Basic Atomic Structure & Interatomic Bonding Some of the important properties of solid materials depend on geometrical atomic arrangements the interactions that exist among the constituent atoms or molecules rizam@kukum.edu.my

Atomic structure Fundamental Concept Atoms are the basic structural unit of all engineering materials. It is the basic unit of an element that can undergo chemical change Each atom consists of a very small nucleus composed of protons & neutrons which is encircled by moving electrons.

Atomic structure Atomic number, Z - Number of protons. In a neutral atom the atomic number is equal to the number of electrons (Z~p=e). Atomic mass, A - Total mass of proton and neutron in the nucleus ( A=Z+N ). Isotope - atoms that have two or more atomic mass. Same number of proton but different number of neutron. 1 atomic mass unit (a.m.u) – 1/12 of the atomic mass of carbon 1 mole= 6.023 x 1023 atoms ( Avogadro’s number NA ). 1 amu/atom = 1g/mol

Example 1

Example 2

Periodic Table Classified all the elements – electron configuration in the periodic table The element situated with increasing atomic number in seven horizontal rows called periods. All the elements arrayed in 1 column or group have similar : valence electron chemical properties physical properties

The periodic table inert gases accept 1e accept 2e give up 1e 2e 3e He Ar Kr Xe Rn inert gases accept 1e accept 2e give up 1e 2e 3e F Li Be Metal Nonmetal Intermediate H Na Cl Br I At O S Mg Ca Sr Ba Ra K Rb Cs Fr Sc Y Se Te Po

Periodic Table Electropositive elements: Electronegative elements: Metallic in nature & give up electrons in chemical reactions to produce positive ions (cations). Most electropositive ~ groups 1A & 2A Electronegative elements: Nonmetallic in nature & accept electrons in chemical reactions to produce negative ions (anions). Most electronegative ~ groups 6A & 7A Groups 4A & 7A can behave either electropositive electronegative manner

Oxidation numbers of the elements with respect to their positions in periodic table

Smaller electronegativity Larger electronegativity is the degree to which an atom attracts electrons to itself. Large values: tendency to acquire electrons. Ranges from 0.7 to 4.0, Smaller electronegativity Larger electronegativity

Summary of some of the electronic structure-chemical property relationships for metals & nonmetals Have few electrons in outer shell, usually 3 or less Have 4 or more electrons in outer shell Form cations by losing electrons Form anions by gaining electrons Have low electronegativities Have high electronegativities

Interatomic Bonding In general, why does bonding between atoms occur? Bonding between atoms generally occurs because the atoms’ energies are lowered through the bonding process.

Types of atomic & molecular bonds Chemical bond between atoms Primary (strong bond) Ionic bonds Covalent bonds Metallic bonds Secondary (weak bond) Permanent dipole bonds Fluctuating dipole bonds Depends on their valence electrons.

Ionic bonds eg; NaCl Ionic bonding arises from the electrostatic attraction between oppositely charged ions. In the process of ion formation, an electron or a number of electrons may be transferred from a highly electropositive element (eg; Na) to a highly electronegative one(eg; Cl). The ionic bond in solids is nondirectional.

Ionic Bonding In compounds made of metals and nonmetals. (Give and take creates bonding !) e.g. Ceramics, NaCl Nondirectional bonding: Bond magnitude is equal in all directions around an ion…THUS, all positive ions should be surrounded by negative ions in 3D. Ionic materials: Hard, Brittle, Electrically and thermally insulative (no free electrons).

Ionic bonding Involves metal & nonmetal elements Metal elements donate e- & nonmetal element gain electron Large difference in electronegativity required. Large different of electronegativity between element means strong bonding

Ionic bonding

Example 3 Describe the ionic bonding process between a pair of Na and Cl atoms. Which electrons are involved in the bonding process? Solution The ionic bonding process between a pair of Na and Cl atoms involves a transfer of the outer 3s1 electron of the Na atom to the 3p vacancy in the Cl atom. Thus, the Na ion formed has the Ne electron configuration while the Cl ion has the Kr electron configuration.

Example 4 After ionization, why is the sodium ion smaller than the sodium atom? Solution After ionization to the Na+, the Na atom becomes smaller because the electron-to-proton ratio of the Na atom is decreased when the Na+ ion forms. Also, the outer third shell no longer exists once the 3s 1 electron is lost by the Na atom.

Example 5 After ionization, why is the chloride ion larger than the chlorine atom? Solution After ionization, the Cl- ion is larger because the electron-to-proton ratio of the chlorine atom is decreased by the ionization process.

Fattractive + FRepulsive = 0 Bonding forces As two atoms approach each other, two forces exist (each is function of separation distance: Attractive force (Depends on type of bonding). Repulsive force (created as atoms approach each other, outer electron shells overlap). At equilibrium: The two atoms will counteract any attractive or repulsive forces: Fattractive + FRepulsive = 0 Atomic centers become separated by r0 (equilibrium spacing).

Interionic Forces for an Ion Pair Fnet = Fattractive + Frepulsive

Interionic Forces for an Ion Pair

Example 6

Bonding energies (cont.) Bonding energy (Eo): Energy at equilibrium separation, i.e., energy required to separate the two atoms to an infinite separation (i.e., break them apart). Magnitude of bonding energy & shape of energy vs. interatomic separation curve depends on material & type of atomic bonding.

Interionic Energies for an Ion Pair Enet = Eattractive + Erepulsive

Example 7 Calculate the net potential energy for a K+Br- pair by using the b constant calculated from example 8. Assume n = 9.5.

Ion arrangements in Solid ionic bond is nondirectional in character. Ionic solid is governed by the geometric arrangement of ions Electrical Neutrality Ionic packing arrangements in (a) CsCl and (b) NaCl. 8 Cl- ions can pack around Cs:, but only 6 Cl- can pack around a Na+ ion.

Ion arrangements in Solid ionic bond is nondirectional in character. Ionic solid is governed by the geometric arrangement of ions Electrical Neutrality Ionic packing arrangements in (a) CsCl and (b) NaCl. 8 Cl- ions can pack around Cs:, but only 6 Cl- can pack around a Na+ ion.

Covalent Bonding Covalent bonding is a primary type of bonding which arises from the reduction in energy associated with the overlapping of half-filled orbitals of two atoms. In this bond, there is an electron exchange interaction. The covalent bond is a directional type of bond.

Covalent Bonding Occur between atoms with small differences in electronegativity & close to each other in the periodic table. Atoms share their outer s & p electrons with other. In a single covalent bond, each of two atoms contributes one electron to form an electron pair bond = energy small ~ more stable. In multiple electron-pair bonds can be formed by one atom with itself or other atoms.

Covalent bonding

Covalent Bonding (Cont.) Materials with covalent bonding: Electrically and thermally insulative. Covalently-bonded materials can be: Hard, High melting temperature. Weak, low melting temperature.

Covalent Bonding Covalent Bonding in the Hydrogen molecule

Covalent Bonding 2. Covalent Bonding in Other Diatomic Molecules

Covalent Bonding 2. Covalent Bonding by Carbon

Covalent Bonding Covalent Bonding by Carbon-containing Molecules Benzene

Partially ionic Partially covalent Few compounds exhibit pure ionic or pure covalent bonding. For a compound, the degree depends on relative positions of atoms in periodic table (i.e, electronegativity). Close elements bond covalently. Far elements bond ionically. (Distant entities need to sacrifice to get bonded !)

Metallic bonding Metallic bonding is a primary type of bonding involving the interaction of the valence electron or electrons of one atom with many surrounding atoms. This interaction leads to a reduction in energy of the system considered. The valence bonding electrons of these bonds are sometimes regarded as an “electron gas” bonding the positive ion cores (atoms less their valence electrons) of atoms. The metallic bond is nondirectional

Metallic bonding e.g., all metals Weak or strong Good conductors for electricity and heat (free electrons).

Metallic bonding Atomic arrangement in a metallic copper crystal. Each copper atom is coordinated with 12 other atoms, producing a crystal structure called face centered-cubic (fcc) structure. The atoms are bonded together by an “electron gas” of delocalized valence electron

Metallic bonding Two-dimensional schematic diagram of metallically bonded atoms. The circles with the inner positive signs represent positive-ion cores, The charge clouds around the ion cores represent the dispersed valence electrons

Metallic bonding

SECONDARY BONDING Formed as a result of the interaction of the electric dipoles contained in atoms or molecules Can be divided by: (1) Fluctuating Dipoles (2) Permanent Dipoles

Secondary bonding (Van der Waal) Exists between all atoms or molecules Electric dipole exists whenever there is some separation of positive and negative portions of an atom or molecule. Evident for inert gases & molecules covalently bonded. Arises from atomic or molecular dipoles. Special case: Hydrogen bonding.

Fluctuating Dipoles Fluctuating dipole bonding is a secondary type of bonding between atoms which contain electric dipoles. These electric dipoles, formed due to the asymmetrical electron charge distribution within the atoms, change in both direction and magnitude with time. This type of bond is electrostatic in nature, very weak and nondirectional.

Fluctuating Dipoles Electron charge cloud distribution in a noble-gas atom Idealized symmetrical electron charge cloud distribution Real case with symmetrical electron charge cloud distribution that changes with time, creating a Fluctuating electric dipoles

Permanent Dipoles Permanent dipole bonding is also a secondary type of bonding between molecules possessing permanent electric dipoles. The bonds, formed by the electrostatic attraction of the dipoles, are directional in nature.

Permanent Dipoles Hydrogen bonding among water molecules due to permanent dipole attraction Permanent dipole nature of the water molecule

Secondary bonding Arises from interaction between dipoles • Fluctuating dipoles • Permanent dipoles-molecule induced -general case: -ex: liquid HCl -ex: polymer

SUMMARY: PRIMARY BONDS Ceramics Large bond energy large Tm large E small a (Ionic & covalent bonding): Metals Variable bond energy moderate Tm moderate E moderate a (Metallic bonding): Polymers Directional Properties Secondary bonding dominates small T small E large a (Covalent & Secondary):

SUMMARY: BONDING Type Bond Energy Comments Ionic Large Nondirectional (ceramics) Covalent Variable large-Diamond small-Bismuth Directional (semiconductors, ceramics, polymer chains) Metallic large-Tungsten small-Mercury Nondirectional (metals) Secondary smallest inter-chain (polymer) inter-molecular

Mixed Bonding Ionic-covalent eg; GaAs Metallic-covalent eg; transition metal Metallic-Ionic eg; intermetallic – NaZn13

Example 8 Explain the following types of primary bonding: (a) ionic, (b) covalent, and (c) metallic. -refer lecture note 2) Explain the following types of secondary bonding: (a) fluctuating dipole, and (b) permanent dipole.

Example 9 Describe the hybridization process for the formation of four equivalent sp3 hybrid orbitals in carbon during covalent bonding. Use orbital diagrams. Solution

Example 10 Describe the hybridization process for the formation of four equivalent sp3 hybrid orbitals in carbon during covalent bonding. Use orbital diagrams. Solution

Example 11 Why is diamond such a hard material? Solution Diamond is extremely hard because its carbon atoms are covalently bonded by single sp3 hybrid bonds in a three dimensional arrangement.

Example 12 How can the high electrical and thermal conductivities of metals be explained by the “electron gas” model of metallic bonding? Ductility? Solution The high electrical and thermal conductivities of metals are explained by the mobility of their outer valence electrons in the presence of an electrical potential or thermal gradient. The ductility of metals is explained by the bonding “electron gas” which enables atoms to pass over each other during deformation, without severing their bonds.

Example 13 Why is diamond such a hard material? Solution Diamond is extremely hard because its carbon atoms are covalently bonded by single sp3 hybrid bonds in a three dimensional arrangement.

Example 14 Describe the covalent bonding process between a pair of hydrogen atoms. What is the driving energy for the formation of a diatomic molecule? Solution The covalent bonding in the hydrogen molecule involves the interaction and overlapping of the 1s orbitals of the hydrogen atoms. The covalent bond forms between the two hydrogen atoms because their energies are lowered by the bonding process.