ELECTRON ARRANGEMENT IN AN ATOM - · PDF file7/5/2013 · ELECTRON ARRANGEMENT IN AN ATOM. ... Electrons in atoms arranged in discrete levels. An electron absorbs energy to...

Engr. Yvonne Ligaya F. Musico 1

ELECTRON ARRANGEMENT IN AN

ATOM

Characteristics of Electrons

Extremely small mass

Located outside the nucleus

Moving at extremely high speeds in a sphere

Have specific energy levels



Energy of Electrons

When atoms are heated, bright lines appear

called line spectra

Electrons in atoms arranged in discrete levels.

An electron absorbs energy to “jump” to a

higher energy level.

When an electron falls to a lower energy level,

energy is emitted.


Loss and Gain of Energy

G

a

I

n

L

o

s

s

Bohr Model

• First model of the electron structure

• Gives levels where an electron is most likely

to be found

• Incorrect today, but a key in understanding

the atom


Bohr Model

Eng. Yvonne Ligya F. Musico 6

Bohr Model

• e- exist only in orbits with specific amounts of energy called energy levels

• Therefore…

– e- can only gain or lose certain amounts of energy

– only certain photons are produced



Bohr’s Model

1

23

45

6

Energy of photon depends on the difference in energy levels

Bohr’s calculated energies matched the IR, visible, and UV lines for the H atom


Other Elements

Each element has a unique bright-line emission spectrum.

“Atomic Fingerprint”

Helium

Bohr’s calculations only worked for hydrogen!

The Quantum Mechanical Model

• Describes the arrangement and space

occupied by electrons in atoms

• This model is based on Quantum Theory,

which says that matter also has properties

associated with waves.

10

Quantum Theory

• State that it is impossible to know an electron’s exact position and momentum at the same time (Uncertainty of Principle).

• So the scientist had to develop the concept of orbitals (sometimes called electron clouds), volumes of space in which an e- is likely present (certainty was replaced with probability).

Quantum Mechanical Model

Describes the arrangement of electrons in

atoms in terms of:

Main or principal energy levels (n)

Energy subshells

Orbitals (space occupied within the atom)


13

Quantum Numbers

The principal quantum number, n

• describes the average distance of the orbital from the nucleus and the energy of the electron in an atom.

• (n=1) is the energy level nearest to the nucleus and the succeeding energy level is located farther from the nucleus.

• Electrons in energy levels at increasing distances from the nucleus have increasingly higher energies.


Maximum No. of Electrons / Energy LevelPrincipal Energy

Level, n

Allowed per Energy Level = 2n2

1 2 x (1)2 = 2

2 2 x (2)2 = 8

3 2 x (3)2 = 18

4 2 x (4)2 = 32

5 2 x (5)2 = 50

6 2 x (6)2 = 72

Quantum Numbers

• The principal quantum number, n


Quantum Numbers

• The azimuthal quantum number, l sometimes called the Orbital Angular Momentum

Quantum Number

designates the overall shape of the orbital within a shell

affects orbital energies (bigger l = higher energy)

all electrons in an atom with the same value of l are said to

belong to the same subshell


Subshells - Orbitals that have the same value of

n but different values of l

n = 4

n = 3

n = 2

n = 1

4s 4p 4d 4f

3s 3p 3d

2s 2p

1s

Quantum Numbers

18

• The azimuthal quantum number, l

Letter Designation of the Subshells

Value of l Letter Subshell Name

# of electrons

that can exist

0 s sharp 2

1 p principal 6

2 d diffuse 10

3 f fundamental 14

Quantum Numbers

• The azimuthal quantum number, l


s p d f

Quantum Numbers

• The magnetic quantum number, ml

– determines the orientation of orbitals within a

subshell

– does not affect orbital energy (except in magnetic

fields!)

– only integer values between -l and +l are allowed

– the number of m values within a subshell is the

number of orbitals within a subshell


Quantum Numbers



The number of possible ml values determines the number of orbitals in a subshell.

l possible values of ml Number of orbital in this subshell

0 0 1

1 -1, 0, +1 3

2 -2, -1, 0, +1, +2 5

3 -3, -2, -1, 0, +1, +2, +3 7

Quantum Numbers



Quantum Numbers



px py pz

Quantum Numbers



Orbitals combine to form a spherical shape.

2s

2pz2py

2px

Quantum Numbers

• The spin quantum number, ms

several experimental observations can be

explained by treating the electron as though it were

spinning

spin makes the electron behave like a tiny magnet

spin can be clockwise or counterclockwise

spin quantum number can have values of

+1/2 or -1/2


Quantum Numbers


• The spin quantum number, ms

Electronic Configuration

• The statement of how many electrons an atom

has in each of its subshells.

• a list showing how many electrons are in each

orbital or subshell in an atom or ion


Arrangement of Electrons

• Aufbau Principle

• Pauli Exclusion Principle

• Hund’s Rule

Aufbau Principle


– Electrons fill the

lowest energy orbital

first.

– “Lazy Tenant Rule”

Methods of Illustrating Electronic Configuration

• s p d f Notation

the distribution of electrons using this method

indicates the energy level and sublevels that

are filled.




Energy sequence

1s 2s 3s 4s 5s 6s 7s

2p 3p 4p 5p 6p 7p

3d 4d 5d 6d 7d

4f 5f 6f 7f

Note:

s ≤ 2

p ≤ 6

d ≤ 10

f ≤ 14

A way to remember the filling order of subshells, write subshells

designations and follow the diagonal arrows starting from left.




1s2

main shell

subshell

number of electrons



Example:

1.) 9F 1s2 2s2 2p5

2.) 13Al 1s2 2s2 2p6 3s2 3p1

3.) 2He 1s2

4.) 12Mg 1s2 2s2 2p6 3s2


Pauli Exclusion Principle

• Limits the number of electrons in any orbital to

not more than two

• Each orbital can hold TWO electrons with

opposite spins.


Hund’s Rule

• Priciple of Maximum Multiplicity

• Within a sublevel, place one e- per orbital before pairing them.

• “Empty Bus Seat Rule”


RIGHTWRONG


• Orbital diagram

using arrows to represent electrons and boxes

for orbitals. (Pauli exclusion principle and

Hund’s Rule should be followed)



• Orbital diagram


↑↓ ↑↓ ↑↓ ↑ ↑

1s 2s 2p


Orbital diagram

1. write the electron configuration in subshell notation

2. draw a box for each orbital.

• Remember that s, p, d, and f subshells contain 1, 3, 5, and 7 degenerate orbitals, respectively.



Orbital diagram

• Remember that an orbital can hold 0, 1, or 2 electrons only, and if there are two electrons in the orbital, they must have opposite (paired) spins (Pauli principle )

3. within a subshell (depicted as a group of boxes), spread the electrons out and line up their spins as much as possible (Hund's rule )



• Orbital diagram

Example:

1.) 9F

2.) 13Al

3.) 2He

4.) 12Mg


↑↓ ↑↓↑↓ ↑↓ ↑↓ ↑↓ ↑

↑↓ ↑↓↑↓ ↑↓ ↑↓ ↑↓

↑↓ ↑↓↑↓ ↑↓ ↑

↑↓

Learning Check

Using the periodic table, write the complete

electronic configuration using electron notation

for each:

A. Cl

B. Sr

C. I


Solution

Using the periodic table, write the complete electronic configuration using electron notation for each:

A. Cl

1s2 2s2 2p6 3s2 3p5

B. Sr

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2

C. I

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5


Paramagnetic and Diamagnetic

Paramagnetic

- When there is unpaired electron

- configurations with unpaired electrons are attracted to magnetic fields (paramagnetism )

Diamagnetic

- When all electrons are paired

- configurations with only paired electrons are weakly repelled by magnetic fields (diamagnetism )



Some other important terms

Differentiating electron

- the last entering electron in the electron configuration

Valence Shell

- the outermost shell

Valence Electron

-all the electrons in the outermost shell or valence shell

Core and Valence Electron

• chemistry involves mostly the shell with the highest

value of principal quantum number , n, called the

valence shell

• the noble gas core under the valence shell is

chemically inert

• simplify the notation for electron configurations by

replacing the core with a noble gas symbol in square

brackets:


Example of Electron Configuration written with the

Core and Valence Electron

Atom Full configuration Core valence configuration

Full configuration using core/valence

notation

O 1s2 2s2 2p4 He 2s2 2p4 [He] 2s2 2p4

Cl 1s2 2s2 2p6 3s2 3p5 Ne 3s2 3p5 [Ne] 3s2 3p5

Al 1s2 2s2 2p6 3s2 3p1 Ne 3s2 3p1 [Ne] 3s2 3p1


Psuedocore Electrons

• electrons in d and f subshells outside the noble

gas core are called pseudocore electrons


Examples of electron configurations containing

pseudocore electrons

Atom Core Psuedocore Valence Full Configuration

Fe Ar 3d6 4s2 [Ar] 3d6 4s2

Sn Kr 4d10 5s2 5p2 [Kr] 4d10 5s2 5p2

Hg Xe 4f14 5d10 6s2 [Xe] 4f14 5d10 6s2

Pu Rn 5f6 7s2 [Rn] 5f6 7s2


ENGR. YVONNE LIGAYA F. MUSICO

ELECTRON ARRANGEMENT IN AN ATOM - · PDF file7/5/2013 · ELECTRON ARRANGEMENT IN AN ATOM. ... Electrons in atoms arranged in discrete levels. An electron absorbs energy to...

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