Material Science: Chemical Bonding

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  1. structure of atom
    1. Atomic number
    2. Atomic mass
    3. Isobar
    4. Isotopes
    5. Isotones
  2. Periodic table
    1. Rare earth metals
    2. Actinide series
  3. Chemical bondings
    1. Primary bonds
      1. Ionic bonding
      2. Covalent bonding
      3. Metallic bonding
    2. Secondary bonds
      1. Induced dipole bonding
      2. Polar dipole bonding
      3. Permanent bonding
    3. Anomalous expansion of water
  4. FAQs

1. Structure of Atom

Each atom consists of a very small nucleus composed of protons and neutrons, which is encircled by moving electrons.

Particles in an atom

Particle Proton Neutron Electron
Position in atom Inside of atomic nucleus Inside of atomic nucleus Revolves around the nucleus
Electric charge Positively charged with value  

1.60 X10-19 C

No charge, Electrically neutral Negatively charged with value

1.60 X10-19 C

Mass Same mass as neutron 9.11X10-27 kg. Same mass as proton 9.11X10-27 kg. Less than proton and neutron

9.11X10-31 kg.

Spin Inside nucleus Inside nucleus Around nucleus

Electron configuration

  • When all the electrons occupy the lowest possible energies in accord with the foregoing restrictions, an atom is said to be in its ground state.
  • The valence electrons are those that occupy the outermost shell.
  • These electrons are extremely important, because they participate in the bonding between atoms to form atomic and molecular aggregates.

1.1 Atomic number(Z)

Each chemical element is characterized by the number of protons in the nucleus, which is called as the atomic number (Z).

  • For an electrically neutral or complete atom, the atomic number also equals the number of electrons.
  • This atomic number ranges in integral units from 1 for hydrogen to 92 for uranium, the highest of the naturally occurring elements.

1.2 Atomic mass (A)

             Atomic mass = mass of protons mass of neutrons

  • Although the number of protons is the same for all atoms of a given element, the number of neutrons (N) may be variable.

1.3 Isobars

These are atoms of different elements having the same atomic mass but different atomic number.

  • Since their number of electrons is different, their chemical properties are different.
  • The light nuclei have unstable isobars.
  • Heavy nuclei have stable isobars and these occur in pairs.

How to calculate atomic weight if more than two atomic masses exists?

  • Atomic weight is the weighted average of the atomic masses of the atom’s naturally occurring isotopes.
  • Otherwise, atomic mass unit (amu) scale has been established where 1 amu is defined as 1/12th of the atomic mass of the most common isotope of carbon, carbon 12 C12  

1.4 Isotopes

Atoms of some elements have two or more different atomic masses, which are called isotopes.

  • They have the same atomic number because the number of protons inside their nuclei remains the same.
  • The difference in their mass number is due to the difference in their number of neutrons.
  • Isotopes are chemically same and physically different

Since they are neutral isotopes are elements having same number of electrons, which make them to possess identical chemical properties. Let us see some examples 1H11H21H3 are all isotopes of hydrogen.

1.5 Isotones

Isotones are elements having the same number of neutrons. Examples of isotones are Chlorine – 37 and Potassium – 39. Both have 20 neutrons in their nuclei.

2. The period table

All the elements have been classified according to electron configuration in the periodic table

How elements are arranged?

  • Elements are situated, with increasing atomic number, in seven horizontal rows called periods.
  • All elements arrayed in a given column or group have similar valence electron structures, as well as chemical and physical properties.
  • These properties change gradually, moving horizontally across each period and vertically down each column.

Periodic table(from callister)

Different groups:

  • Group 0, the rightmost group, are the inert gases, which have filled electron shells and stable electron configurations.
  • Group VIIA and VIA elements are one and two electrons deficient, respectively, from having stable structures.
  • The Group VIIA elements (F, Cl, Br, I, and At) are sometimes termed the halogens.
    • The term “halogen” means “salt-former” and compounds containing halogens are called “salts”.
  • The alkali and the alkaline earth metals (Li, Na, K, Be, Mg, Ca, etc.) are labeled as Groups IA and IIA, having, respectively, one and two electrons in excess of stable structures.
  • The elements in the three long periods, Groups IIIB through IIB, are termed the transition metals, which have partially filled d electron states and in some cases one or two electrons in the next higher energy shell.
  • Groups IIIA, IVA, and VA (B, Si, Ge, As, etc.) display characteristics that are intermediate between the metals and nonmetals by virtue of their valence electron structures.

Elements Properties:

  • As may be noted from the periodic table, most of the elements really come under the metal classification.
  • electropositive elements
    • capable of giving up their few valence electrons to become positively charged ions.
  • Electronegative elements
    • they readily accept electrons to form negatively charged ions
    • electronegativity increases in moving from left to right and from bottom to top.

2.1 Rare earth elements

A rare-earth elements are set of seventeen chemical elements, the fifteen lanthanides, as well as scandium and yttrium.

How name came?

  • Despite their name, rare-earth elements are relatively plentiful in Earth’s crust, with the exception of the radioactive promethium.
  • However, because of their geochemical properties, rare-earth elements are typically dispersed and not often found concentrated as rare-earth minerals in economically exploitable ore deposits.

Applications:

Neodymium

  • powerful magnets used in loudspeakers and computer hard drives to enable them to be smaller and more efficient.
  • Magnets containing neodymium are also used in green technologies such as the manufacture of wind turbines and hybrid cars.

Lanthanum

  • in camera and telescope lenses.
  • Compounds containing lanthanum in studio lighting and cinema projection.

Cerium

  • In catalytic converters in cars.

Praseodymium

  • Used to create strong metals for use in aircraft engines.
  • Also in Special sort of glass, used to make visors to protect welders and glassmakers.

Gadolinium

  • Used in X-ray and MRI scanning systems, and also in television screens.

Yttrium, terbium, europium

  • Making televisions and computer screens and other devices that have visual displays as they are used in making materials that give off different colours.
  • Europium is also used in making control rods in nuclear reactors.

2.2 Actinide series

It is a series of radioactive metallic elements in Group 3 of the periodic table

3. Chemical Bondings

In order to understand the why materials behave like they do and why they differ in properties, it is necessary that one should look at atomic level.

The study primarily concentrates on two issues:

  • what made the atoms to cluster together, and
  • how atoms are arranged.

Atoms are bound to each other by number of bonds.

Octet rule:

According to this, atoms can combine either by transfer of valence electrons from one atom to another(gaining or losing) or by sharing of valence electrons in order to have an octet(stable electronic arrangement) in their valence shells.

These inter-atomic bonds are primarily of two kinds

i. Primary bonds

a). Ionic bonding

Bonding Mechanism:

This bond exists between two atoms when one of the atoms is negative (has an extra electron) and another is positive (has lost an electron).

Properties:

  • Then there is a strong, direct Coulomb attraction.
  • Basically ionic bonds are non-directional[FAQ] in nature.

Example:

Take bonding in NaCl (rock salt).

Ionic bonding

  • In the molecule, there are more electrons around Cl, forming Cl and fewer electrons around Na, forming Na+.
  • Ionic bonds are the strongest bonds.
  • In real solids, ionic bonding is usually exists along with covalent bonding.

 

b). Covalent bond

In covalent bonding, stable electron configurations are assumed by the sharing of electrons between adjacent atoms.

Bonding mechanism:

  • Two atoms that are covalently bonded will each contribute at least one electron to the bond, and the shared electrons may be considered to  belong to both atoms.

Example:

Bonding in Cl2 molecule

  • Cl atom is having 17 electrons and outermost orbit with seven electrons but need one more electron to reach octet state(inert configuration of Argon).
  • Condition of other chlorine atom also same and these two atoms reach an agreement where each atom contribute an electron fro the pooling.
  • As in the figure these two pooled electrons are now belongs to both the atoms to achieve Argon configuration

Properties:

  • Typically, covalent bonds are very strong.
  • The covalent bond is directional
    • that is, it is between specific atoms and may exist only in the direction between one atom and another that participates in the electron sharing
  • Hardness of diamond
    • It is a result of the fact that each carbon atom is covalently bonded with four neighboring atoms
    • And each neighbor is bonded with an equal number of atoms to form a rigid three-dimensional structure.

c). Metallic Bonding

It is found in metals and their alloys.

Bonding Mechanism:

  • Metallic materials have one, two, or at most, three valence electrons(bond forming electrons).
  • With this model, these valence electrons are not bound to any particular atom in the solid and are more or less free to drift throughout the entire metal.
  • They may be thought of as belonging to the metal as a whole, or forming a “sea of electrons” or an “electron cloud.”  
  • The remaining non valence electrons and atomic nuclei form what are called ion cores, which possess a net positive charge equal in magnitude to the total valence electron charge per atom.

  • These free electrons act as a “glue” to hold the ion cores together.  

Properties:

  • The metallic bond is non directional in character.
  • Metallic bonding is found in the periodic table for Group IA and IIA elements and, in fact, for all elemental metals.
  • at room temperature metallically bonded materials are ductile and fail after certain amount of elongation.

Type of primary bondings between atoms

  • It is possible to have interatomic bonds that are partially ionic and partially covalent, and, in fact, very few compounds exhibit pure ionic or covalent bonding.

For a compound, the degree of either bond type depends on the relative positions of the constituent atoms in the periodic table.

Which bonded materials are electric conductors?

  • Metals are good conductors of both electricity and heat, as a consequence of their free electrons
  • By way of contrast, ionically and covalently bonded materials are typically electrical and thermal insulators, due to the absence of large numbers of free electrons.

ii. Secondary bonding or Van der waals bonding

These are also called as physical bonds that are weak compared to primary bonds. Bonding energies are also very less. Secondary bonding exists between virtually all atoms or molecules,

Secondary bonding forces arise from atomic or molecular dipoles.

What are Dipoles?

  • An electric dipole exists whenever there is some separation of positive and negative portions of an atom or molecule.
  • The bonding results from the coulombic attraction between the positive end of one dipole and the negative region of an adjacent one.

a). Fluctuating Induced Dipole Bonds

A dipole may be created or induced in an atom or molecule that is normally electrically symmetric

  • That means the overall spatial distribution of the electrons is symmetric with respect to the positively charged nucleus.

Bonding Mechanism:

Stable and Vibrating molecules

  • One of these dipoles can in turn produce a displacement of the electron distribution of an adjacent molecule or atom when it experience a vibrational motion.
  • This will induces the second one also to become a dipole that is then weakly attracted or bonded to the first, which is called as induced dipole bond.

Properties:

  • For the materials with induced dipole bonding, melting and boiling temperatures are extremely low.
  • These are the weakest of all possible intermolecular bonds.

Examples:

  • The liquefaction and, in some cases, the solidification of the inert gases and other electrically neutral and symmetric molecules such as H2 and Cl2 are realized because of this type of bonding.

b). Polar Molecule-Induced Dipole Bonds

Polar molecules:

  • Permanent dipole moments exist in some molecules by virtue of an asymmetrical arrangement of positively and negatively charged regions
  • Such molecules are termed as polar molecules.

Mechanism Example:

HCl(Hydrogen chloride molecule)

  • A permanent dipole moment arises from net positive and negative charges that are respectively associated with the hydrogen and chlorine ends of the HCl molecule.
  • Polar molecules can also induce dipoles in adjacent nonpolar molecules, and a bond will form as a result of attractive forces between the two molecules.

Properties:

  • The magnitude of this bond will be greater than for fluctuating induced dipoles.

c). Permanent Dipole Bonds

Van der Waals forces will also exist between adjacent polar molecules. The associated bonding energies are significantly greater than for bonds involving induced dipoles.

Mechanism example:

The strongest secondary bonding type, the hydrogen bond, is a special case of polar molecule bonding.

  • It occurs between molecules in which hydrogen is covalently bonded to fluorine (as in HF), oxygen (as in H2O), and nitrogen (as in NH3).
  • The single hydrogen electron is shared with the other atom thus forming a strong attractive force with the negative end of an adjacent molecule.

Properties:

  • The magnitude of the hydrogen bond is generally greater than that of the other types of secondary bonds.
  • Melting and boiling temperatures for hydrogen fluoride and water are abnormally high in light of their low molecular weights, as a consequence of hydrogen bonding.

iii. Water (Volume Expansion Upon Freezing)

Most substances experience an increase in density,up on freezing(i.e transforming from a liquid to a solid with cooling)

One exception is water, which exhibits the anomalous and familiar expansion upon freezing—approximately 9 volume percent expansion.

This behavior may be explained on the basis of hydrogen bonding.

H2O structure in Ice:

  • Each H2O molecule has two hydrogen atoms that can bond to oxygen atoms
  • Its single O atom can bond to two hydrogen atoms of other H2O molecules.

Thus, for solid ice, This is a relatively open structure—i.e., the molecules are not closely packed together—and, as a result, the density is comparatively low.

H2O structure in liquid:

This structure is partially destroyed, upon melting and water molecules become more closely packed together. This leads to an increase in density.

Applications of this phenomenon:

  • This phenomenon explains why icebergs float.
  • Why, in cold climates, it is necessary to add antifreeze to an automobile’s cooling system (to keep the engine block from cracking).

 

4. FAQs

Q: What is a directional and non-directional bond?

In directional bonding one bonding atom prefer specific orientations in space relative to one another bonding atom. As a result, molecules in which atoms are bonded covalently have definite shapes.

In non-directional bonding charge is uniform in all direction and because of this an ion is surrounded by other ion from all the direction . This generally happen in case of ionic bonding.

References

  1. William D. Callister, Jr.David G. Rethwisch: Materials Science and Engineering: An Introduction, Wiley publication, 2014
  2. NCERT , Class 11: Chemistry part-1

7 Comments

  1. Deepak Bhojwani says:

    Basic but most important thanks sir for sharing…

  2. Yacon Root says:

    Awesome Website. Really enjoyed reading.

  3. R K singh says:

    Water molecules align themselves in a magnetic field.TRUE OR FALSE.

  4. R K singh says:

    Please cover the topics”Electrical and magnetic properties of materials” under material science.

  5. Mandakini shyam says:

    Awsum work…it is so beautifully drafted …iam spell bound..thank you sir

  6. Anjani bhardwaj says:

    Molecular orbital theory is to be studied or not??

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