Structure of an Atom

Atomic number
Atomic number (z) is the number of protons in an atom; it is also equal to the number of electrons in the atom.
Atomic number = Number of protons 
Example: The atomic number of an element is 12 then its atom contains 12 protons and 12 electrons.

Mass Number
The mass number (A) is defined as the sum of the number of protons and neutrons present in the nucleus of an atom. Example: Mass number of Nitrogen atom is 14 then it contains 7 protons and 7 neutrons.
Mass number = Protons + Neutrons

Notation of element
_{Atomic number} Element ^{Mass number}  ; _Z X^A
Example: Nitrogen atom has notation as  ₇ N ¹⁴ .
Question: An atom X has mass number 40 and atomic number 18.Find out the number of protons, number of electrons and number of neutrons present in the atom X?
Solution: 
Number of protons:
Number of protons = Atomic number
                               = 18
Number of electrons:
In an atom number of electrons = Number of protons
                                                    = 18
Number of neutrons = Mass number – Atomic number
                                 = 40-18 = 22

Electronic configuration
Electrons are placed in fixed energy levels called shells. These shells are also called orbits. These orbits or shells are represented by the letters K, L, M, N,… or the numbers n=1, 2, 3, 4,….
The arrangement of electrons in the shells is known as electronic configuration.

Rules for accommodating electrons in various shells (Bohr-bury scheme)
The maximum number of electrons that can be accommodated in any energy level of the atom is given by a formula 2n². Where n is the number of that energy level.
According to Bohr’s model of an atom, the first energy level K can have 2 electrons, the maximum number of electrons which can be accommodated in shell L is 2X2²  = 8.
Similarly, for M and N shells, maximum number of electrons that can be accommodated are 2X3²= 18 and 2X4²= 32 respectively.
Electrons in an atom do not occupy a new shell unless all inner shells like K, L, M, N….. are completely filled. Hence, filling of electrons in an atom follows a stepwise approach.
The maximum number of electrons that can be filled in a valence shell is eight even if it has capacity to accommodate more than eight.

After a series of experiments and a detailed study by scientists like louis de broglie, schrodinger, somerfeld and others proved that shells or energy levels have sub shells within them. Electrons are distributed in these sub-shells.

The various sub shells are s, p, d and f.
Every sub-shell can accommodate a fixed number of electrons.
"s" sub shell can hold a maximum of 2 electrons,
"p" sub shell can hold a maximum of 6 electrons,
"d" sub shell can hold a maximum of 10 electrons,
"f" sub shell can hold a maximum of 14 electrons.

Structure of Sodium atom
Sodium has atomic number 11 and mass number 23.
The nucleus of sodium has 11protons and 12 neutrons and it is surrounded by 11 electrons.
The electronic configuration of sodium: 
K shell or I shell = 2 electrons
L- shell or II shell = 8 electrons
M-Shell or III shell = 1 electron
∴ Electronic configuration of sodium atom = 2, 8, 1



Valence electrons
The electrons, which are present in the outermost shell of an atom are called valence electrons.
Example: Sodium.
Atomic number of sodium is 11
Electronic configuration of sodium is 2 8 1
In sodium 3^rd shell is the outermost shell (valence shell) .In this shell it has 1 electron. Hence the number of valence electrons present in sodium is 1.

The chemical properties of an atom are dependent on these valence electrons. Since they are ones that are participate in a chemical reaction.
Elements having valence electrons 1, 2 or 3 are called metals.
Exception: Hydrogen has 1 valence electron but it is not consider as a metal.
These elements lose electrons easily and form a positively charged ion called cation.
Example: Na - e^- → Na⁺
Elements having valence electrons 4, 5, 6 or 7 are called as Non-metals
These elements gain electrons and forms a negatively charged ion called anion.
Example: F + e^- → F^-
Elements with same number of valence electrons will have similar chemical properties. Whereas the elements with different valence electrons will have different chemical properties.

Octet configuration
An element with 8 electrons in an outermost-shell is said to possess an octet.
Elements with octet configuration are stable.
Example: Except helium all other noble gases possess octet configuration.

Valency
The combining capacity of the atoms to form molecules either with same or different elements is defined as valency.
Atom contains less than four electrons in its outermost shell, the valency of an atom is equal to the number of electrons present in the valence shell.
Example: Sodium has one electron in its outermost shell, so the valency of sodium is 1.
Calcium has two electrons in its outermost shell, so the valency of calcium is 2.
Aluminum has three electrons in its outermost shell, so the valency of aluminum is 3.
If the outer shell has more than four electrons, the valency = 8 - the number of electrons in the outer shell.

Atomic mass:  The total number of protons and neutrons present in one atom of an element is called atomic mass.
Helium has two protons and two neutrons.
Mass number of he = 2 + 2=4
Mass of He atom 2 + 2 =4u (unified mass).

Isotopes:  Atoms of the same element with the same atomic number but different mass numbers are called isotopes.
Examples: ₁H¹, ₁H², ₁H³ are the isotopes of hydrogen,₆ C¹² ;₆C¹³  are isotopes of carbon.
The chemical properties of all the isotopes of an element are the same. This is due to the presence of same number of electrons.

Uses of isotopes:
An isotope of uranium is used as fuel in a nuclear reactor.
An isotope of cobalt is used in the treatment of cancer.
For treating goitre, an isotope of iodine is used.

Isobars:  Atoms of different elements with different atomic numbers but have the same mass number are called isobars.
Example: ₁₈ Ar⁴⁰, ₁₉ K⁴⁰ and ₂₀Ca⁴⁰.

Reasons for chemical activity of an atom: The chemical activity of an atom is dependent on the number valence electrons. An atom with complete octet configuration is chemically inert and it does not participate in chemical reactions.
Example: Noble gases.

The atoms of element with incomplete octet configuration are chemically active. These elements combine with others to attain stable electronic configuration (octet configuration).
Example 1: Hydrogen has one electron and it requires one more electron to attain the nearest inert gas configuration. To achieve this each hydrogen atom contributes an electron and form hydrogen molecule.
Example 2: Chlorine atom has 7 electrons in its valence shell and it requires 1 more electron to attain the stable octet configuration. To achieve this each chlorine atom contributes an electron and form chlorine molecule.