Introductory Chemistry, 2nd Edition Nivaldo Tro Chapter 9 Electrons in Atoms and the Periodic Table

Tro's Introductory Chemistry, Chapter 9 Why do Blimps Float? Because they are filled with a gas less dense than air Early blimps used hydrogen gas; hydrogen’s flammability led to the Hindenburg disaster Blimps now use helium, a nonflammable gas – in fact it doesn’t undergo any chemical reactions This chapter investigates models of the atom we use to explain the differences in the properties of the elements Tro's Introductory Chemistry, Chapter 9

Electromagnetic Radiation Light is one of the forms of energy Light is one type of a more general form of energy called electromagnetic radiation Electromagnetic radiation travels in waves Tro's Introductory Chemistry, Chapter 9

Characteristics of a Wave Wavelength = distance from peak to peak Amplitude = height of the peak Frequency = the number of wave peaks that pass in a given time Speed = rate the waves travel Tro's Introductory Chemistry, Chapter 9

Tro's Introductory Chemistry, Chapter 9 Particles of Light Scientists in the early 20th century showed that electromagnetic radiation was composed of particles we call photons Max Planck and Albert Einstein photons are particles of light energy Each wavelength of light has photons that have a different amount of energy the longer the wavelength, the lower the energy of the photons Tro's Introductory Chemistry, Chapter 9

The Electromagnetic Spectrum Light passed through a prism is separated into all its colors = continuous spectrum; colors blend into each other Color of light is determined by its wavelength Tro's Introductory Chemistry, Chapter 9

Electromagnetic Spectrum Visible light is a very small portion of the electromagnetic spectrum Tro's Introductory Chemistry, Chapter 9

Light’s Relationship to Matter Atoms can absorb energy, but they must eventually release it When atoms emit energy, it is released in the form of light = emission spectrum Atoms don’t absorb or emit all colors, only very specific wavelengths; the spectrum of wavelengths can be used to identify the element Tro's Introductory Chemistry, Chapter 9

Emission Spectrum or Line Spectrum Tro's Introductory Chemistry, Chapter 9

Tro's Introductory Chemistry, Chapter 9 Line Spectra = specific wavelengths are emitted; characteristic of atoms Tro's Introductory Chemistry, Chapter 9

The Bohr Model of the Atom Nuclear Model of atom does not explain how atom can gain or lose energy Neils Bohr developed a model to explain how structure of the atom changes when it undergoes energy transitions Bohr postulated that energy of the atom was quantized, and that the amount of energy in the atom was related to the electron’s position in the atom quantized means that the atom could only have very specific amounts of energy Tro's Introductory Chemistry, Chapter 9

Bohr Model of Atom: Electron Orbits In the Bohr Model, electrons travel in orbits or energy levels around the nucleus The farther the electron is from the nucleus the more energy it has

The Bohr Model of the Atom: Orbits and Energy Each orbit (energy level) has a specific amount of energy Energy of each orbit is symbolized by n, with values of 1, 2, 3 etc; the higher the value the farther it is from the nucleus and the more energy an electron in that orbit has Tro's Introductory Chemistry, Chapter 9

The Bohr Model of the Atom: Energy Transitions Electrons can move from a lower to a higher (farther from nucleus) energy level by absorbing energy When the electron moves from a higher to a lower (closer to nucleus) energy level, energy is emitted from the atom as a photon of light Tro's Introductory Chemistry, Chapter 9

The Bohr Model of the Atom Ground and Excited States Ground state – atoms with their electrons in the lowest energy level possible; this lowest energy state is the most stable. Excited state – a higher energy state; electrons jump to higher energy levels by absorbing energy Atom is less stable in an excited state; it will release the extra energy to return to the ground state Tro's Introductory Chemistry, Chapter 9

Electron Energy Levels: Energy Level How many e fit? (2n2) 3rd 18 electrons 2 x 32 2nd 8 electrons 2 x 22 1st 2 electrons 2 x 12 Each energy level has a maximum # of electrons it can hold. H has one electron; it is in the 1st energy level. Bohr model H Tro's Introductory Chemistry, Chapter 9

Bohr Model for Atom Electrons fill the Lowest energy levels first Bohr Model for C with 6 electrons Tro's Introductory Chemistry, Chapter 9

The Bohr Model of the Atom Success and Failure The Bohr Model very accurately predicts the spectrum of hydrogen with its one electron It is inadequate when applied to atoms with many electrons A better theory was needed Tro's Introductory Chemistry, Chapter 9

The Quantum-Mechanical Model Orbitals Erwin Schrödinger used mathematics to predict probability of finding an electron at a certain location in the atom Result is a map of regions in the atom that have a particular probability for finding the electron Orbital = a region with a very high probability of finding the electron when it has a particular amount of energy Tro's Introductory Chemistry, Chapter 9

The Quantum-Mechanical Model Each principal energy level or shell has one or more subshells # of subshells same as the principal quantum number or shell The subshells are often represented as a letter s, p, d, f Each kind of subshell has orbitals with a particular shape Tro's Introductory Chemistry, Chapter 9

Tro's Introductory Chemistry, Chapter 9 Shells & Subshells Tro's Introductory Chemistry, Chapter 9

Probability Maps & Orbital Shape s orbitals are spherical Tro's Introductory Chemistry, Chapter 9

Probability Maps & Orbital Shape p orbitals Tro's Introductory Chemistry, Chapter 9

Subshells and Orbitals The subshells of a principal shell have slightly different energies the subshells in a shell of H all have the same energy, but for multielectron atoms the subshells have different energies s < p < d < f Each subshell contains one or more orbitals s subshells have 1 orbital p subshells have 3 orbitals d subshells have 5 orbitals f subshells have 7 orbitals Tro's Introductory Chemistry, Chapter 9

The Quantum Mechanical Model Energy Transitions As in Bohr Model, atoms gain or lose energy as electron moves between orbitals in different energy shells and subshells The ground state of the electron is the lowest energy orbital it can occupy Excited state = when an electron moves to a higher energy orbital Tro's Introductory Chemistry, Chapter 9

The Bohr Model vs. The Quantum Mechanical Model Both the Bohr and Quantum Mechanical models predict the spectrum of hydrogen very accurately Only the Quantum Mechanical model predicts the spectra of multielectron atoms Tro's Introductory Chemistry, Chapter 9

Electron Configurations Electron configuration = distribution of electrons into the various energy shells and subshells in an atom in its ground state Each energy shell and subshell has a maximum number of electrons it can hold s = 2, p = 6, d = 10, f = 14 Tro's Introductory Chemistry, Chapter 9

Writing Electron Configurations We place electrons in the energy shells and orbitals in order of energy, from low energy up: Aufbau Principle (order of filling of orbitals) The d and f orbitals overlap into the higher energy levels Tro's Introductory Chemistry, Chapter 9

Tro's Introductory Chemistry, Chapter 9 5f 4s 4p 4d 4f 3s 3p 3d Energy 2s 2p Relative Energy of Orbitals in the Quantum Mechanical Model 1s Tro's Introductory Chemistry, Chapter 9

Order of Subshell Filling in Ground State Electron Configurations Start by drawing a diagram putting each energy shell on a row and listing the subshells, (s, p, d, f), for that shell in order of energy, (left-to-right) 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 7s next, draw arrows through the diagonals, looping back to the next diagonal each time Tro's Introductory Chemistry, Chapter 9

Filling the Orbitals in a Subshell with Electrons Energy shells fill from lowest energy to high Subshells fill from lowest energy to high s → p → d → f A single orbital can hold a maximum of 2 electrons (Pauli’s exclusion principle); orbitals that are in the same subshell have the same energy When filling orbitals that have the same energy, place one electron in each before completing pairs (Hund’s rule) Tro's Introductory Chemistry, Chapter 9

Electron Configuration of Atoms in their Ground State Electron configuration = order of filling with electrons; number of electrons in that subshell written as a superscript Kr = 36 electrons = 1s22s22p63s23p64s23d104p6 Shorthand way: use the symbol of the previous noble gas in brackets to represent all the inner electrons, then just write the last set Rb = 37 electrons = 1s22s22p63s23p64s23d104p65s1 = [Kr]5s1 Tro's Introductory Chemistry, Chapter 9

Electron Configurations how many electrons in that orbital Nitrogen: 1s22s22p3 energy level orbital (atomic number = 7) Tro's Introductory Chemistry, Chapter 9

Mg, Z = 12, so Mg has 12 protons and 12 electrons Example – Write the Ground State Orbital Diagram and Electron Configuration of Magnesium. Determine the atomic number of the element from the Periodic Table This gives the number of protons and electrons in the atom Mg, Z = 12, so Mg has 12 protons and 12 electrons Tro's Introductory Chemistry, Chapter 9

Tro's Introductory Chemistry, Chapter 9 Example – Write the Ground State Orbital Diagram and Electron Configuration of Magnesium. Draw 9 boxes to represent the first 3 energy levels s and p orbitals 1s 2s 2p 3s 3p Tro's Introductory Chemistry, Chapter 9

Tro's Introductory Chemistry, Chapter 9 Example – Write the Ground State Orbital Diagram and Electron Configuration of Magnesium. Add one electron to each box in a set, then pair the electrons before going to the next set until you use all the electrons When pairing, put in opposite arrows          1s 2s 2p 3s 3p Tro's Introductory Chemistry, Chapter 9

Tro's Introductory Chemistry, Chapter 9 Example – Write the Ground State Orbital Diagram and Electron Configuration of Magnesium. Use the diagram to write the electron configuration Write the number of electrons in each set as a superscript next to the name of the orbital set 1s22s22p63s2 = [Ne]3s2 1s 2s 2p 3s 3p  Tro's Introductory Chemistry, Chapter 9

Tro's Introductory Chemistry, Chapter 9 Valence Electrons Valence electrons = electrons in all the subshells with the highest principal energy shell (outermost shell) Core electrons = in lower energy shells Valence electrons responsible for both chemical and physical properties of atoms. Valence electrons responsible for chemical reactions Tro's Introductory Chemistry, Chapter 9

Tro's Introductory Chemistry, Chapter 9 Valence Electrons Rb = 37 electrons = 1s22s22p63s23p64s23d104p65s1 The highest principal energy shell of Rb that contains electrons is the 5th, therefore Rb has 1 valence electron and 36 core electrons Kr = 36 electrons = 1s22s22p63s23p64s23d104p6 The highest principal energy shell of Kr that contains electrons is the 4th, therefore Kr has 8 valence electrons and 28 core electrons Tro's Introductory Chemistry, Chapter 9

How many valence electrons does each atom have? carbon: 1s22s22p2 chlorine: 1s22s22p63s23p5 Tro's Introductory Chemistry, Chapter 9

How many valence electrons does each atom have? carbon: 1s22s22p2 = 4 chlorine: 1s22s22p63s23p5 = 7 Tro's Introductory Chemistry, Chapter 9

Electron Configurations and the Periodic Table Tro's Introductory Chemistry, Chapter 9

Electron Configurations from the Periodic Table Elements in the same period (row) have valence electrons in the same principal energy shell The number of valence electrons increases by one as you progress across the period Elements in the same group (column) have the same number of valence electrons and they are in the same kind of subshell Tro's Introductory Chemistry, Chapter 9

Electron Configuration & the Periodic Table Elements in the same column have similar chemical and physical properties because their valence shell electron configuration is the same The number of valence electrons for the main group elements is the same as the group number Tro's Introductory Chemistry, Chapter 9

The Explanatory Power of the Quantum-Mechanical Model The properties of the elements are largely determined by the number of valence electrons they contain Since elements in the same column have the same number of valence electrons, they show similar properties Tro's Introductory Chemistry, Chapter 9

The Noble Gas Electron Configuration The noble gases have 8 valence electrons except for He, which has only 2 electrons Noble gases are especially unreactive He and Ne are practically inert Reason noble gases are unreactive is that the electron configuration of the noble gases is especially stable Tro's Introductory Chemistry, Chapter 9