IB Chemistry Power Points Topic 02 Atomic Structure Atomic Structure

2.1: The nuclear atom 2.1 The nuclear atom 2.1 Application and skills Understandings: • Atoms contain a positively charged dense nucleus composed of protons and neutrons (nucleons). • Negatively charged electrons occupy the space outside the nucleus. • The mass spectrometer is used to determine the relative atomic mass of an element from its isotopic composition. Use of the nuclear symbol notation 𝑋𝑋 𝑍𝑍 𝐴𝐴 to deduce the number of protons, neutrons and electrons in atoms and ions. Calculations involving non-integer relative atomic masses and abundance of isotopes from given data, including mass spectra.

ATOM.. Atomos An atom is the smallest particle of an element that retains its identity in a chemical reaction. Although early philosophers and scientists could not observe individual atoms, they were still able to propose ideas about the structure of atoms.

Once upon a time… Democritus atomic theory Democritus reasoned that atoms were indivisible and indestructible. Although, Democritus’s ideas agreed with later scientific theory, they did not explain chemical behavior They also lacked experimental support because Democritus’s approach was not based on the scientific method.

Democritus who? John Daltons Atomic theory By using experimental methods, Dalton transformed Democritus’s ideas on atoms into a scientific theory. Dalton studied the ratios in which elements combine in chemical reactions. All elements are composed of tiny indivisible particles called atoms. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds. Chemical reactions occur when atoms are separated from each other, joined, or rearranged in different combinations. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction.

Thomson’s Plum Pudding model Discovered Electrons So maybe the atom is divisible after all Thomson discovered the “electron” by conducting his cathode ray experiment. He then proposed the plum pudding model of the atom.

Rutherford’s Gold-Foil Experiment Rutherford’s results were that most alpha particles went straight through, or were slightly deflected. What was surprising is that a small fraction of the alpha particles bounced off the gold foil at very large angles. Some even bounced straight back toward the source. Which lead to the discovery of the nucleus.

Review – Basic Atomic Structure NUCLEUS NUCLEUS ELECTRONS ELECTRONS PROTONS PROTONS NEUTRONS NEUTRONS NEGATIVE CHARGE POSITIVE CHARGE POSITIVE CHARGE NEUTRAL CHARGE

Review – Basic Atomic Model Subatomic components Relative Mass Charge Proton 1 +1 Neutron Electron 5 x 10-4 -1

A-Z notation mass number A element symbol atomic number Z © Addison-Wesley Publishing Company, Inc. The atomic number equals the number of protons. Each element has a unique atomic number.

Mass Number mass number A = protons + neutrons always a whole number NOT the value given on the Periodic Table! © Addison-Wesley Publishing Company, Inc.

Practice: determine the required values and write the chemical symbol in A-Z notation. Chlorine-37 atomic #: mass #: # of protons: # of electrons: # of neutrons: 17 37 20

p+ > e- p+ < e- cation anion Ions ions are electrically charged atoms Neutral atom lose electrons gain electrons positive ion negative ion p+ > e- p+ < e- cation anion

Practice: determine the required values for the negative chloride ion 37 Cl -1 atomic #: mass #: # of protons: # of electrons: # of neutrons: 17 37 18 20

Practice: determine the required values for the positive calcium ion 40 Ca +2 atomic #: mass #: # of protons: # of electrons: # of neutrons: 20 40 18

carbon-12 and carbon-14 are isotopes Isotopes: Atoms of the same element with different mass numbers. carbon-12 and carbon-14 are isotopes © Addison-Wesley Publishing Company, Inc. stable similar chemical properties radioactive

Radioisotopes and Their Uses Radioisotopes are unstable isotopes that undergo radioactive decay. Radioisotopes have a number of uses: U-235 is used as fuel in nuclear reactors Co-60 is used in cancer radiation therapy C-14 is used as a tracer and for archeological dating Am-241 is used in smoke detectors

Mass Spectrometer A mass spectrometer is used to detect, identify and measure the abundance of different atoms, molecules or molecular fragments. Mass spectrometer studies are used to determine the average atomic mass for an element. The operation of a mass spectrometer can be divided into 5 steps: Vaporization Ionization Acceleration Deflection Detection

Vaporization: the element to be analyzed is heated and vaporized (gaseous form). => http://www.magnet.fsu.edu/education/tutorials/java/singlesector2/index.html Chapter 12

Ionization: the gaseous element is injected slowly into a vacuum chamber where the atoms are bombarded by electrons. This forms ions positive ions X (g) + e-  X+(g) + 2 e- => http://www.magnet.fsu.edu/education/tutorials/java/singlesector2/index.html Chapter 12

Acceleration: the gaseous ions are accelerated through an electric field (towards a negative plate) => http://www.magnet.fsu.edu/education/tutorials/java/singlesector2/index.html Chapter 12

Deflection: Ions are deflected in an adjustable magnetic field oriented at right angles to the path. Heavier ions are deflected less. => http://www.magnet.fsu.edu/education/tutorials/java/singlesector2/index.html Chapter 12

=> Detection: ions of a specific mass are counted Chapter 12 http://www.magnet.fsu.edu/education/tutorials/java/singlesector2/index.html Chapter 12

A sample mass spectrograph Output provides the abundances of the elemental isotopes of different relative mass

Atomic Mass is Relative 12C atom = 1.992 × 10-23 g atomic mass unit (amu) 1 amu = 1/12 the mass of a 12C atom 1 p = 1.007276 amu 1 n = 1.008665 amu 1 e- = 0.0005486 amu © Addison-Wesley Publishing Company, Inc.

Average Atomic Mass Avg. Atomic Mass a weighted average of all isotopes of an element based on the % abundance data from mass spectrometer this value is found on the Periodic Table Avg. Atomic Mass

Average Atomic Mass EXAMPLE: Calculate the average atomic mass of chlorine if its abundance in nature is 75.77% 35Cl, and 24.23% 37Cl. Avg. Atomic Mass 35.48 amu

Average Atomic Mass Gallium has two naturally occurring isotopes, Ga-69 and Ga-71, with masses of 68.9257 amu and 70.9249 amu, respectively. Calculate the percent abundances of these isotopes Average relative mass of Ga 69.7231 amu Solve to get 60.1% Ga-69 and 39.9% Ga-71

2.2 Electron Configuration Understandings Emission spectra are produced when photons are emitted from atoms as excited electrons return to a lower energy level. The line emission spectrum of hydrogen provides evidence for the existence of electrons in discrete energy levels, which converge at higher energies. The main energy level or shell is given an integer number, n, and can hold a maximum number of electrons, 2n2. A more detailed model of the atom describes the division of the main energy level into s, p, d and f sub-levels of successively higher energies. Sub-levels contain a fixed number of orbitals, regions of space where there is a high probability of finding an electron. Each orbital has a defined energy state for a given electronic configuration and chemical environment and can hold two electrons of opposite spin. Applications -and Skills Description of the relationship between colour, wavelength, frequency and energy across the electromagnetic spectrum. Distinction between a continuous spectrum and a line spectrum. Description of the emission spectrum of the hydrogen atom, including the relationships between the lines and energy transitions to the first, second and third energy levels. Recognition of the shape of an s atomic orbital and the px, py and pz atomic orbitals. Application of the Aufbau principle, Hund’s rule and the Pauli exclusion principle to write electron configurations for atoms and ions up to Z = 36.

All EM radiation is fundamentally the same All EM radiation is fundamentally the same. The only difference between a gamma ray and a radio wave is the frequency/wavelength/energy.

Visible light is one category of EM radiation Visible light is one category of EM radiation. The visible light spectrum is subdivided into six “colors”. White Light Prism RED ORANGE YELLOW GREEN BLUE VIOLET

A continuous spectrum includes all wavelengths of radiation in a given range. When white light is passed through a prism a continuous spectrum is produced.

Colored lights do not emit all the wavelengths of the visible light spectrum. For example, a red light emits mostly wavelengths from the red end of the spectrum. An energized gas sample will emit light of specific wavelengths characteristic of the gas. This is called a line spectrum

Emission spectra are unique for each element

a central dense positive nucleus composed of protons and neutrons. The Bohr model of the atom was developed using information from hydrogen emission spectrum studies. Bohr envisioned an atomic model with: a central dense positive nucleus composed of protons and neutrons. negative electrons at specific energies orbit the nucleus mostly empty space. Nucleus is 10-5 times smaller than atom.

Bohr further stated that the orbiting electrons occupy discrete energy levels. Electrons can only “jump” between energy levels if they absorb or emit a specific amount of energy.

Bohr saw the line spectrum of hydrogen as a direct result of energized electrons releasing a specific amount of energy by emitting a photon of light at a certain wavelength. The different lines in the hydrogen spectrum were evidence for a number of different energy levels.

Visible spectrum for hydrogen atom lower energy longer wavelength higher energy shorter wavelength Visible spectrum for hydrogen atom convergence

Electrons fill the lowest energy orbitals first. Lower energy = more stable electron orbit Electrons fill the lowest energy orbitals first. Each orbital has a maximum possible number of electrons. As you should recall: 1st energy level (ground state) = 2 electrons 2nd energy level = 8 electrons

s p d f 1A 2A 3B 4B 5B 6B 7B 8B 1B 2B 3A 4A 5A 6A 7A 8A group # = # valence (outside) e- s 1 2 3 4 5 6 7 p Row = # shells d f

Electron Configuration 1s1 group # # valence e- possibilities are: s: 1 or 2 p: 1-6 d: 1-10 f: 1-14 Total e- should equal Atomic # row # shell # possibilities are 1-7 7 rows subshell possibilities are s, p, d, or f 4 subshells What element has an electron configuration of 1s1?

Practice: Ask these questions every time you have to write an electron configuration Lithium: find the element on the periodic table what is the period number? how many shells? what is the group number? how many valence electrons? what subshell(s) does Li have? what is the electron configuration? atomic # = 3 2 2 1 1 s 1s2 2s1

Practice: Ask these questions every time you have to write an electron configuration Boron: find the element on the periodic table what is the row #? how many shells? what is the group #? how many valence electrons? what subshell(s) does B have? what is the electron configuration? atomic # = 5 2 2 3 3 p 1s2 2s2 2p1

Order of Electron Subshell Filling: It does not go “in order” 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f14 6s2 6p6 6d10 7s2 7p6 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f14 6d10 7p6

Subshells d and f are “special” group # = # valence e- 1 2 3 4 5 6 7 3d d period # = # e- shells 4d 5d 6d f 4f 5f

Electron Configuration 1s1 group # # valence e- possibilities are: s: 1 or 2 p: 1-6 d: 1-10 f: 1-14 Total e- should equal Atomic # row # shell # possibilities are 1-7 7 rows subshell possibilities are s, p, d, or f 4 subshells What element has an electron configuration of 1s1?

Practice: Ask these questions every time you have to write an electron configuration Lithium: find the element on the periodic table what is the row #? how many shells? what is the group #? how many valence electrons? what subshell(s) does Li have? what is the electron configuration?

Practice: Ask these questions every time you have to write an electron configuration Boron: find the element on the periodic table what is the row #? how many shells? what is the group #? how many valence electrons? what subshell(s) does B have? what is the electron configuration?

Order of Electron Subshell Filling: It does not go “in order” 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f14 6s2 6p6 6d10 7s2 7p6