Atomic Structure

Objective Know the atomic structure Know the orbital electrons and its shells Know the forces that binds the electron

to the nucleus. Know the relationship between the

electron and the periodic table.

Nucleus of Atom The nucleus of an atom is very small.

Less than 0.001% of the volume of an atom is occupied by matter.

The nucleus containing the neutrons and protons of the atom contains most of its mass.

Atom is mostly empty space. The proton and neutron of the atom is in the nucleus which contains most of its mass.

OrbitsElectron orbits are grouped into

different shells. specifically, the number and arrangement

of electrons in the outermost shell determine the chemical behavior of an atom. The outermost shell is called valence shell or valence electron.

Orbital has two type of orbits or shell. Shell Subshell

Shell and Subshell Shell and Subshell are defined by the

quantum number, not by the distance of its electron from the nucleus or even their overall energy.

Shells are labeled as K, L, M, N, O, P and Q or 1, 2, 3, 4, 5, 6 and 7. Going from innermost shell to the outermost shell. Electron in outer shells have higher

average energy and travel further from the nucleus than those in the inner shells.

SubshellsEach shell is composed of subshells

labeled s, p, d, f and g.The maximum number of electrons in

shell 1 is 2 x 12 = 2 x 1 = 2.The maximum number of electrons in

shell 2 is 2 x 22 = 2 x 4 = 8.The maximum number of electrons in

shell 3 is 2 x 32 = 2 x 9 = 18.The maximum number of electrons in

shell 4 is 2 x 42 = 2 x 16 = 32.

Electron Arrangement The maximum number of electrons that can

exist in each shell increases with the distance of the shell from the nucleus. The electron limit per shell can be calculated

from the following formula, where (n) is the shell number.Electron limit per shell = 2xn2

Physicist call the shell number (n) the principal quantum number (the shell number e.g. K,L,M,N,O,P and Q or 1,2,3,4,5,6 and 7). Every electron in every atom can be precisely identified by the principal quantum number.

Periodic table The number of the outermost occupied electron

shell of an atom is equal to its period in the periodic table.

The number of electron in the outermost shell is equal to its group. Oxygen (O) has 8 electrons.

8O – 2 K shell – 6 L shell

Means that Oxygen is in the second period and in the sixth group.

Example

Aluminum (13Al) has the following electron configuration:K shell = 2L shell = 8M shell = 3

Maximum number of electrons that can occupy each electron shell.

Shell Number Shell Symbol Number of Electrons

1 K 2

2 L 8

3 M 18

4 N 32

5 O 50

6 P 72

7 Q 98

Orbital Names Orbital are given names in the form of

X typey Where as X is the energy level corresponding to

the principal quantum number n(it is the shell symbol of an electron or orbits in layman's term).

type is a lower-case letter denoting the shape or subshell of the orbital ( s, p, d and f subshell).

y is the number of electron in that orbital. Example: 1s2 which means that the orbital has

two electrons and is the lowest energy level (n=1) with a subshell “s”.

Atomic structure Examples Hydrogen (H)

1 p, 1 e K shell 1

Helium (He) 2 p, 2 e, 2 n

K shell 2

Lithium (Li) 3 p, 3 e, 4 n

K shell 2, L shell 1

Periodic table All atoms having one electron in the outer shell lie in group I

of the periodic table. Atoms with two electrons in the outer shell falls in group II etc.

Atoms with eight electrons in its outer shell are in group VIII and are very stable chemically.

No outermost shell can contain more than 8 electrons. The orderly scheme of atomic progression from smallest to

largest atom is interrupted in the fourth period. Instead of adding atom to the next outer shell, electrons are added to an inner shell. Atoms associated with this phenomenon are called

transitional elements or transitional metals. Transitional elements tends to have high tensile strength,

density and melting and boiling point. Due to the d orbital electrons ability to delocalize within the metal lattice.

Force that binds the electron to the nucleus

The closer an electron is to the nucleus, the more tightly it is bound.

The attraction of electron and protons are due to the law of electricity, which states that opposite charges attract each other.

The opposing forces cause electron to maintain their distance from the nucleus, and thus electrons travel in a circular or elliptical path.

Electron binding energyK shell is the closes to the nucleus which

in turn has a high electron binding energy. While Q shell is the farthest which has low electron binding energy.

The complexity of the electron configuration of atoms increases as one progresses through the periodic table to the most complex naturally occurring element.

Ionized Atoms If an atom has an extra electron or has

an electron removed, it is ionized. An ionized atom is not electrically neutral but carries a charge equal in magnitude to the difference between the number of electrons and protons.

Atoms cannot be ionized by addition or subtraction of protons, since that changes the atom from one element to another.