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Chapter 4Arrangement of Electrons in Atoms
THE DEVELOPMENT OF A NEW ATOMIC MODEL
The Rutherford model of the atom was an improvement over previous models, but it was incomplete. It did not explain how the atom’s nega?vely charged electrons are distributed in the space surrounding its posi?vely charged nucleus. AAer all, it was well known that oppositely charged par?cles aCract each other. So what prevented the nega?ve electrons from being drawn into the posi?ve nucleus?
In the early 20th century, a new atomic model evolved as a result of inves?ga?ons into the absorp?on and emission of light by maCer. The studies revealed a rela?onship between light and an atom’s electrons. This new understanding led directly to a revolu?onary view of the nature of energy, maCer, and atomic structure.
Properties of Light• Before 1900, scien?sts thought light behaved as a wave • Changed when it was discovered that light also had par?cle-‐like characteris?cs
• Review of wavelike proper?es will help understanding theory of light as it was at the beginning of the 20th century
• Visible light is a kind of electromagne+c radia+on à form of energy that exhibits wavelike behavior as it travels through space • X-‐rays, UV, infrared, etc.
• Together, all forms of electromagne?c radia?on form the electromagne+c spectrum
Properties of EM Radiation• All forms of EM radia?on travel at a constant speed (3.0 × 108 meters per second (m/s)) through a vacuum and at slightly slower speeds through maCer
• It has a repe??ve nature, which can be described by the measurable proper?es à wavelength and frequency
• Wavelength (λ) à the distance between corresponding points on neighboring waves
• Frequency (ν) à the number of waves that pass a given point in a specific 6me, usually one second
λ
λ
Frequency and Wavelength• Frequency and wavelength are mathema?cally related to each other
• This rela?onship is wriCen as follows
c = λν
• c is the speed of light • λ is the wavelength of the electromagne?c wave • ν is the frequency of the electromagne?c wave
• Because c is the same for all electromagne?c radia?on, the product λν is a constant
The Photoelectric Effect• Early 1900s, scien?sts conducted two experiments involving rela?ons of light and maCer that could not be explained by the wave theory of light
• One experiment involved a phenomenon known as the photoelectric effect
• Photoelectric effect à the emission of electrons from a metal when light shines on the metal
The Mystery• For a given metal, no electrons were emiCed if the light’s frequency was below a certain minimum—regardless of how long the light was shone
• Light was known to be a form of energy, capable of knocking loose an electron from a metal
• Wave theory of light predicted light of any frequency could supply enough energy to eject an electron
• Couldn’t explain why light had to be a minimum frequency in order for the photoelectric effect to occur
The Particle Description of Light• Explana?on à 1900, German physicist Max Planck was studying the emission of light by hot objects
• Proposed a hot object does not emit electromagne?c energy con?nuously, as would be expected if the energy emiCed were in the form of waves
• Instead, Planck suggested the object emits energy in small, specific amounts called quanta
• Quantum à the minimum quan6ty of energy that can be lost or gained by an atom
• Planck projected the following rela?onship between a quantum of energy and the frequency of radia?on
E = hν
• In the equa?on, • E is the energy, in joules, of a quantum of radia?on • ν is the frequency of the radia?on emiCed • h is a constant now known as Planck’s constant
• h = 6.626 × 10−34 J• s
1905 Albert Einstein• Expanded on Planck’s theory by introducing the idea that electromagne?c radia?on has a dual wave-‐par+cle nature
• Light displays many wavelike proper?es, it can also be thought of as a stream of par?cles
• Einstein called these par?cles photons • Photon à a par6cle of electromagne6c radia6on having zero mass and carrying a quantum of energy
• The energy of a specific photon depends on the frequency of the radia?on
Ephoton = hν
The Hydrogen-‐Atom Line-‐Emission
• When current is passed through a gas at low pressure, the poten?al energy of some of the gas atoms increases
• The lowest energy state of an atom is its ground state
• A state in which an atom has a higher poten6al energy than it has in its ground state is an excited state
Returning to Ground State• When an excited atom returns to its ground state, it gives off the energy it gained in the form of electromagne?c radia?on
• Example of this Excited neon atoms emit light when falling back to the ground state or to a lower-‐energy excited state.
Line-‐Emission Spectrum• Scien?sts passed electric current through a vacuum tube containing hydrogen gas at low pressure, they observed the emission of a characteris?c pinkish glow
• EmiCed light was shined through a prism, it was separated into a series of specific frequencies of visible light
• The bands of light were part of what is known as hydrogen’s line-‐emission spectrum
Quantum Theory• The hydrogen atoms should be excited by whatever amount of energy was added to them
• Scien?sts expected to see the emission of a con?nuous range of frequencies of electromagne?c radia?on, that is, a con+nuous spectrum
• Why had the hydrogen atoms given off only specific frequencies of light?
• ACempts to explain this observa?on led to an en?rely new theory of the atom called quantum theory
• Whenever an excited hydrogen atom falls back from an excited state to its ground state or to a lower-‐energy excited state, it emits a photon of radia?on
• The energy of this photon (Ephoton = hν) is equal to the difference in energy between the atom’s ini?al state and its final state
• Changes of energy (transi?on of an electron from one orbit to another) is done in isolated quanta
• Quanta are not divisible
• There is sudden movement from one specific energy level to another, with no smooth transi?on
• There is no ``in-‐between‘‘
• Hydrogen atoms emit only specific frequencies of light à showed that energy differences between the atoms’ energy states were fixed
• The electron of a hydrogen atom exists only in very specific energy states
Bohr Model of the Hydrogen • Solved in 1913 by the Danish physicist Niels Bohr • Proposed a model of the hydrogen atom that linked the atom’s electron with photon emission • According to the model, the electron can circle the nucleus only in allowed paths, or orbits
• When the electron is in one of these orbits, the atom has a definite, fixed energy
• The electron, and therefore the hydrogen atom, is in its lowest energy state when it is in the orbit closest to the nucleus
• This orbit is separated from the nucleus by a large empty space where the electron cannot exist
• The energy of the electron is higher when it is in orbits farther from the nucleus
The Rungs of a Ladder• The electron orbits or atomic energy levels in Bohr’s model can be compared to the rungs of a ladder
• When you are standing on a ladder, your feet are on one rung or another
• The amount of poten?al energy that you possess relates to standing on the first rung, the second rung…
• Your energy cannot relate to standing between two rungs because you cannot stand in midair
• In the same way, an electron can be in one orbit or another, but not in between
Moving up in the Orbit…• Explaining spectral lines: • While in a given orbit, electron isn’t gaining or losing energy • Can move to higher-‐energy orbit by gaining energy equal to difference in energy between higher-‐energy orbit and ini?al lower-‐energy orbit
• When hydrogen is excited, electron is in higher-‐energy orbit • When electron falls to lower energy level, a photon is emiCed in process called emission
• Photon’s energy is equal to energy difference between ini?al high energy and final lower energy level
• To move electron from low energy to high energy level, energy must be added to atom in process called absorp6on
• When a hydrogen atom is in an excited state, its electron is in a higher-‐energy orbit
• When the atom falls back from the excited state, the electron drops down to a lower-‐energy orbit
• In the process, a photon is emiCed that has an energy equal to the energy difference between the ini?al higher-‐energy orbit and the final lower-‐energy orbit
• Based on different wavelengths of H spectrum, Bohr calculated allowed energy levels for H atom
• Then related possible energy-‐level changes to lines in spectrum • Lyman series shows result of electron dropping from levels E6, E5, E4, E3, E2 to ground level E1
• Bohr’s model explained spectral lines so well many scien?sts thought it could be applied to all atoms
• Did not explain spectra of atoms with more than one electron
• Did not explain the chemical behavior of atoms
SECTION 2 – THE QUANTUM MODEL OF THE ATOM
To the scien?sts of the early 20th century, Bohr’s model of the hydrogen atom contradicted common sense. Why did hydrogen’s electron exist around the nucleus only in certain allowed orbits with definite energies? Why couldn’t the electron exist in a limitless number of orbits with slightly different energies? To explain why atomic energy states are quan?zed, scien?sts had to change the way they viewed the nature of the electron.
Electrons as Waves• Photoelectric effect and H spectrum showed light behaving as wave and par?cle
• Could electrons have dual wave-‐par?cle nature? • 1924 Louis de Broglie proposed answer that revolu?onized our understanding of maCer
• DeBroglie pointed out behavior of electrons in Bohr’s orbits similar to known behavior of waves
• Any wave in limited space has only certain frequencies • Electrons should be considered as being in limited space around the nucleus
• So electron waves could exist only at specific frequencies • According to E = hv, frequencies will correspond to specific energies – the quan?zed energies of Bohr’s orbits
• Other parts of DeBroglie’s hypothesis confirmed by experiment • Electrons, like light, can be bent or diffracted • Diffrac6on is bending a wave as it passes by edge of object or through small opening
• Diffrac?on experiments showed beams of electrons can interfere with each other
• Interference happens when waves overlap • Overlapping reduces energy in some areas and increases in other areas
The Heisenberg Uncertainty
• If electrons are both par?cles and waves, then where are they in the atom?
• To answer this ques?on, it is important to consider a proposal first made in 1927 by the German theore?cal physicist Werner Heisenberg
• Heisenberg’s idea involved detec?on of electrons by interac?on with photons
• Photons have about same energy as electrons • Trying to locate specific electron with photon knocks electron off course
• So there is always uncertainty trying to locate an electron • Heisenberg uncertainty principle à states that it is impossible to determine simultaneously both the posi6on and velocity of an electron or any other par6cle
Schrodinger Wave Equation• Used hypothesis that electrons have dual nature to develop equa?on that treated electrons in atoms as waves
• Bohr assumed quan?za?on was fact • Schrodinger didn’t assume – it was a natural result of his equa?on
• Only waves of specific energies (frequencies) provided solu?ons to equa?on
• With Heisenberg uncertainty, wave equa?on laid founda?on for modern quantum theory
• Quantum theory – describes mathema6cally the wave proper6es of electrons and other small par6cles
Orbitals• Electrons do not travel around the nucleus in neat orbits • Instead, they exist in certain regions called orbitals • Orbital à a three-‐dimensional region around the nucleus that indicates the probable loca6on of an electron
Atomic Orbitals and Quantum Numbers• Bohr’s model – electrons of increasing energy occupy orbits farther and farther from nucleus
• Schrodinger equa?on – electrons in orbitals have quan?zed energies
• Energy level is not the only characteris?c of an orbital shown by solving Schrodinger equa?on
Atomic Orbitals and Quantum • In order to completely describe orbitals, scien?sts use quantum numbers
• Quantum numbers à specify the proper6es of atomic orbitals and the proper6es of electrons in orbitals
• First 3 numbers result from solu?ons to Schrodinger’s equa?on
• Indicate energy level, shape, and orienta?on of each orbital
The Quantum Numbers1. Principal quantum number à the main energy level
occupied by the electron
2. Angular momentum quantum number à the shape of the orbital
3. Magne+c quantum number à the orienta?on of the orbital around the nucleus
4. Spin quantum number à the two fundamental spin states of an electron in an orbital
Principal Quantum Number
• Symbolized by n • Indicates the main energy level occupied by the electron • Values are posi?ve integers only • As n increases, the electron’s energy and its average distance from the nucleus increase
Angular Momentum Quantum
• Except at the first main energy level, orbitals of different shapes— known as sublevels—exist for a given value of n
• The angular momentum quantum number, symbolized by l, indicates the shape of the orbital
• The values of l allowed are zero and all posi?ve integers less than or equal to n − 1 • n = 2 can have one of two shapes • l = 0 and l = 1
n = 1 • l = 0 only • For l = 0, the corresponding leCer designa?on for the orbital shape is s
• Shape of s is a sphere
n = 2• l = 0 (s), l = 1 (p) • For l = 1, , the corresponding leCer designa?on for the orbital shape is p
• Shape of p is
n = 3• l = 0 (s), l = 1 (p), l = 2 (d) • For l = 2, the corresponding leCer designa?on for the orbital shape is d
• Shape of d is
n = 4• l = 0 (s),1 (p), 2 (d), 3 (f) • For l = 3, the corresponding leCer designa?on for the orbital shape is f
• Shape is too complicated to picture
• Each atomic orbital is designated by the principal quantum number followed by the leCer of the sublevel
• For example, the 1s sublevel is the s orbital in the first main energy level, while the 2p sublevel is the set of p orbitals in the second main energy level
Magnetic Quantum Number• Atomic orbitals can have the same shape but different orienta?ons around the nucleus
• The magne+c quantum number, symbolized by m, indicates the orienta6on of an orbital around the nucleus
S orbital• Because an s orbital is spherical and is centered around the nucleus, it has only one possible orienta?on
• This orienta?on corresponds to a magne?c quantum number of m = 0
• There is therefore only one s orbital in each s sublevel
P orbital• p orbital can extend along the x, y, or z axis • There are therefore three p orbitals in each p sublevel, which are designated as px, py , and pz orbitals
• The three p orbitals occupy different regions of space and correspond, in no par?cular order, to values of m = −1, m = 0, and m = +1
D orbital• There are five different d orbitals in each d sublevel
• The five different orienta?ons, including one with a different shape, correspond to values of m = −2, m = −1, m = 0, m = +1, and m = +2
Spin Quantum Number• An electron in an orbital can be thought of as spinning on an axis
• It spins in one of two possible direc?ons, or states
• The spin quantum number has only two possible values — +1/2 , −1/2
• Indicate the two fundamental spin states of an electron in an orbital
• ***A single orbital can hold a maximum of two electrons, which must have opposite spins***
• You can think of an atom as a very bizarre house with the nucleus living on the ground floor, and then various rooms (orbitals) on the higher floors occupied by the electrons
• On the first floor there is only 1 room (the 1s orbital) • On the second floor there are 4 rooms (the 2s, 2px, 2py and 2pz orbitals)
• On the third floor there are 9 rooms (one 3s orbital, three 3p orbitals and five 3d orbitals); and so on
• But the rooms aren't very big . . . Each orbital can only hold 2 electrons
SECTION 3 – ELECTRON CONFIGURATIONS
• Quantum model of atom beCer than Bohr model b/c it describes the arrangements of electrons in atoms other than hydrogen
• Electron configura+on à the arrangement of electrons in an atom
• b/c atoms of different elements have different numbers of e-‐, the e-‐ configura?on for each element is unique
• Electrons in atoms assume arrangements that have lowest possible energies
• Ground-‐state electron configura+on à lowest-‐energy arrangement of electrons for each element
• Some rules combined with quantum numbers let us determine the ground-‐state e-‐ configura?ons
Rules Governing Electron
• To build up electron configura?ons for the ground state of any par?cular atom, first the energy levels of the orbitals are determined
• Then electrons are added to the orbitals one by one according to three basic rules
1. Au{au principle 2. Pauli exclusion principle 3. Hund’s rule
Aufbau Principle• The first rule shows the order in which electrons occupy orbitals
• According to the AuLau principle, an electron occupies the lowest-‐energy orbital that can receive it
• The orbital with the lowest energy is the 1s orbital
• Ground-‐state hydrogen atom à electron in this orbital
• The 2s orbital is the next highest in energy, then the 2p orbitals
• Beginning with the third main energy level, n = 3, the energies of the sublevels in different main energy levels begin to overlap
• The 4s sublevel is lower in energy than the 3d sublevel
• Therefore, the 4s orbital is filled before any electrons enter the 3d orbitals
• (Less energy is required for two electrons to pair up in the 4s orbital than for a single electron to occupy a 3d orbital)
Pauli Exclusion Principle• The second rule reflects the importance of the spin quantum number • According to the Pauli exclusion principle, no two electrons in the same atom can have the same set of four quantum numbers
Hund’s Rule• The third rule requires placing as many unpaired electrons as possible in separate orbitals in the same sublevel • Electron-‐electron repulsion is minimized à the electron arrangements have the lowest energy possible • Hund’s rule à orbitals of equal energy are each occupied by one electron before any orbital is occupied by a second electron, and all electrons in singly occupied orbitals must have the same spin
Orbital Notation
H He
C
Electron-‐Configuration Notation• Electron-‐configura?on nota?on eliminates the lines and arrows of orbital nota?on
• Instead, the number of electrons in a sublevel is shown by adding a superscript to the sublevel designa?on
• The hydrogen configura?on is represented by 1s1
• The superscript indicates that one electron is present in hydrogen’s 1s orbital
• The helium configura?on is represented by 1s2
• Here the superscript indicates that there are two electrons in helium’s 1s orbital
Sample ProblemThe electron configura+on of boron is 1s22s22p1. How many electrons are present in an atom of boron? What is the atomic number for boron? Write the orbital nota+on for boron.
Practice Problems• The electron configura?on of nitrogen is 1s22s22p3. How many electrons are present in a nitrogen atom? What is the atomic number of nitrogen? Write the orbital nota?on for nitrogen.
Practice ProblemsThe electron configura?on of fluorine is 1s22s22p5. What is the atomic number of fluorine? How many of its p orbitals are filled? How many unpaired electrons does a fluorine atom contain?
• Answer • 9 • 2 • 1
Elements of the Second Period• According to Au{au principle, aAer 1s orbital is filled, the next electron occupies the s sublevel in the second main energy level
• Lithium, Li, has a configura?on of 1s22s1
• The electron occupying the 2s level of a lithium atom is in the atom’s highest, or outermost, occupied level
• The highest occupied level à the electron-‐containing main energy level with the highest principal quantum number
• The two electrons in the 1s sublevel of lithium are no longer in the outermost main energy level
• They have become inner-‐shell electrons à electrons that are not in the highest occupied energy level
• The fourth electron in an atom of beryllium, Be, must complete the pair in the 2s sublevel because this sublevel is of lower energy than the 2p sublevel
• With the 2s sublevel filled, the 2p sublevel, which has three vacant orbitals of equal energy, can be occupied
• One of the three p orbitals is occupied by a single electron in an atom of boron, B
• Two of the three p orbitals are occupied by unpaired electrons in an atom of carbon, C
• And all three p orbitals are occupied by unpaired electrons in
Elements of Third Period• AAer the outer octet is filled in neon, the next electron enters the s sublevel in the n = 3 main energy level
• Atoms of sodium, Na, have the configura?on 1s22s22p63s1
• Once past Neon, can use Noble-‐gas nota?on
Noble-‐Gas Notation• The Group 18 elements (helium, neon, argon, krypton, xenon, and radon) are called the noble gases
• To simplify sodium’s nota?on, the symbol for neon, enclosed in square brackets, is used to represent the complete neon configura?on:
[Ne] = 1s22s22p6
• This allows us to write sodium’s electron configura?on as [Ne]3s1, which is called sodium’s noble-‐gas nota?on
Elements of the Fourth Period• Period begins by filling 4s orbital (empty orbital of lowest energy)
• First element in fourth period is potassium, K • E-‐ configura?on [Ar]4s1
• Next element is calcium, Ca • E-‐ configura?on [Ar]4s2
• With 4s sublevel filled, 4p and 3d sublevels are next available empty orbitals • Diagram shows 3d sublevel is lower energy than 4p sublevel
• Total of 10 electrons can fill 3d orbitals
• These are filled from element scandium (Sc) to zinc (Zn)
• Scandium has e-‐ configura?on [Ar]3d14s2 • Titanium, Ti, has configura?on [Ar]3d24s2 • Vanadium, V, has configura?on [Ar]3d34s2
• Up to this point, 3 e-‐ with same spin added to 3 separate d orbitals (required by Hund’s rule)
• Chromium, Cr, has configura?on [Ar]3d54s1
• We added one electron to 4th 3d orbital • We also took a 4s electron and added it to 5th 3d orbital • Seems to be against Au{au principle
• In reality, [Ar]3d54s1 is lower energy than [Ar]3d44s2
• Having 6 outer orbitals with unpaired electrons in 3d orbital is more stable than having 4 unpaired electrons in 3d orbitals and forcing 2 electrons to pair in 4s
• For tungsten, W, (same group as chromium) having 4 e-‐ in 5d orbitals and 2 e-‐ paired in 6s is most stable arrangement
• No easy explanan?on
• Manganese, Mn, has configura?on [Ar]3d54s2 • Added e-‐ goes to 4s orbital, completely filling it and leaving 3d half-‐filled
• Star?ng with next element, e-‐ con?nue to pair in d orbitals
• So iron, Fe, has configura?on [Ar]3d64s2 • Cobalt, Co, has [Ar]3d74s2 • Nickel, Ni, has [Ar]3d84s2
• With copper, Cu, an electron from 4s moves to pair with e-‐ in 5th 3d orbital
• Configura?on: [Ar]3d104s1 (most stable)
• For zinc, Zn, 4s sublevel filled to give [Ar]3d104s2
• For next elements, 1 e-‐ added to 4p orbitals according to Hund’s rule
Elements of Fifth Period• In 18 elements of 5th period, sublevels fill in similar way to 4th period elements
• BUT they start at 5s level instead of 4s level
• Fill 5s, then 4d, and finally 5p
• There are excep?ons just like in 4th period
Practice ProblemWrite both the complete electron-‐configura?on nota?on and the noble-‐gas nota?on for iron, Fe.
Write both the complete electron configura?on nota?on and the noble-‐gas nota?on for iodine, I. How many inner-‐shell electrons does an iodine atom contain?
• 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5 • [Kr] 4d10 5s2 5p5 • 46
How many electron-‐containing orbitals are in an atom of iodine? How many of these orbitals are filled? How many unpaired electrons are there in an atom of iodine?
• 27 • 26 • 1
Write the noble-‐gas nota?on for ?n, Sn. How many unpaired electrons are there in an atom of ?n?
• [Kr] 5s2 4d10 5p2 • 2
Without consul?ng the periodic table or a table in this chapter, write the complete electron configura?on for the element with atomic number 25.
• 1s2 2s2 2p6 3s2 3p6 4s2 3d5
Elements of 6th and 7th periods• 6th period has 32 elements • To build up e-‐ configura?ons for elements in this period, e-‐ first added first to 6s orbital in cesium, Cs, and barium, Ba
• Then in lanthanum, La, e-‐ added to 5d orbital
• With next element, cerium, Ce, 4f orbitals begin to fill • Ce: [Xe]4f15d16s2
• In next 13 elements, 4f orbitals filled
• Next 5d orbitals filled
• Period finished by filling 6p orbitals
• (some excep?ons happen)
Practice ProblemWrite both the complete electron-‐configura?on nota?on and the noble-‐gas nota?on for a rubidium atom.
• 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1 • [Kr]5s1
Write both the complete electron configura?on nota?on and the noble-‐gas nota?on for a barium atom.
• 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 • [Xe]6s2