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2014-‐05-‐06 The Atom The concept of a discrete unit that makes up all Atomic Theory Chemistry 11 matter has been around for centuries. These ideas were based on philosophical reasoning rather than experimentation and empirical observations. This concept has been accepted by scientists since it elegantly explains new discoveries in the field of chemistry. In addition… Aristotle proposed that all matter is made up of 4 elements with 4 different properties: The fifth element is Aether, the material that fills the region of the universe above the terrestrial sphere. Atomic Number and Atomic Mass The elements are differentiated from one another by the numbers of protons in the nucleus. Atomic Number: The number of protons in the nucleus. A neutral atom has no charge, therefore: In a neutral atom: Number of Protons = Number of Electrons For Example: How many electrons are possessed by the following? N3- 10 electrons Ca2+ 18 electrons Br- 36 electrons 1 2014-‐05-‐06 Since both neutrons and protons have a mass of 1.0 u, the total atomic mass of an atom will be found by their combined totals. For Example: Find the number of protons, neutrons, and electrons possessed by the following: What about the electrons? Species having the same atomic number, but different atomic masses (same # of protons, different number of neutrons). 1 1 H = ORDINARY HYDROGEN (called “protium”). 2 1 H = DEUTERIUM (sometimes call “heavy” hydrogen). 3 1 H = TRITIUM (called “radioactive” hydrogen). 13 protons, 14 neutrons, 13 electrons 75 33 As 33 protons, 42 neutrons, 33 electrons For Example: Experiments show that chlorine is a mixture which is 75.77% Cl-35, and 24.23% Cl-37. If the precise molar mass of Cl-35 is 34.968853 g/mol and of Cl-37 is 36.965903 g/mol, what is the average molar mass of the chlorine atoms in such a mixture? Homework: You may also use the atomic mass to calculate the average. The average mass will be less exact, but still satisfactory. Al The molar masses given on the periodic table are found by calculating the average mass of a sample containing a mixture of isotopes. Isotope: For Example: 27 13 Do: Introduction to Atomic Theory W.S. 2 2014-‐05-‐06 The Periodic Table! As real elements became discovered, the Greek ideas of Air, Earth, Fire, and Water had to be abandoned. Scientists needed an elegant, easy to use method of accessing all the information about the elements. Major Divisions Within the Periodic Table Two important trends appear in the periodic table: Period: The set of elements in a given row going across the table. Group or Family: The set of elements in a given column going up and down the table. There are several groups, rows, and “blocks” of elements: In summary: 3 2014-‐05-‐06 In 1913, Niels Bohr proposed a model that The Electronic Structure of the Atom When a hydrogen atom is irradiated by energy, some of the energy is absorbed then reemitted as light. explained why the observed line spectrum for Hydrogen looks the way it does. He proposed that: The electron in hydrogen can only exist If the light is passed through a prism, a “line in specific energy states. These energy states are associated with specific circular orbits which the electron can occupy around the atom. Listen here, I say! spectrum” is observed. When an electron absorbs energy, it instantaneously moves from one orbit to another. The greater the energy, the farther the orbit is from the nucleus. ENERGY LEVEL: A specific amount of energy which an electron in an atom can possess. The energy levels of hydrogen have the pattern below (“n” is the number of the energy level). The observed spectrum represents energy level differences occurring when an electron gives off energy and drops from a higher energy level. The energy difference between two different energy levels is called the QUANTUM of energy associated with the transition between the two levels. The Energy Level Diagram for Hydrogen The lowest sets of energy levels for hydrogen are as follows: A few years after Bohr published his theories, several changes were made to his ideas. The idea of electrons orbiting along a specific path in a well defined orbit had to be abandoned. Instead, different electrons, depending on their energies, occupy particular regions of space called “orbitals”. 4 2014-‐05-‐06 Each dash represents the energy possessed by a particular orbital in the atom. The letter s, p, d, and f refer to the four “types” of orbitals (more to come later). Shell: The set of all orbitals having the same n value. For Example: Some notes… All the orbitals for a hydrogen atom with a given value of n have the same energy (not true for atoms with more than one electron). Rules governing which types of orbitals can occur: For a given value of “n”, certain types of orbitals are possible The 3rd shell consists of the 3s, 3p, and 3d orbitals. Subshell: A set of orbitals of the same type. For Example: The set of five 3d orbitals in the 3rd shell is a subshell. For For For For n= n= n= n= 1: 2: 3: 4: only the s type is possible the s and p types are possible the s, p, and d types are possible the s, p, d, and f types are possible. An s type subshell consists of ONE s orbital A p type subshell consists of THREE p orbitals. A d type subshell consists of FIVE d orbitals. An f type subshell consists of SEVEN f orbitals. The Energy Level Diagram for Polyelectronic Atoms The energy level diagram must be modified to describe any other atom. The following diagram applies to ALL polyelectronic atoms (atoms having more than one electron). 5 2014-‐05-‐06 ELECTRON CONFIGURATIONS The addition of electrons to the orbitals follows three simple rules: Aufbau Principle: As atomic number increases, electrons are added to the available Writing Electron Configurations for Neutral Atoms ELECTRON CONFIGERATION: Describes which orbitals in an atom contain electrons and how many electrons are in each orbit. orbitals. To ensure LOWEST POSSIBLE ENERGY for the atom, electrons are added to the orbitals having the lowest energy FIRST. How do we do this? Pauli Exclusion Principle: A maximum of TWO electrons can be placed in each subshell. Hunds Rule: When electrons occupy subshells of equal energy, they must be singly occupied with electrons having parallel spins. 2nd electrons are then added to each subshell so each electron has opposite spin. Tryski… Core Notation Predict the electron configuration of the following: Si 1s22s22p63s23p2 Tc 1s22s22p63s23p64s23d104p65s24d5 Ca 1s22s22p63s23p64s2 Zr 1s22s22p63s23p64s23d104p65s24d2 Ga 1s22s22p63s23p64s23d104p1 The OUTER electrons are all those outside the core. Since the core electrons are not involved in chemical reactions, they are excluded from the electron configuration. For Example: Al 1s22s22p63s23p1 The electrons belonging to an atom can be broken into two subsets: The CORE electrons. The OUTER electrons. The CORE of an atom is the set of electrons with the configuration of the nearest noble gas having an atomic number LESS than that of the atom being considered. Write the following using core notation: Zr ([Kr]5s24d2) Ga ([Ar]4s23d104p1) Co ([Ar]4s23d7) becomes: [Ne]3s23p1 6 2014-‐05-‐06 Homework: Do: Writing Electron Configurations for Ions Anions: The Periodic Table and Stuff W.S. #1-4 Add electrons to the last unfilled subshell, starting where the neutral atom left off. For Example: Oxygen: [He] 2s2 2p4 + 2e- → O2-([He]2s22p6) Sulphur: [Ne] 3s2 3p4 + 2e- → S-([Ne]3s23p6) For Example: Cations: 2 Rules: 1. Electrons in the outermost shells (largest n value) are removed first. 2. If there are electrons in both the s and p orbitals of the outermost shell, the electrons in the p orbitals are removed first. Tin: [Kr] 5s2 4d10 5p2 → 2e- + Sn2+([Kr]5s24d10) Tin: [Kr] 5s2 4d10 5p2 → 4e- + Sn4+([Kr]4d10) p electrons BEFORE s electrons BEFORE d electrons Outermost electrons are removed preferentially. Also, e- in the highest energy outermost orbital require the least amount of energy to be completely removed from the atom. I pity the fool who doesn’t do these examples! Ru3+ [Kr]4d5 Sb3+ [Kr]5s24d10 to Kr: Cr ([Ar] 4s2 3d4) → “3d4” is one e- short of a half S2- [Ne]3s23p6 Cu ([Ar] 4s2 3d9) → “3d9” is one e- short of a filled N3- [He]2s22p6 2 exceptions to the configurations of elements up filled subshell. subshell. 7 2014-‐05-‐06 The actual configurations for Cr and Cu are found to be: Cr ([Ar] 4s1 3d5) → “4s1” and “3d5” are two half filled subshells. Cu ([Ar] 4s1 3d10) → “4s1” is a half filled subshell, and “3d10” is a filled subshell. Because of this extra stability, an atom or ion that is one e- short of a “d5” or “d10” configuration will shift an e- from the s- subshell having the highest energy into the unfilled d- subshell. Therefore: A filled or exactly half filled d- subshell is especially stable. Predicting Number of Valence Electrons Valence Electrons: Electrons that can take place in chemical reactions. Are all the electrons in the atom EXCEPT: Core electrons. In filled d or f subshells. For Example: Al([Ne] 3s2 3p1) has 3 valence electrons: → “3s2 3p1” Ga([Ar] 4s2 3d10 4p1) has 3 valence electrons: → Omit “3d10” b/c filled Pb([Xe] 6s2 4f14 5d10 6p2) has 4 valence electrons: → Omit “4f14” and “5d10” b/c filled Xe([Kr] 5s2 4d10 5p6) has ZERO valence electrons: → Noble gas configuration Homework: Do: The Periodic Table and Stuff W.S. #5-7 Study for your quiz!!! Electron Configurations What is on the Exam? History of the Atom The Atom Atomic Number and Mass # of Protons, Neutrons, and Electrons Isotopes The Electronic Structure of the Atoms Theory Configurations (Neutral, Ions, Core) Exceptions Valence Electrons The Periodic Table 8