Atomic Structure and Periodicity

Schrodinger Wave Equation

22

2 28dh EVm dx

Equation for probability of a single electron being found along a single axis (x-axis)Erwin Schrodinger

Pauli Exclusion Principle

No two electrons in an atom can have the same four quantum numbers.

Wolfgang Pauli

Heisenberg Uncertainty Principle

You can find out where the electron is, but not where it is going.

OR…You can find out where the electron is going, but not where it is!

“One cannot simultaneously determine both the position and momentum of an electron.”

WernerHeisenberg

Quantum NumbersEach electron in an atom has a unique set of 4 quantum numbers which describe it.

Principal quantum number Angular momentum quantum number

Magnetic quantum number Spin quantum number

Principal Quantum NumberGenerally symbolized by n, it denotes the shell (energy level) in which the electron is located.

Tell the energyNumber of electrons that can fit in a shell:

2n2

N= 1, 2, 3, ….

Angular Momentum Quantum NumberThe angular momentum quantum number,

generally symbolized by l, denotes the orbital (subshell) in which the electron is located.

n2= the number of orbitals on an energy level

All orbitals with the same value forN are said to be degenerate.

Magnetic Quantum NumberThe magnetic quantum number, generally symbolized by m, denotes the orientation of the electron’s orbital with respect to the three axes in space.

Assigning the Numbers The three quantum numbers (n, l, and

m) are integers. The principal quantum number (n)

cannot be zero. n must be 1, 2, 3, etc. The angular momentum quantum

number (l ) can be any integer between 0 and n - 1.

For n = 3, l can be either 0, 1, or 2. The magnetic quantum number (ml) can

be any integer between -l and +l. For l = 2, m can be either -2, -1, 0, +1,

+2.

Principle, angular momentum, and magnetic quantum numbers: n, l, and ml

Spin Quantum NumberSpin quantum number denotes the behavior (direction of spin) of an electron within a magnetic field.

Possibilities for electron spin:

12

12

Magnetic Properties• Although an electron behaves like a tiny

magnet, two electrons that are opposite in spin cancel each other. Only atoms with unpaired electrons exhibit magnetic susceptibility – A paramagnetic substance is one that is

weakly attracted by a magnetic field, usually the result of unpaired electrons.

– A diamagnetic substance is not attracted by a magnetic field generally because it has only paired electrons.

Half of the distance between nucli in covalently bonded diatomic molecule

"covalent atomic radii"

Periodic Trends in Atomic Radius Radius decreases across a period

Increased effective nuclear charge dueto decreased shielding

Radius increases down a group Addition of principal quantum levels

Determination of Atomic Radius

Bond Radius

Table of Atomic Radii

Zeff constantn ↑r ↑

n constant

Zeff ↑r ↓

I.E. ↓

I.E. ↑

II/

Tend to be positive values endothermic process A + energy = A+ + e-

Ionization Energy: the energy required to remove an electron from an atom

John A. SchreifelsChemistry 211

Chapter 8-18

HIGHER IONIZATION ENERGIES

big jump in I.E. when core electrons start to be removed:

electrons from a lower main shell start to get removed. II/

Increases for successive electrons taken from the same atom Tends to increase across a period

Electrons in the same quantum level do not shield as effectively as electrons in inner levels

    Irregularities at half filled and filled sublevels due to extra repulsion of electrons paired in orbitals, making them easier to remove Tends to decrease down a group

Outer electrons are farther from thenucleus

Ionization Energy: the energy required to remove an electron from an atom

Trends in Electron Affinity

• The first occurs between Groups IA and IIA.– Added electron must

go in p-orbital, not s-orbital.

– Electron is farther from nucleus and feels repulsion from s-electrons.

Trends in Electron Affinity

• The second occurs between Groups IVA and VA.– Group VA has no empty

orbitals.– Extra electron must go

into occupied orbital, creating repulsion.

A + e- + energy → A-

A + e- → A- + energy

Trends in Electron Affinity

.

Can be an endothermic or exothermic process

Affinity tends to decrease as you go down in a period

Electrons farther from the nucleusexperience less nuclear attraction

Some irregularities due to repulsive forces in the relatively small p orbitals

Electron Affinity - the energy change associated with the addition of an electron

Affinity tends to increase across a period

Affinity tends to decrease as you go down in a period

Electrons farther from the nucleusexperience less nuclear attraction

Some irregularities due to repulsive forces in the relatively small p orbitals

Electron Affinity - the energy change associated with the addition of an electron

Table of Electron Affinities

A measure of the ability of an atom in a chemicalcompound to attract electrons

Electronegativities tend to increase across a period Electronegativities tend to decrease down a group or remain the same

Electronegativity

ElectronegativityWhat is it?Electronegativity is the power

of an atom to attract electron densityin a covalent bond

ElectronegativityPauling’s electronegativity scale

The higher the value, the more electronegative the element

Fluorine is the most electronegative elementIt has an electronegativity value of 4.0

ElectronegativityPauling’s electronegativity scale

Electronegativity

Periodic Table of Electronegativities

Cations Positively charged ions

Smaller than the corresponding

atomAnions Negatively charged ions Larger than the corresponding

atom

Ionic Radii

Sizes of ions• Ions are atoms that have either gained or lost

electrons (so that the # of electrons is not equal to the # of protons)

• The size of an atom can change dramatically if it becomes an ion

• E.g. when sodium loses its outer electron to become Na+ it becomes much smaller. Why?

• Na+ is smaller than Na because it has lost its 3s electron. Its valence shell is now 2s22p6 (it has a smaller value of n)

• Changing n values is one explanation for the size of ions. The other is …

Sizes of ions: electron repulsion• Valence electrons push each other away

9

+

• When an atom becomes a –ve ion (adds an electron to its valence shell) the repulsion between valence electrons increases without changing ENC

• Thus, F– is larger than F

Table of Ion Sizes

Summary of Periodic Trends electronegativity

electronegatvity

Ionic radius

Ioni

c ra

dius