Atoms,Molecules and Bonding

Course: DiplomaSubject: Applied science(Chemistry)

Unit: I

Introduction Chemistry is the study of matter and study of the changes that matter undergoes.Pure Chemistry

Applied Chemistry

Technology

Branches Analytical Chemistry

Physical Chemistry

Biochemistry

Organic Chemistry

Inorganic Chemistry

Chemistry's presenceIn our surroundingsMedicineIndustryResearchEnvironmental Issues

Atoms..A microscopic small particle that could not be made any smaller and still behave as a chemical system. Atoms are the smallest particles that can exist . Atoms cannot be created or destroyed.

Atomic Structure

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ATOMIC PARTICLES:

Particle Mass

(g) Charge

(Coulombs) Charge (units)

Electron (e-) 9.1 x 10-28 -1.6 x 10-19 -1

Proton (p) 1.67 x 10-24 +1.6 x 10-19 +1

Neutron (n) 1.67 x 10-24 0 0

Atom

Proton Neutron Electron

ELECTRONS

Small negatively charged particle Orbit, circle, around the nucleusAtoms are neutralNumber of electrons = number of protons

Protons ..

small, positively charged particles reside in the nucleus along with the neutron make up most of the mass of the atom the number of protons is what defines the type of a particular atom.number of protons = number of electrons

Neutrons

small particles with no charge reside in the nucleus along with the proton, make up most of the mass of the atom

Molecules A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds.

H2

H2O NH3

CH4

A Diatomic molecule contains only two atoms H2, N2, O2, Br2, HCl, CO

A Polyatomic molecule contains more than two atoms.

O3, H2O, NH3, CH4

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Examples of Molecules

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Ions..An ion is an atom, or group of atoms, that has a net positive or negative charge.

Ions

Cations

Anions

Cation – ion with a positive chargeIf a neutral atom loses one or more electronsit becomes a cation.

Na 11 protons11 electrons

Na+ 11 protons10 electrons

Anion – ion with a negative chargeIf a neutral atom gains one or more electronsit becomes an anion.

Cl 17 protons17 electrons

Cl- 17 protons18 electrons

A monatomic ion contains only one atom

Na+, Cl-, Ca2+, O2-, Al3+, N3-

A polyatomic ion contains more than one atom

OH-, CN-, NH4+, NO3

-

Ionic compounds

ionic compounds consist of a cation and an anion

• the formula is always the same as the empirical formula

• the sum of the charges on the cation and anion in each formula unit must equal zero

Example of Ionic compound

NaCl

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Chemical NomenclatureIonic Compounds

often a metal + nonmetal

anion (nonmetal), add “ide” to element name

BaCl2 barium chloride

K2O potassium oxide

Mg(OH)2

KNO3

magnesium hydroxide

potassium nitrate

Atomic number (Z) = number of protons in nucleus

Mass number (A) = number of protons + number of neutrons

= atomic number (Z) + number of neutrons

Isotopes are atoms of the same element (X) with different numbers of neutrons in the nucleus.XA

Z

H11 H (D)21 H (T)3

1

U23592 U238

92

Mass Number

Atomic NumberElement Symbol

Molecular FormulaMolecular Formula Definition: An expression which states the number and type of atoms present in a molecule of a substance.

Example: There are 6 C atoms and 14 H atoms in a hexane molecule, which has a molecular formula of C6H14.

Chemical Bond Molecules of chemical substances are made of two or more atoms joined togather by some force, acting between them. This force which results from the interaction between two or more atoms that go to form a stable molecule, is referred to as a chemical Bond.

A Chemical bond is defined as a force that acts between two or more atoms to hold them together as a stable molecule.

Valence The term valence or valency is often used to state that potential or capacity of an element to combine with other elements.

For an example in HCl ,one atom of chlorine is combined with one atom of hydrogen and the valency of chlorine is 1.

Valence or valency is the number of bonds formed by an atom in a molecule.

The valence electrons which actually involves in bond formation are called Bonding electrons.The remaining valence electrons are referred to as non bonding electrons.

Valence

BondingElectrons

Non bonding Electrons

Electronic theory of valenceG.N.Lewis and w.Kossel gave electronic theory of valence and explained why atoms join to form molecules.

Noble gas atoms had a stable electronic configuration and others have unstable or incomplete configuration.

It states that: In chemical bond formtion ,atoms interact by losing,gaining,or sharing of electrons so as to acquire a stable noble gas configuration.

Rule of Octet Atoms of noble gas possess a stable outer shell of 8 electrons or octet.Electronic theory of valence also termed as octet theory of valence/ Rule of octet.

It may be stated as :Atoms interact by electron sharing or electron transfer, so as to achieve the stable outer shell of eight electrons.It also known as Rule of eight.

Types of bonds Ionic Bond

Covalent Bond

Coordinate Bond

Hydrogen bond

Metallic Bond

The type of chemical bond developed between the two combining atoms depends upon the way these atoms acquire a stable noble gas configuration.

Elements may combine through any one of the following ways to form stable compounds.

i. By the transfer of electrons from the atom of an element to the atom or atoms of another. This gives rise to an ionic (or electrovalent) bond.

ii. By mutually sharing the electrons. This gives rise to a covalent bond.

iii. By one-sided sharing of electrons. This gives rise to a coordinate bond.

Ionic BondThis type of bond is established by transfer of an electron from one atom to another.In an ionic bond, one atom loses an electron to another atom, forming a cation and anion, respectively. And, they attract towards each other ,by electrostatic attraction they combined togather.An IONIC BOND is an electrostatic interaction that holds together a positively charged ion (cation) and a negatively charged ion (anion).

Formation of Ions

A has one electron in excess and B has one electron short than the stable octet.Therefore, A transfers an electron to B and in this transaction both atoms acquire stable electronic configuration. And they held togather by electrostatic attraction.

Examples of Ionic CompoundSodium Chloride:NaCl

In table salt, for example, a valence electron from a sodium atom is transferred to a chlorine atom, forming Na+ and Cl-. Because the ions have opposite charges, they are attracted to each other. The loss of a valence electron and the attraction to the atom that took it happen simultaneously.

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Covalent BondsA covalent bond is formed between two atoms (similar or dissimilar) by a mutual sharing of electrons. The shared pairs of electrons are counted towards the stability of both the participating atoms.

A covalent bond is defined as the force of attraction arising due to mutual sharing of electrons between the two atoms.

The combining atoms may share one, two or three pairs of electrons.

When the two atoms combine by mutual sharing of electrons, then each of the atoms acquires stable configuration of the nearest noble gas. The compounds formed due to covalent bonding are called covalent compounds.

CovalencyThe number of electrons which an atom contributes towards mutual sharing during the formation of a chemical bond is called its covalency in that compound. Thus, the covalency of hydrogen in H2 (H - H) is one; that of oxygen in O2 is two (O = O), and that of nitrogen in N2 is three (N º N).

Single Covalent Bond A covalent bond formed by mutual sharing of one pair of electrons is called a single covalent bond, or simply a single bond. A single covalent bond is represented by a small line (-) between the two atoms.

ExamplesFormation of ammonia (NH3)

The electronic configurations of nitrogen and hydrogen are           N           1s2 2s2 2p3           or           2,5           H           1s1                       or            1 Thus, each nitrogen atom requires three more electrons to acquire a stable noble gas configuration. On the other hand, each H-atom requires only one electron to achieve the stable helium configuration. This is done by mutually sharing three pairs of electrons between one nitrogen and three hydrogen atoms, as shown below.

MULTIPLE COVALENT BONDS:

The covalent bonds developed due to mutual sharing of more than one pairs of electrons are termed multiple covalent bonds. These are,

Double covalent bond. The bond formed between two atoms due to the sharing of two electron-pairs is called a double covalent bond or simply a double bond. It is denoted by two small horizontal lines (=) drawn between the two atoms, e.g., O = O, O = C = O etc.

Triple covalent bond. Bond formed due to the sharing of three electron-pairs is called a triple covalent bond or simply a triple bond. Three small horizontal lines between the two atoms denote a triple bond,

e.g., N º N, and H - C º C H (acetylene).

Comparison..

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Coordinate BondCoordinate bond is formed when the shared electron-pair is provided by one of the combining atoms. The atom which provides the electron-pair is termed as the donor atom, while the other atom which accepts it, is termed as the acceptor atom.

The bond formed when one-sided sharing of electrons take place is called a coordinate bond. Such a bond is also known as dative bond. A coordinate bond is represented by an arrow (®) pointing towards the acceptor atom.

Ammonium (NH4+) ion

During the formation of ammonium ion, nitrogen is the donor atom, while H+ is the acceptor ion as shown below.

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Polar and nonpolar covalent bondsPolar covalent Bond A covalent bond in which electrons are shared unequally and the bonded atoms acquire a partial positive and negative charge ,is called a polar covalent bond.

Nonpolar covalent bond A covalent bond in which electrons are equally distributed ,the + and – charge are equal proportion which makes atoms neutral ,is called nonpolar covalent bond.

Hydrogen BondingIf a hydrogen atom is bonded to a highly electronegative element such as fluorine, oxygen, nitrogen, then the shared pair of electrons lies more towards the electronegative element. This leads to a polarity in the bond in such a way that a slight positive charge gets developed on H-atom, viz.,

H+ d : O- d H+ d : F- d H+ d : N- d This positive charge on hydrogen can exert electrostatic attraction on the negatively charged electronegative atom of the same or the other molecule forming a bridge-like structure such as

Xd - - Hd+ × × × × × × Y -d - Hd+ where X and Y are the atoms of strongly electronegative elements. The bond between the hydrogen atom of one molecule and a more electronegative atom of the same or another molecule is called hydrogen bond.

ExampleHydrogen fluoride (HF).

Water (H2O).

Types of Hydrogen BondingH-Bonding

Intermolecular H-bonding

Interamolecular H-bonding

(a) Intermolecular hydrogen bonding

When the hydrogen bonding is between the H-atom of one molecule and an atom of the electronegative element of another molecule, it is termed as intermolecular hydrogen bonding. For example, hydrogen bonding in water, ammonia etc., is intermolecular hydrogen bonding.

(b) Intramolecular hydrogen bonding

The intramolecular hydrogen bonding is between the hydrogen of one functional group, and the electronegative atom of the adjacent functional group in the same molecule. For example, the molecule of o-nitrophenol, shows intramolecular hydrogen bonding. The p-nitrophenol shows intermolecular hydrogen bonding.

Metallic BondThe peculiar type of bonding which holds the atoms togather in metal crystal is called The Metallic Bonding.

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Vanderwals bondThe relatively weak attractive forces that act on neutral atoms and that arise because of the electric polarization induced in each of the particles by the presence of other particles.

Inter molecular: between molecules (not a bond)

 

Intra molecular: bonds within molecules(stronger)

Types of Van der Waals Forces

1)    dipole-dipole

2)  dipole-induced dipole

3) dispersion

1-Dipole-Dipole-Two polar molecules align so that d+ and d- are matched (electrostatic attraction)

Ex: ethane (C2H6) vs. fluromethane (CH3F)

Fluoromethane (CH3F)

H H

H C F H C F

H H

d- d-+d +d

Ethane (C2H6)

H H H H

H C C H H C C H

H H H H

Dipole-Dipole

NOT Dipole-Dipole

2-Dipole-Induced DipoleA dipole can induce (cause) a temporary dipole to form in a non-polar molecule

The molecules then line up to match d+ and d- charges.

Example:

H Cld+ -d Are-

e-e-

e-

e-

e-

e-

e- e-

e-

e- e-

e-

e-

e-

e-e-

e-

A DIPOLE (it’s polar)

non-polarINDUCEDDIPOLE

-d+d

Dipole – Induced Dipole (weak and short-lived)

3-Dispersion ForcesA temporary dipole forms in a non-polar molecule…

which leads to…a temporary dipole to form in ANOTHER non-polar molecule

Dispersion is the ONLY intermolecular attraction that occurs between non-polar molecules

Example:

Cl-Cle-

e-

e-

e-

e-

e-

e- e-e-

e-

e-e-

e-

e-e-

e-e-

e-

non-polarINDUCEDDIPOLE

-d+d

TEMPORARY DIPOLEnon-polar

Cl-Cle-

e-e-

e-

e-

e-e- e-

e-

e-

e-e-

e-e-

e-

e- e--d+d

Dispersion (weakest and very short-lived)

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References1. Essentials of Physical chemistry by Bahl &

Tuli2. Elements of Physical chemistry by Glass ton

& Lewis3.http://www.howstuffworks.com4.http://www.wpclipart.com/energy/atom/

atomic_structure.png.html5.http://www.en.wikipedia.org6. http://meddic.jp/metallic_bond