Chapter 2. Atomic structure and interatomic bonding2.1. Atomic structure

2.1.1.Fundamental concepts2.1.2. Electrons in atoms2.1.3. The periodic table

2.2. Atomic bonding in solids2.2.1. Bonding forces and energies

2.2.2. Primary interatomic bonds

2.2.3. Secondary bonding

2.2.4. Molecules

2.1. Atomic structure

2.1.1.Fundamental concepts

Atom consists of a nucleus (protons and neutrons) and electrons that move in their orbits.e : 1.6 X 10-19 CMp = mn = 1.67 X 10-27 kgMe = 9.11 X 10-31 kg

A chemical element hasAtomic number (Z) : the number of proton in the nucleus,Atomic mass (A) : the sum of the masses of protons and neutrons

in the nucleus,Neutron number (N) : the number of neutron may vary for a given

elements.

Isotopes : elements with two or more different atomic masses

Atomic weight (amu)A ≈ Z + N

2.1.2. Electrons in atoms

Bohr atomic model

Electrons revolve aroundan atomic nucleus indiscrete orbitals

An electron is an particle

Wave-mechanical model Electrons are considered as both wave-like and particle-like

Every electron in an atom is characterized by four parameterscalled quantum numbers. Principal quantum number, n

n = 1 → K; n = 2 → L; and so onSecond quantum number, l

s, p, d, fThird quantum number, ml (number of energy states)

s → ml = 1; p → ml = 3 ; d → ml = 5 ; f → ml = 7Fourth quantum number, ms (spin orientation when a magneticfield is applied)

ms = + ½ and – ½

The schematic of the relative energies of the electronsfor various shells and subshells

a. Bohr atomic modelElectrons are in fixed positionsand energy (quantized energylevels).

b. Wave-mechanical modelElectrons’ position is consideredto be the probability of anelectron’s being at variouslocations around the nucleus.

Comparison between the two models

Pauli exclusion principleEach electron state can hold no more than two electrons,which must have opposite spins

Electron configurations

Ground state is a level where electrons occupy the lowestpossible energy according to Pauli exclusion principle

Electron configurations represent the manner in which thesestates are occupied.Examples: Hydrogen (H) 1s1

Helium (He) 1s2

Sodium (Na) 1s2 2s2 2p6 3s1

Calcium (Ca) 1s2 2s2 2p6 3s2 3p6 4s2

2.1.3. The periodic table

The periodic table consists of all elements that have beenclassified according to electron configuration.Groups IA and IIA (alkali and alkaline earth metals)

Elements with one and two electrons in excess of stable structures.Groups IIIA, IVA, and VA

Elements between metals and nonmetals by virtue of their valenceelectron structures.

Groups VIA and VIIAElements with two and one electron in deficient of stable structures.(Group VIIA is called the halogens)

Group VIIIA (Inert gases)Elements which have filled electron shells and stable configurations.

2.2. Atomic bonding in solids

2.2.1. Bonding forces and energies

Interatomic forces determine the physical properties of materials.There are two types of forces:the attractive and the repulsive forcesMost of time, it is more convenient to work with potentialenergies, which are attractive and the repulsive energies.Both terms depend on the distance between the centre oftwo atoms.The equilibrium distance, roFor many atoms, ro = 0.3 nm (3Å )

Bonding force

FN = FA + FR

The net force, FN

In equilibrium:

FA + FR = 0

Bonding energy

EN = EA + ER

The net energy, EN

The potential energybetween two atoms

∫= FdrE

Types of bonding:

A. Primary bonding or chemical bondingThis bonding is found in solids and involves the valence electrons.This type of bonding is strong (» 100 kJ/mol)

Examples: ionic, covalent, and metallic bondsB. Secondary bonding or physical bonding or van der Waals

This bonding is found in most solids and arises from atomic ormolecular dipoles.This type of bonding is weak ( ≅ 10kJ/mol)

Examples: fluctuating induced dipole bonds, polar molecule-Induced dipole bonds, and pemanent dipole bonds

A. Primary bonding or chemical bonding

Ionic bondingIt is always found in compounds that are composed of both metallicand nonmetallic elements. Atoms of a metallic element easily giveup their valence electrons to the nonmetallic atoms.This bonding is a nondirectional bonding, the magnitude of the bondis equal in all directions around an ion.

Coulombic bonding force

Attractive energy:

Repulsive energy:

r

AEA −=

rE

n

B

B=

A, B, n = constants,n ~ 8

A. Primary bonding or chemical bonding

Covalent bondingIt is usually found in many nonmetallic elemental molecules (H2, Cl2, F2)and molecules containing dissimilar atoms (CH4, H20, HNO3, HF)This bonding is formed on stable electron configurations by sharing of electrons between adjacent atoms.

A very strong covalent bond

Diamond with a very highmelting temperature(713 kJ/mol; 3550 ºC)

A very weak covalent bond

Bismuth with a very lowmelting temperature(270 ºC)

A. Primary bonding or chemical bondingMetallic bondingIt is found in many metals and their alloys (group IA and IIA). Metallic materials have 1, 2 or at most 3 valence electrons.These valence electrons are not bound to any particular atom to anyParticular atom in the solid and are free to drift throughout theentire metal.

“sea of electrons”or “electron cloud”Net negative charge

Ion coresNet positive charge

Weak metallic bondHg (68 kJ/mol; -39 ºC)

Strong metallic bondW (850 kJ/mol; 3410 ºC)

B. Secondary bonding or physical bonding or van der Waals

Fluctuating induced dipole bondsAll atoms have constant vibrational motion and it causes electricalsymmetry and creates small electric dipoles

B. Secondary bonding or physical bonding or van der Waals

Polar molecule-induced dipole bondsIt causes by virtue of an asymmetrical arrangement of positivelyand negatively charged regions

B. Secondary bonding or physical bonding or van der Waals

Permanent dipole bondsIt exist between adjacent polar molecules. The hydrogen bond is the strongest secondary bonding type.