Tài liệu Types of Atomic & Molecular Bonds pptx

Objectives of the class Develop a sense of history of scientific thinking Explain the types of bonds You should be able to have develop a “feeling” for the mechanical properties based

Typescof Atomic &

Molecular-Bonds

Objectives of the class

Develop a sense of history of scientific thinking

Explain the types of bonds You should be able to have develop a “feeling” for the mechanical properties based on the type of bond

Greek philosopher Leucippus used the word

atom, which means indivisible (400 BC) Lots of work done by the alchemists

Lavoisier: chemical reaction leads to no net

loss or gain of matter

Dalton: concept of atomic mass (1808)

Mendelev: periodic table of elements (1869)

Moseley: showed that the number of electrons

= number of protons (atomic number)

Wilhelm Conrad Roentgen

first Nobel prize in physics (1901)

Ruterford: experiments with alpha

particle (1907-11)

Max Planck: quanta (1900)

Louis de Broglie: all matter has both

wave and particle properties (1923).

Primary Atomic Bonds

Covalent Bonds Metallic Bonds

Secondary Atomic & Molecular

Bonds

Permanent Dipole Bonds Fluctuating Dipole Bonds

lonic Bonding

Large interatomic forces are created by the

“coulombic” effect produced by positively

and negatively charged ions

lonic bonds are “nondirectionafl’

The “cation” has a + charge & the “anion” has

Historical Note

lonic Bonding in Solids

Because the ionic bond is

nondirectional the ions pack

together in a solid in ways

which are governed by their

relative sizes

Another important factor is that

the ions must be arranged so

that their is local charge

neutrality [Note the structure

of NaCl.].

Covalent Bonding

Large interatomic forces are created by the sharing of electrons to form directional bonds

The atoms have small differences in electronegativity

& close to each other in the periodic table

The atoms share their outer s and p electrons so that each atom attains the noble-gas electron

ore

Covalent Bonding in Carbon

A carbon atom can form form sp°

orbitals directed symmetrically

toward the corners of a

tetrahedron [Note the examples

below |

>> sp” hybrid bonds

⁄ Positive ion cores

Valence electrons in the form

of electron charge clouds

It is convenient for many purposes to regard an atom

in a metal as having a definite size, which may be defined by the distance between its center and that

of its neighbor

This distance is that at which the various forces acting

on the atom are in equilibrium

In a metal, the forces can be considered as

— (a) the attractive forces between electrons &

positive ions,

— (b) the repulsion between the complete electron shells of the positive ions, &

— (c) the repulsion between the positive ions as a

result of their similar positive charges

Permanent Dipole Bonds

— Weak intermolecular bonds are formed between molecules

which possess permanent dipoles

— Adipole exists in a molecule if there is asymmetry in its electron density distribution

Fluctuating Dipole Bonds

— Weak electric dipole bonding can take place among atoms due to an instantaneous asymmetrical distribution of

electron densities around their nuclei

— This type of bonding is termed fluctuation since the

electron density is continuously chanaina

Van der Waals bonds

This approach can be called the “hard sphere" model

of an atom

The radius of an atom (or ion) determined for a

particular crystal structure is not a real

characteristic of that atom, because when the same

atom appears in different crystal structure it displays

different radii

The radius of an atom (or ion) can be determined for a

particular metal by using the dimensions of the unit

cell of the crystal structure it forms

If ionic forces are so strong, how

come does salt (NaCI) dissolves so

quickly in water???

Water Molecule

+