrons How then are atoms of one element different than atoms of another element? Why is an atom of C different from an atom of H? How then are atoms of one element different than atoms of another element? Why is an atom of C different from an atom of H? Elements are different because they contain different numbers of protons Elements are different because they contain different numbers of protons The of an element is the number of protons in the nucleus of an atom of that element The atomic number of an element is the number of protons in the nucleus of an atom of that element # of protons in an atom = the number of electrons # of protons in an atom = the number of electrons Atoms are electrically neutral Atoms are electrically neutral

34. 33 Atomic Number Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element Element # of Protons Atomic Number Carbon66 Phosphorus1515 Gold7979

35. 34 Mass Number The majority of mass of an atom is concentrated in the nucleus The majority of mass of an atom is concentrated in the nucleus The mass of an atom depends on the number of protons and neutrons The mass of an atom depends on the number of protons and neutrons Total number of protons and neutrons in an atom is called the mass number Total number of protons and neutrons in an atom is called the mass number The number of neutrons in an atom is the difference between the mass number and atomic number The number of neutrons in an atom is the difference between the mass number and atomic number Number of neutrons = mass number – atomic number

36. 35 Complete Symbols We began looking at symbols for some of the elements, however: We began looking at symbols for some of the elements, however: Complete symbols contain the symbol of the element, the mass number and the atomic number Complete symbols contain the symbol of the element, the mass number and the atomic number X Mass Number Atomic Number Superscript → Subscript →

37. 36 Complete Symbols Au Find each of the following: Number of protons Number of protons Number of neutrons Number of neutrons Number of electrons Number of electrons Atomic number Atomic number Mass number Mass number Atoms can also be referred to by using the mass and the name of the element – gold – 197 Atoms can also be referred to by using the mass and the name of the element – gold – 197 197 79 118 79 197

38. 37 Complete Symbols If an element has an atomic number of 34 and a mass number of 78, what is the: If an element has an atomic number of 34 and a mass number of 78, what is the: Number of protons? Number of protons? Number of neutrons? Number of neutrons? Number of electrons? Number of electrons? Name of element? Name of element? Complete symbol? Complete symbol? 34 44 34 Selenium Se 78 34

39. 38 Complete Symbols If an atom has 91 protons and 140 neutrons what is the: If an atom has 91 protons and 140 neutrons what is the: Atomic number? Atomic number? Mass number? Mass number? Number of electrons? Number of electrons? Complete symbol? Complete symbol? Name of the element? Name of the element? 91 91+140 = 231 91 Pa Protactinium 231 91

40. 39 Isotopes So, if two atoms have different number of protons, they are different elements. So, if two atoms have different number of protons, they are different elements. What if two atoms have different number of neutrons? What if two atoms have different number of neutrons? Atoms with different number of neutrons are isotopes of the same element Atoms with different number of neutrons are isotopes of the same element They have different mass numbers They have different mass numbers Despite these differences, isotopes are chemically alike because they have identical numbers of protons and electrons Despite these differences, isotopes are chemically alike because they have identical numbers of protons and electrons Protons and electrons are responsible for chemical behavior Protons and electrons are responsible for chemical behavior

41. 40 Isotopes Another way to say it: Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons Another way to say it: Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons There are three known isotopes of hydrogen There are three known isotopes of hydrogen IsotopeProtonsElectronsNeutronsNucleus Hydrogen–1 (protium) (protium) Hydrogen-2(deuterium) Hydrogen-3(tritium) 11 11 11 1 2 0

42. 41 Isotopes Elements occur in nature as mixtures of isotopes Elements occur in nature as mixtures of isotopes

43. 42 Atomic Mass How heavy is an atom of oxygen? How heavy is an atom of oxygen? It depends, because there are different kinds of oxygen atoms (different isotopes of Oxygen) It depends, because there are different kinds of oxygen atoms (different isotopes of Oxygen) We are more concerned with the average atomic mass. We are more concerned with the average atomic mass. This is based on the abundance (percentage) of each isotope of that element in nature. This is based on the abundance (percentage) of each isotope of that element in nature.

44. 43 Determining Atomic Mass We don’t use grams for this mass because the numbers would be too small We don’t use grams for this mass because the numbers would be too small Instead of grams, the unit we use is the Atomic Mass Unit (amu) Instead of grams, the unit we use is the Atomic Mass Unit (amu) It is defined as one-twelfth the mass of a carbon-12 atom It is defined as one-twelfth the mass of a carbon-12 atom Carbon-12 was chosen because of its isotope purity. Carbon-12 was chosen because of its isotope purity. Each isotope has its own atomic mass, thus we determine the average from percent abundance Each isotope has its own atomic mass, thus we determine the average from percent abundance

45. 44 Calculating Atomic Mass To calculate the average atomic mass of an element we need to know 3 things: To calculate the average atomic mass of an element we need to know 3 things: What are the stable isotopes of the element What are the stable isotopes of the element The mass of each isotope of the element The mass of each isotope of the element The natural percent abundance of each isotope of the element The natural percent abundance of each isotope of the element

46. 45 Calculating Atomic Mass Sample Steps for An Element with 3 Naturally Occurring Isotopes Sample Steps for An Element with 3 Naturally Occurring Isotopes mass isotope1 x natural abundance isotope1 = avg. mass isotope1 mass isotope1 x natural abundance isotope1 = avg. mass isotope1 mass isotope2 x natural abundance isotope2 = avg. mass isotope2 mass isotope2 x natural abundance isotope2 = avg. mass isotope2 mass isotope3 x natural abundance isotope3 = avg. mass isotope3 mass isotope3 x natural abundance isotope3 = avg. mass isotope3 avg. mass isotope1 + avg. mass isotope2 + avg. mass isotope3 = avg mass of avg. mass isotope1 + avg. mass isotope2 + avg. mass isotope3 = avg mass of atom for given atom for given element element The resulting sum is the weighted average mass of the atoms of the element that occur in nature The resulting sum is the weighted average mass of the atoms of the element that occur in nature

47. 46 Atomic Mass Practice Problem Practice Problem The element copper has naturally occurring isotopes with mass numbers of 63 and 65. The relative abundance and atomic masses are 69.2% for mass = 62.93 amu, and 30.8% for mass = 64.93 amu. Calculate the average atomic mass of copper The element copper has naturally occurring isotopes with mass numbers of 63 and 65. The relative abundance and atomic masses are 69.2% for mass = 62.93 amu, and 30.8% for mass = 64.93 amu. Calculate the average atomic mass of copper

48. 47 Periodic Table – A Preview The periodic table is an arrangement of elements in which the elements are separated into groups based on a set of repeating properties The periodic table is an arrangement of elements in which the elements are separated into groups based on a set of repeating properties The periodic table allows you to easily compare the properties of one element to another The periodic table allows you to easily compare the properties of one element to another

49. 48 Periodic Table – A Preview Each element is identified with its symbol placed in a square Each element is identified with its symbol placed in a square The elements are listed in order of increasing atomic number from left to right and top to bottom The elements are listed in order of increasing atomic number from left to right and top to bottom

50. 49 Periodic Table – A Preview Each horizontal row (there are 7 of them) is called a period Each horizontal row (there are 7 of them) is called a period The properties of the elements vary as you move across it from element to element The properties of the elements vary as you move across it from element to element The pattern then repeats as you move to the next period The pattern then repeats as you move to the next period Each vertical column is called a group or family Each vertical column is called a group or family Elements in a group have similar chemical and physical properties Elements in a group have similar chemical and physical properties Identified with a number and either an “A” or a “B” Identified with a number and either an “A” or a “B”