Atomic structure & interatomic

bonding

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Chapter two

Atomic StructureMass Charge

Proton 1.67 х 10-27 kg + 1.60 х 10 -19 C

Neutron 1.67 х 10-27 kg Neutral

Electron 9.11 х 10-31 kg - 1.60 х 10 -19 C

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�Electron mass = 1/1836 that of a proton

�Radius of an atom= 0.1 nm = 0.1 x10 -9 m (1Angstrom)

�50,000,000 atoms lined up measure 10mm !!!

�Nucleus takes up 10 -14 of the total volume of atom

and has diameter of 4 -15 fm (femtometer = 10 -15 m)

Precision: How closely measurements of the same quantity come to each other.Accuracy: How close an experimental observation lies to the true value.

� Atomic mass (A) = mass of protons + mass of neutrons

�# of protons gives chemical identification of the element

�# of protons = atomic number (Z)

�# of neutrons (N) defines isotope number

General Notes:

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�The atomic mass unit (amu) is often used to express

atomic weight. 1 amu is defined as 1/12 of the atomic

mass of the most common isotope of carbon atom that

has 6 protons (Z=6) and six neutrons (N=6).

mproton ≈ mneutron = 1.67 х 10-27 kg = 1 amu.

The atomic mass of the 12C atom is 12 amu.

� Atomic mass (A) ≈ atomic number (Z) + # of neutrons (N)

�The atomic weight is often specified in mass per mole.

�A mole is the amount of matter that has a mass in

grams equal to the atomic mass in amu of the atoms (A

mole of carbon has a mass of 12 grams).

�Atomic weight = weighted average of the atomic

masses of the atoms naturally occurring isotopes.

Atomic weight of carbon is 12.011 amu.

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�The number of atoms in a mole is called the Avogadro

number, Nav = 6.023 × 10 23.Nav = 1 gram/1 amu.

mole of carbon has a mass of 12 grams).

� Atomic weight of Fe = 55.85 amu/atom = 55.85 g/mol

�The number of atoms per cm3, n, for material of

density ρ (g/cm3) and atomic mass A (g/mol):

n = Nav × ρ / A

Graphite (carbon): ρ = 2.3 g/cm3, A = 12 g/mol

n = 6.023 × 1023 atoms/mol × 2.3 g/cm3 / 12 g/mol =

11.5 × 10 22 atoms/cm3

Diamond (carbon): ρ = 3.5 g/cm3, A= 12 g/mol

n = 6.023 × 1023 atoms/mol × 3.5 g/cm3 / 12 g/mol =

17.5 × 10 22 atoms/cm3

Examples:

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17.5 × 10 22 atoms/cm3

Water (H2O) ρ = 1 g/cm3, Aw= 18 g/mol

n = 6.023 × 1023 molecules/mol × 1 g/cm3 / 18 g/mol =

3.3 × 10 22 molecules/cm3

For material with n = 6 × 10 22 atoms/cm3 calculate

mean distance between atoms L .

Atomic Models:

But quantum mechanics

tells us that this analogy

is not correct !!

The electrons form a cloud around the nucleus. This

picture looks like a mini planetary system.

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is not correct !!

Electrons move not in circular

orbits, but in 'fuzzy‘ orbits.

Actually, we cannot tell how it

moves, but only can say what is

the probability of finding it at some distance from the

nucleus. Only certain “orbits” or shells of electron

probability densities are allowed.

�The shells & electrons are identified by four

quantum number, n, l, ml and ms

�The quantum numbers arise from solution of

Schrodinger’s equation.

�Pauli Exclusion Principle: only one electron

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Pauli Exclusion Principle: only one electron

can have a given set of the four quantum

numbers.

�Now we can give a short description for the

quantum numbers.

Primary Quantum Number

� n

� Can have values from 1 to infinity, but they

can only be integers

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� K, L, M, N

� Represents the energy of the orbital, which is

also related to the size of the orbital

� An orbital is the region of space where you

are likely to find the electron

Angular Momentum Quantum

Number� l

� Shape of the orbital

� Can have values from 0 to n-1

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� Can have values from 0 to n-1

� s, p, d, f, g, h…..

� If there is more than one electron present, the

angular momentum quantum number also

affects the orbital energy (also called the

azimuthal quantum number)

Magnetic Quantum Number

� ml

� Can have integer values from –l to +l

� Thus, if n=1, l =0, and ml must equal 0

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� Thus, if n=1, l =0, and ml must equal 0

� In other words, it can only have one value

� If n=2, then l can equal either 0 or 1

� If it equals 1, then mlcan equal –1, 0 or +1

� It can have three values

Remember

� s orbitals correspond to l = 0

� p orbitals correspond to l = 1

� d orbitals correspond to l = 2

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3

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� d orbitals correspond to l = 2

� f orbitals correspond to l = 3

� How many orbitals are possible for each of these types?

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Spin Quantum Number

� ms

� +1/2

� -1/2

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� -1/2

� Two electrons of opposite spin fill each orbital

� The first three quantum numbers define an orbital

� You need all four to define an electron

Shorthand Notation

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� Germanium has 32 protons and 32 electrons

� 1s22s22p63s23p63d104s24p2

Shorthand Notation

Electron Shells

� Bonding occurs only with the electrons in the outer most shells – called the valence electrons

Inner electrons are called the core

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� Inner electrons are called the core electrons

� The valence electrons are those in the outer s and p orbitals, and any unfilledd and p orbitals.

1s22s22p63s23p63d104s24p2

Core electrons Valence

electrons

1s22s22p63s23p63d24s2

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1s22s22p63s23p63d24s2

Chemistry happens in the valence shell

Fundamental Concepts� First ionization energy (IE): it’s also called

ionization potential, it is the energy required

to remove the most weakly bound electron

from an isolated gaseous atom

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Atom (g) + IE = positive ion (g) + e-

and can be calculated from the equation:

IE = 13.6 Z2 / n2

Fundamental Concepts

� Electron Affinity (EA): the reverse process to the ionization energy, it is the energy change associated with an isolated gaseous atom accepting one electron

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accepting one electron

Atom (g) + e- = negative ion (g)

EA : positive if energy released.

: negative if energy required.

Fundamental Concepts

� Atomic and ionic radii : in general, positive ions are smaller than neutral atoms, while negative ions are larger.

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atoms, while negative ions are larger.

Fundamental Concepts� Electronegativity (χ) : independent measure

for atom attraction to electrons from another atom in a bond forming. It can be calculated from:

(χ) = (IE + EA) / 2

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(χ) = (IE + EA) / 2

or from:

(χ) = [ {0.31 (n + 1 ± c) } / r ] + 0.5

n: # of valence electrons.

c: any formal valence charge on the atom.

r: covalent radius.

Periodic TableS-block d-block p-block

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f-block

Trends in The Periodic Table

(IE) (EA)

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Atomic & ionic radii Electronegativity

Atomic Bonding:

� There are both attractive and repulsive

Binding Energy:

The bond is an electrostatic force that bind atoms

or molecules together.

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� There are both attractive and repulsive forces acting on atoms

� When they are balanced a bond is formed

� When the total energy of a pair of atoms is minimized, a bond is formed

Interatomic separation Force

Attraction

Repulsion

Net force FN ( FA+FR)

ro

Attraction force FA

Repulsion force FR

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Interatomic separation

Repulsion force FR

Potential energy; E

Attraction

Repulsion

Attraction energy

Repulsion energy

Net energy (E= min)Eo

Types of bonds:

Primary secondary

Strong weak

Eo : Bonding energy.

r0 : Bond length.

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Strong weak

Chemical physical

Primary bond is created when there is direct interaction

of electrons between two or more atoms.

Secondary bond occur due to indirect interaction of

electrons in adjacent atoms or molecules.

Electronegativity controls how elements combine

(bond) with each other because it provides a

measure of the excess binding energy between atoms A and B, ∆A-B (in kJ/mol) :

Primary bonds:

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The excess binding energy is related to the

energy required to separate two bonded atoms,

bond dissociation energy, DEAB :

∆A-B = 96.5 ( χA – χB ) 2

∆A-B = DEAB – [ (DEAA) (DEBB)] 1/2

Types of Primary bonds:Electronegativity difference > 2.0 Ionic bond

Electronegativity difference < 0.4 covalent bond

0.4 < Electronegativity difference < 2.0 Polar

covalent bond

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covalent bond

Special types of primary bonds is metallic bond

Ionic Bonds

� Metal-Nonmetal

� Cation-anion

� Non-directional

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� Non-directional

� Poor electrical conductivity

� Poor thermal conductivity

� Ceramics are formed from ionic bonds

� What is a molecule?

Covalent Bonds

� Nonmetal – nonmetal

� Directional bonds

� Poor electrical conductivity

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� Poor electrical conductivity

� Poor thermal conductivity

� Polymers are covalently bonded

� Compounds

Partial Ionic character:

Polar covalent bond:

A bond neither truly ionic nor totally covalent.

(HF)

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Partial Ionic character:

Ionic character % = 100 [ 1- exp { -0.25 ( χA – χB )2}]

Metallic Bonds

� Metal-Metal

� Non-directional

� Electrons are free to move around

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� Electrons are free to move around

� Good electrical conductivity

� Good thermal conductivity

� What is a molecule?

Mixed bonding

� 2 or more metals may form an intermetallic

compound

� A mixture of ionic and metallic bonds

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� Ceramics (nonmetal – metal) are usually a

mixture of ionic and covalent

� As the electronegativity difference increases,

the bond becomes more ionic

Secondary Bonding

� Metallic compounds and ionic compounds form crystals.

� But how do molecules of covalently

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� But how do molecules of covalently bonded elements stick together?

� Secondary Bonds

� Van der Waal’s: Dipole-dipole forces.

� Dipole-induced dipole

� London dispersion forces

Types of Secondary Forces

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� London dispersion forces

� Hydrogen bonding

What is dipole?

Atom or molecule that have some separation

of positive and negative portions

Van der Waal’s

� Interaction between permanent dipoles

� The interaction of permanent dipoles (analogous to magnets but having an

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(analogous to magnets but having an electrostatic dipole moment).

CH3Cl CH3Cl

Dipole induced dipole

A permanent dipole moment can induce a

dipole in a neighboring molecule in

which the unperturbed centers of positive

and negative charge are otherwise

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and negative charge are otherwise

coincident

CH4 CH3Cl

London dispersion forces:

•No permanent dipole

•It does have an instantaneous dipole moment

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•These instantaneous dipoles orient

themselves with their neighbors to give an

overall force of attraction.

H-bonding

� Hydrogen bonding: can be viewed as the interaction between very strong dipoles in - OH or -NH2 groups. It is

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dipoles in - OH or -NH2 groups. It is reflected in the very high boiling point of water compared with molecules of similar size.

H2O H2O

Bonding Energy

� How does bonding energy relate to melting point?

� Modulus of Elasticity?

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� Modulus of Elasticity?

� Coefficient of Thermal Expansion?

� Hint: The higher the bonding energy the more tightly the atoms are held together.