Estructura atomica Resumen

8/13/2019 Estructura atomica Resumen

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Atomic Structure.

The structure of a material may be examined at four levels:

Atomic Structure.

Atomic Arrangement.

Microstructure.

Macrostructure.

Atomic Structure influences how atoms are bonded together, an understanding of which in turn

helps us to categorize materials as metals, semiconductors, ceramics, an polymers and permit us

to draw some general conclusions concerning the mechanical properties and physical behavior of

these four classes of materials.


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An atom is composed of a nucleus surrounded by electrons. The nucleus contains neutrons and

positively charged protons and carries a net positive charged. The negatively charged electrons are

held to the nucleus by an electrostatic attraction.

Electronic Structure of the Atom.

Electrons occupy discrete energy levels within the atom. Each electron possesses a particular

energy, with no more than two electrons in each atom, having the same energy. This also implies

that there is a discrete energy difference between any two energy levels.

Quantum Numbers. The energy level to which each electron belongs is determined by 4

quantum numbers. The number of possible energy level is determined by the first three quantum

numbers.

Principal (Main) Quantum Number n.

http://www.medciencia.com/primera-imagen-real-de-un-atomo-de-hidrogeno/


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Azimuthal Quantum Number l.It is determined by using the formula: l= n-1 for each quantum shell. And is designated by

lowercase letters:

Magnetic Quantum Number m.It is determined by using the formula: m= 2l+1

Spin Quantum Number ms.Is assigned values +1/2 and -1/2 to reflect the different spins. Also is based on Pauli Exclusion

Principle; this principle specifies that no more than two electrons, with opposing electronic

spins, may be presented in each orbital.

s forl

= 0 d forl

=2

p for l= 1 f for l


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Atomic Bonding.

The sum of all the Forces (attraction and repulsion) interacting in the atom must be in equilibrium,

in order to achieve this is required a bonding energy.

There are four important mechanisms by which atoms are bonded in solids.

1. Ionic Bond2. Covalent Bond3. Metallic Bond4. Van der Waals Bond

In three of the four mechanisms, bonding is achieved when the atoms fill their outer s and p levels.

These bonds are known as primary bondsand they are relatively strong between adjacent atoms.

Van der Waals bonds are secondary bonds, caused by weak electrostatic attraction, as a result of

this, the bond is relatively weak.

Ionic Bond.

It occurs when one atom donates its valence electrons to a different atom, filling the outer energy

shell of the second atom. Both atoms now have filled (or empty) outer energy levels, but both

have acquired an electrical charge an behave as ions (cation and anion). The oppositely charged

ions are then attracted to one another and produce the ionic bond.


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Covalent Bond.

Materials with covalent bond share electrons among two or more atoms. In order for the covalent

bonds to be formed, the atoms must be arranged so the bonds have a fixed directional

relationship with one other. Although covalent bonds are very strong, materials bonded in this

manner typically have poor ductility and poor electrical and thermal conductivity.

For example a silicon atom, which has a covalence of four, contains eight electrons in its outer

energy shell by sharing its electrons with four surrounding silicon atoms. Each instance of sharing

represents one covalent bond; thus, each silicon atom is bonded to four neighboring atoms by

four covalent bonds. Other examples are semiconductors and ceramics like glass and bricks.


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Metallic Bond.

This bond is found in metals and their alloys. Metallic elements have a low electronegativity; give

up their valence electrons to form a sea of electrons surrounding the atoms. The valence

electrons move freely within the electron sea and become associated with several atom cores. The

positively charged atom cores are held together by mutual attraction to the electron, thusproducing a strong metallic bond. Because their electrons are not fixed in any position, metals are

good electrical conductors at relative low temperatures.

Van der Waals Bonding.

These bonds join the molecules or groups of atoms by weak electrostatic attraction. The origin of

the Van der Waals Forces between atoms and molecules is of quantum mechanics nature. This

kind of bond is a secondary bond, like an auxiliary bond, and is formed because of inducted or

permanent dipolar moments inside atoms and molecules.

There are three kinds of interactions:

1. London Forces: interactions between two inducted dipoles. Ex. Carbon Tetrachloride.2. Debye Forces: When inducted dipole interacts with permanent dipolar molecule. Ex.

CarbonTetrachloride and water molecules.

3. Keesom Forces: Interactions between permanent polar molecules. Ex.Water molecules.Hydrogen bonding

Estructura atomica Resumen

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