Atomic structure

Subject classification: this is a physics resource .

Atomic theory

An Atom is a small part of element that takes part in chemical reactions. It is made up of three subatomic structures called Protons, Neutrons, and Electrons.

Subatomic structures:

    __________________________________ 
    Particle:               Charge:              Mass:            
    Proton                     +1                1 *u  
    Neutron                     0                1 *u
    Electron                   -1              1 / 1836*u

The Nucleus, in the center of atom, consist of protons and neutrons. Orbiting around the nucleus are the electrons.

Structure of an atom

The Nucleus, in the center of atom, consist of protons and neutrons. Orbiting around the nucleus are the electrons.

Each unique element has an Atomic Number equal to the number of Protons it contains. There are 92 naturally occuring Elements (1-92) and others which have been artificially created (93+..)

Each element also has an Atomic Weight for the most commonly found Isotope. Atomic Weight = number of Protons + number of Neutrons.

See: The Periodic Table

In a stable/ uncharged Atom the number of Electrons will equal the number of Protons. If the number of Electrons is changed the Atom will become Ionized and gain either a positive (fewer Electrons) or negative (greater Electrons) charge.

Isotopes

The same element can exist in different forms, each form having the same atomic number, but different mass numbers. These forms are called isotopes. Isotopes that cannot decay during a defined period are called stable isotopes. And isotopes that can decay during a defined period are called unstable (or radioactive) isotopes. For example :

        1               2                        3   
        1H             1H                       1H                                                     
        Protium.        Deuterium.        Tritium.

Tritium is an unstable isotope of hydrogen.

Bohrs model

Electrons are considered to move around the nucleus in fixed shells (orbits), at various energy levels. These levels may be designated K L M N ........ or 1 2 3 4 ........... . The first level can only contain 2 electrons. The second level can hold 8 electrons.

When the electrons are excited, then they can transfer between the shells. As we move away from the nucleus, the energy levels increase.

Excited state of atom

The configuration of electrons occupying the least amount of space (Bohrs model) is called the ground state. But when electrons are excited (by getting electricity, heat), they jump to a higher level.

This condition of the atom is called an excited state. But when the electrons return to the ground state, they give off energy.

Levels and sublevels

Some energy levels contain sublevels known as s p d f. The number of sublevels depends on the number of levels.

Orbital

The regions around a nucleus where the probability of finding an electron of a particular energy level is highest,are called orbitals.
An Orbital can hold only two electrons. They are spinning opposite ways. They are called orbital pairs.'n' represents principal quantum number or simply the number of the shell. So nth shell contains n^2 number of orbitals.And hence 2*n^2 number of electrons.
The shape of orbital depends of sublevel.

Valence

Valence electrons revolve in valence shells.
Electrons, that can enter to the reaction are only at the last level.
These electrons are called, valence electrons. The maximum number

of valence electrons is 8. The Valence determinate, how many electrons,does the atom have to borrow or to lend. All parts of atoms except last orbit are called kernel.

The electron dot formula, represents the valence electrons.

Examples:         . .
                 . N .   The dotted formula of the nitrogen.
                   .

Ionization energy

Ionization energy is the amount of energy needed to remove an electron from a gaseous atom or ion. Ionization energy depends the number of protons in the nucleus and the shielding (screening) of the inner electrons.