Bohr Model of Hydrogen Atom

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1 Bohr Model of Hydrogen Atom electrons move in circular orbits around nucleus orbits can only be of certain radii each radius corresponds to different energy ( only certain energies are allowed) n - defines energy (and orbit radius) Orbitals and Quantum Numbers In reality, electrons are better described as existing in clouds of electron density at varying distances from the nucleus. These clouds are known as orbitals and each orbital has a characteristic energy and shape. 1

2 Principle Quantum Number n integral values of 1, 2, 3,... as n increases orbital becomes larger electron has higher energy electron less tightly bound Azimuthal Quantum Number l integral values from 0 to n-1 defines shape of orbital l = 0 l = 1 l = 2 s orbital p orbital d orbital etc 2

3 Magnetic Quantum Number m l integral values between l and - l defines direction of orbital Orbitals & Quantum Numbers (cont d) orbitals w/same n value make up an electron shell orbitals w/same n & l values make up an electron subshell 3

4 Orbitals & Quantum Numbers (cont d) shell w/quantum number n will have exactly n subshells each subshell consists of a specific # of orbitals (1, 3, 5,...) total number of orbitals in a shell = n 2 Electron Configurations Ground state configuration (most stable) is the arrangement in which the electrons are in their lowest possible energy state. Hund s rule for degenerate orbitals the lowest energy is attained when the number of electrons with the same spin is maximized (electrons repel each other) 4

5 Example 1 Write the electron configuration of chlorine, element 17. 1s 2 2s 2 2p 6 3s 2 3p 5 [Ne] 3s 2 3p 5 Period 4 and Beyond Ti [Ar] 4s 3d 4s fills first then 3d 4s lower in energy 5

6 Example 2 What family of elements is characterized by having ns 2 np 3 outer-electron configuration? Group 5A Electron Shells in Atoms As you move down the periodic table, n increases. As n gets larger, that shell s radius increases. 6

7 Atomic Sizes Atomic size increases as move down the periodic table Atomic size decreases as move right across the periodic table Example 3 Which would be closer to the nucleus, the n = 3 shell in Ar or the n = 3 shell in Kr? Kr - because the number of protons is higher 7

8 Example 4 Predict which will be greater, the P Br bond length in PBr 3 or the P Cl bond length in PCl 3? P Br Example 5 Arrange the following atoms in order of increasing atomic radius: K, Mg, Ca Mg < Ca < K 8

9 Ionization Energy Minimum energy required to remove an electron from the ground state of the isolated atom or ion. The greater the ionization energy, the harder it is to remove an electron. Ionization Energy (cont d) First ionization energy (I 1 ) - energy required to remove first electron from neutral atom. Na (g) Na + (g) + e- Second ionization energy (I 2 ) - energy required to remove second electron. Na + (g) Na2+ (g) + e- I 1 < I 2 < I 3 for a given element 9

10 Trends in Ionization Energy Within a row, I 1 increases with increasing atomic number effective nuclear charge increasing Within each group, I 1 decreases with increasing atomic number average distance from nucleus increasing The transition elements don t exhibit much variation in I 1 Example 6 Predict which of the following atoms has the lowest first ionization energy: C, Si, N, P Si 10

11 Electron Affinities E energy change that occurs when an electron is added to an atom Cl (g) + e - Cl - (g) E = -349 kj /mol in most cases, energy is released when an electron is added to an atom ( E = - ) ionization energy measures tells how easy it is to remove an electron, electron affinity tells how easy it is for an atom to gain an electron Trends in Electron Affinity In general, electron affinity increases as you move left to right across a row filling a shell is energetically favorable half-filling a shell is favorable Within each group, electron affinity decreases as you move down the table average distance from nucleus is increasing 11

12 Metals low ionization energies so they are easily oxidized goal is usually to lose enough electrons to achieve noble gas configuration tend to form ionic compounds with nonmetals metal oxides particularly important 2 Ni (s) + O 2(g) 2 NiO (s) Most Metal Oxides are Basic Metal oxide + water metal hydroxide CaO (s) + H 2 O (l) Ca(OH) 2(aq) Metal oxide + acid salt + water CaO (s) + HCl (aq) CaCl 2(aq) + H 2 O (l) 12

13 Nonmetals nonmetals tend to gain electrons when reacting with metals (reduced) goal is to gain enough electrons to achieve noble gas configuration compounds of only nonmetals - molecular form ionic compounds when reacting with metals Ca (s) + Cl 2(g) CaCl 2(s) Nonmetal Oxides are Acidic Nonmetal oxide + water acid CO 2(g) + H 2 O (l) H 2 CO 3(aq) Nonmetal oxide + base salt + water CO 2(g) + 2 KOH (aq) K 2 CO 3(aq) + H 2 O (l) 13

14 Metalloids properties intermediate between metals and nonmetals 14