1. CHEM 1405: Introductory Chemistry Houston Community College Dr. Laura Jakubowski Chapter 4 – Models of the Atom Textbook “Introductory Chemistry: Concepts and Critical Thinking” Seventh Edition by Charles H. Corwin © 2014 Pearson Education, Inc.

2. Dalton Model of the Atom In 1803, John Dalton proposed the following: 1.An element is composed of tiny, indivisible, indestructible particles called atoms 2.All atoms of an element are identical and have the same properties 3.Atoms of different elements combine to form compounds 4.Compounds contain atoms in small whole number ratios 5.Atoms can combine in more than one ratio to form different compounds Dalton’s atomic theory was an important step toward understanding the nature of matter – but his first two proposals were not entirely correct 2 © 2014 Pearson Education, Inc.

3. Thomson Model of the Atom It was found that atoms are divisible to at least two subatomic particles A negatively charged particle, an electron (e - ) has a -1 charge A positively charged particle, a proton (p + ) has a +1 charge In 1903, J. J. Thomson proposed a model of the atom known as the plum pudding model, where electrons are present in a homogeneous sphere of positive charge (like raisins in a pudding) With the help of Robert Millikan, Thomson was able to determine the masses of protons and electrons Electron is 9.11 × 10 -28 g Proton is 1.67 × 10 -24 g 3 © 2014 Pearson Education, Inc.

4. Rutherford Model of the Atom Ernest Rutherford and his assistant, Hans Geiger did an experiment to discover that at the center of an atom is an atomic nucleus 4 © 2014 Pearson Education, Inc.

5. Rutherford Model of the Atom In 1911, Rutherford proposed a new model of the atom where the negatively charged electrons were distributed around a positively charge nucleus The size of the atom and its nucleus could be estimated from his experiments to be 10 -8 cm and 10 -13 cm, respectively By analogy – if the atom were as large as an NFL football stadium, the nucleus would be the size of a small marble 5 © 2014 Pearson Education, Inc.

6. Rutherford Model of the Atom Due to the “heaviness” of the nucleus, Rutherford predicted that it contained neutral particles in addition to protons In 1932, James Chadwick discovered the neutron (n 0 ) The subatomic particles are summarized in the table: 6 © 2014 Pearson Education, Inc.

7. Atomic Notation Each element has a characteristic number of protons in its atomic nucleus, this value is called the atomic number The total number of protons and neutrons in the nucleus of an atom is called the mass number Atomic notation is a shorthand method to display the number of protons and neutrons in an atom 7 © 2014 Pearson Education, Inc.

8. Atomic Notation Examples The atomic notation for sodium is written as: The atomic number (protons) is 11 and the mass number (protons + neutrons) is 23 From this, the number of neutrons can be determined as (23 - 11) = 12 There is a neutral charge, therefore there are 11 electrons to balance charge How many protons, neutrons, and electrons are there in fluorine atom: Silver (Ag) has 47 protons and 62 neutrons. What is the atomic notation? 8 © 2014 Pearson Education, Inc. Na 23 11 F 19 9

9. Atomic Notation Examples The atomic notation for sodium is written as: The atomic number (protons) is 11 and the mass number (protons + neutrons) is 23 From this, the number of neutrons can be determined as (23 - 11) = 12 There is a neutral charge, therefore there are 11 electrons to balance charge How many protons, neutrons, and electrons are there in fluorine? Silver (Ag) has 47 protons and 62 neutrons. What is the atomic notation? 9 © 2014 Pearson Education, Inc. Na 23 11 F 19 9 Ag 109 47 9 protons, 10 neutrons, and 9 electrons 47 atomic number, and (47 + 62) = 109 mass number

10. Isotopes All atoms of the same element have the same number of protons But, for most elements, the number of neutrons can vary – atoms of the same element that have a different number of neutrons in the nucleus are called isotopes As the number of neutrons vary, so does the mass number Hydrogen has two naturally occurring isotopes protium (1 proton, 0 neutrons) – 99.983% of hydrogen atoms deuterium (1 proton, 1 neutron) – 0.017% of hydrogen atoms 10 © 2014 Pearson Education, Inc. H 1 1 H 2 1

11. Isotopes An isotope is often referred to by stating the name followed by its mass number, for example, carbon-12 The atomic notation should be written as State the number of protons and neutrons in each of the following isotopes, and write out name or show atomic notation accordingly: mercury-202 uranium-238 11 © 2014 Pearson Education, Inc. C 12 6 Cl 37 17 Sn 120 50

12. Isotopes An isotope is often referred to by stating the name followed by its mass number, for example, carbon-12 The atomic notation should be written as State the number of protons and neutrons in each of the following isotopes, and write out name or show atomic notation accordingly: mercury-202 uranium-238 12 © 2014 Pearson Education, Inc. C 12 6 Cl 37 17 Sn 120 50 17 p +, 20 n 0, chlorine-37 80 p +, 202 n 0, 50 p +, 70 n 0, tin-120 92 p +, 146 n 0, Hg 202 80 U 238 92

13. Atomic Mass Atoms are much too small to weigh directly – one carbon atom has a mass of 1.99 × 10 -23 g, instead a relative mass is used With a special instrument, the mass of an atom can be compared to the mass of a standard carbon-12 atom – atomic mass unit (amu) is equal to 1/12 the mass of a carbon-12 atom Simple and weighted averages – important for dealing with isotopes! A company manufactures a 16-lb and 8-lb shot put ball, of the shot put balls, 75% are 16-lb and 25% are 8-lb Simple average: (16 lb + 8 lb) / 2 = 12 lb Weighted average: (16 lb × 0.75) + (8 lb × 0.25) = 14 lb No one shot put ball weighs 14 lb, it would weight either 16 lb or 8 lb – 14 lb represents a theoretical average shot put ball 13 © 2014 Pearson Education, Inc.

14. Atomic Mass of an Element The atomic mass of an element is the weighted average mass of all naturally occurring isotopes State the atomic mass of each element with the following data for its natural isotopes:, 12.000 amu, 98.89% abundance, 13.003 amu, 1.11% abundance, 27.977 amu, 92.21% abundance, 28.976 amu, 4.70% abundance, 29.974 amu, 3.09% abundance 14 © 2014 Pearson Education, Inc. C 12 6 C 13 6 Si 28 14 Si 29 14 Si 30 14 (12.000 amu × 0.9889) + (13.003 amu × 0.0111) 12.01 amu

15. Atomic Mass of an Element The atomic mass of an element is the weighted average mass of all naturally occurring isotopes State the atomic mass of each element with the following data for its natural isotopes:, 12.000 amu, 98.89% abundance, 13.003 amu, 1.11% abundance, 27.977 amu, 92.21% abundance, 28.976 amu, 4.70% abundance, 29.974 amu, 3.09% abundance 15 © 2014 Pearson Education, Inc. C 12 6 C 13 6 Si 28 14 Si 29 14 Si 30 14 (12.000 amu × 0.9889) + (13.003 amu × 0.0111) 12.01 amu (27.977 amu × 0.9221) (28.976 amu × 0.0470) + (29.974 amu × 0.0309) 28.09 amu

16. The atomic number and atomic mass of an element is given in the periodic table In the textbook, here is how the information is given: Of the first 83 elements, 81 have one or more stable isotopes. From the table, it is not possible to know the number and abundance of each isotope – remember atomic mass is a weighted average The Periodic Table 16 © 2014 Pearson Education, Inc.

17. Nuclear Composition from the Periodic Table Refer to the periodic table and determine the atomic number and atomic mass for each of the following elements: iron titanium gold 17 www.periodictable.com iodine neon calcium

18. Nuclear Composition from the Periodic Table Refer to the periodic table and determine the atomic number and atomic mass for each of the following elements: iron 26; 55.845 titanium 22; 47.867 gold 79; 196.97 18 www.periodictable.com iodine 53; 126.90 neon 10; 20.180 calcium 20; 40.078

19. The Wave Nature of Light Light travels in waves – wavelength refers to the distance the light travels to complete one cycle and frequency refers to the number of wave cycles completed in each second – velocity of light is constant! 19 © 2014 Pearson Education, Inc. longer wavelength lower frequency shorter wavelength higher frequency

20. Light – A Continuous Spectrum Light usually refers to radiant energy that is visible Our eyes see light in the visible spectrum (400-700 nm) Wavelengths of radiant energy outside this range are not visible Ultraviolet radiation (below 400 nm) is too short Infrared radiation (above 700 nm) is too long 20 © 2014 Pearson Education, Inc.

21. Light – A Continuous Spectrum The complete radiant energy spectrum is an uninterrupted band, or continuous spectrum, of visible and invisible light 21 © 2014 Pearson Education, Inc.

22. The Quantum Concept In the quantum concept, it is proposed that energy is not radiated continuously, but in small bundles Radiant energy has both a wave nature and a particle nature, an individual unit of light energy is referred to as a photon 22 © 2014 Pearson Education, Inc. Loss of potential energy is continuous Loss of potential energy in steps (quantized amounts)

23. Bohr Model of the Atom In 1913, Niels Bohr speculated that electrons orbit around the atomic nucleus just as planets orbit around the Sun Electrons travel in a fixed-energy orbit referred to as an energy level Electrons are found only in their specific energy levels and nowhere else This model of the atom is referred to as the Bohr atom 23 © 2014 Pearson Education, Inc.

24. Evidence for Energy Levels An experiment was performed in which a voltage was passed through hydrogen gas in a sealed tube The light emitted could be separated out into narrow bands of color (434 nm, 486 nm, and 656 nm), called an emission line spectrum 24 © 2014 Pearson Education, Inc.

25. Evidence for Energy Levels Bohr used the emission line spectrum of hydrogen to support his model The electric current passing through hydrogen temporarily excites an electron to a higher energy orbit This higher energy state is unstable; as the electron drops to a lower energy orbit, a photon of light energy is given off The energy of the photon equals the energy lost by the electron 25 © 2014 Pearson Education, Inc.

26. Emission Spectra as “Atomic Fingerprints” Each element produces a unique set of spectral lines A line spectrum is referred to as an atomic fingerprint and is useful in the identification of atoms 26 © 2014 Pearson Education, Inc.

27. “Neon” Lights 27 © 2014 Pearson Education, Inc. To obtain different colored advertising lights, different noble gases are used

28. Energy Levels and Sublevels Emission spectra of other elements had too many lines for Bohr to explain He proposed that electrons occupy an energy sublevel within a main energy level The sublevels were designated s, p, d, and f – in reference to sharp, principal, diffuse, and fundamental emission spectral lines 28 © 2014 Pearson Education, Inc. neon The number of sublevels in each level corresponds to the number of the main energy level An s sublevel holds 2 electrons p – 6 electrons d – 10 electrons f – 14 electrons

29. Distribution of Electrons by Energy Level 29 © 2014 Pearson Education, Inc.

30. Electron Configuration Electrons are arranged around the nucleus in a systematic order, electrons fill the energy sublevels according to their energy 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s This order is not as expected (more explanation in Chapter 6) For now, use the figure to predict how energy sublevels are filled 30 © 2014 Pearson Education, Inc.

31. Writing Electron Configurations The electron configuration of an atom is a shorthand statement describing the location of electrons by sublevel For example, a 2p level containing 2 electrons – write as 2p 2 Write the electron configuration of an atom, using iron as an example First – how many electrons are there?: iron has 26 protons  26 electrons Second – write the order in which electron sublevels are arranged: 1s 2s 2p 3s 3p 4s 3d... Third – fill out sublevels accordingly, until filled with all 26 electrons: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6 Check your work: sum of superscripts should equal number of electrons: (2 + 2 + 6 + 2 + 6 + 2 + 6) = 26 31 © 2014 Pearson Education, Inc.

32. Electron Configuration Write the predicted electron configuration for each of the following: a)fluorine b)strontium c)argon d)cadmium 32 © 2014 Pearson Education, Inc.

33. Electron Configuration Write the predicted electron configuration for each of the following: a)fluorine – atomic number for F is 9 1s 2 2s 2 2p 5 b)strontium – atomic number for Sr is 38 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 c)argon – atomic number for Ar is 18 1s 2 2s 2 2p 6 3s 2 3p 6 d)cadmium – atomic number for 48 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 33 © 2014 Pearson Education, Inc.

34. Quantum Mechanical Model of the Atom Physicist Werner Heisenberg concluded it was not possible to accurately determine both the location and energy of a small particle (such as an electron) simultaneously – known as the uncertainty principle A new model of the atom formed which kept the idea of quantized energy levels, but incorporated the concept of uncertainty – known as the quantum mechanical atom The energy of an electron can be described in terms of the probability of it being within a spatial volume surrounding the nucleus – the region of high probability (~95%) for finding an electron of given energy is called an orbital 34 © 2014 Pearson Education, Inc.

35. Sizes and Shapes of the Orbitals Orbitals are arranged about the nucleus according to their size and shape – generally electrons having higher energy are in larger orbitals All s orbitals have a spherical shape, the higher the energy of the orbital, the larger its size 35 © 2014 Pearson Education, Inc.

36. Sizes and Shapes of the Orbitals All p orbitals have a dumbbell shape, there are three different 2p orbitals To help understand the concept of orbitals, visualize a fly trapped between two bottles; the fly is free to move around the entire space just as an electron is around an orbital 36 © 2014 Pearson Education, Inc.

37. Sizes and Shapes of the Orbitals The 3d sublevel consists of five 3d orbitals, not all with the same shape A 4f orbital looks even more complex 37 © 2014 Pearson Education, Inc. chemwiki.ucdavis.edu

38. Summary 38 © 2014 Pearson Education, Inc. Matter consists of atoms based on the laws of conservation of mass and definite proportion End of 1800s, there was evidence the atom was divisible into at least two subatomic particles – negative electrons and positive protons In the 1900s, atoms were discovered to have an atomic nucleus and a third type of neutral, subatomic particle called a neutron Atomic notation is used to describe the composition of a nucleus, where the number of protons is the atomic number and the sum of protons and neutrons is the mass number. Atoms with equal atomic number but different mass number are isotopes The relative mass of an atom, compared to carbon-12, is expressed using atomic mass units (amu). The weighted average of all isotopes of an element is called the atomic mass Light travels through space as a wave of radiant energy

39. Summary 39 © 2014 Pearson Education, Inc. The distance between light waves is called the wavelength and the number of cycles completed in a second is the frequency. The radiant energy spectrum is a continuous spectrum and includes the visible spectrum (between 400-700 nm) The energy radiated by an object is not continuous; when an object radiates light it releases a unit of energy called a photon In the Bohr atom, electrons posses a specific energy and occupy an energy level. An emission line spectrum helped support this theory It was later revealed that energy sublevels exist within energy levels of electrons, an s sublevel holds 2 electrons, p 6 electrons, d 10 electrons, and f 14 electrons Electrons fill sublevels in order of increasing energy, the description of this for a specific element is called an electron configuration In the uncertainty principle, Heisenberg proposed that both the location and energy of an electron cannot be known at the same time

40. Summary 40 © 2014 Pearson Education, Inc. This led to the a new model – the quantum mechanical atom, where electrons are present in different sized and shaped orbitals Key terms: electron, proton, atomic nucleus, neutron, atomic notation, atomic number, mass number, isotope, atomic mass unit, atomic mass, light, wavelength, frequency, visible spectrum, radiant energy spectrum, continuous spectrum, photon, energy level, Bohr atom, emission line spectrum, energy sublevels, electron configuration, uncertainty principle, quantum mechanical atom, orbital