atom

All material is made up of matter, and the fundamental unit of matter is the atom.

Atom is:

• The basic unit of matter.
• The smallest particle of an element, which may or may not exist on its own.
• Atom is further divisible into protons, electrons, and neutrons.

Basic Structure of an Atom:

• Atoms are built of sub-atomic particles - protons, electrons, and neutrons.
• Protons and neutrons are found in the nucleus. The nucleus is located at the centre of the atom.
• The electrons revolve around the nucleus in orbits.

P : protons, N : Neutrons, E : Electrons

Proton: Sub-atomic particle with a positive charge(+1) and unit mass(1). The proton is a positively charged particle that is located at the center of the atom in the nucleus of an atom. The hydrogen atom is unique in that it only has a single proton and no neutron in its nucleus. The number of protons in the nucleus of an atom, which is characteristic of a chemical element, determines its place in the periodic table.

Neutron: Sub-atomic particle with no charge(0) and unit mass(1). The neutron doesn’t have any charge. The number of neutrons affects the mass and the radioactivity of the atom.

Electron: Sub-atomic particle with a negative charge(-1) and negligible mass. Electrons are the smallest particles in an atom. They are attracted to the positive charge of the protons, which is why they orbit around the nucleus. Electrons are much smaller than neutrons and protons.

Forces in an atom

The components of an atom are held together by three forces. Protons and neutrons are held together by strong and weak nuclear forces.

Electrical attraction holds electrons and protons. While electrical repulsion repels protons away from each other, the attracting nuclear force is much stronger than electrical repulsion. The strong force that binds together protons and neutrons is 1038 times more powerful than gravity, but it acts over a very short range, so particles need to be very close to each other to feel its effect.

Atomic Number of an atom 

Atomic number of an element is equal to the number of protons in the atom of an element or equal to the number of electrons in the atom of an element.

Therefore, atoms are electrically neutral as the number of protons is equal to the number of electrons.
 

Atomic number = Number of protons = Number of electrons

   

Mass Number of an atom

As the mass of an electron is negligible, the mass of an atom is the sum of the mass of protons and neutrons present in the nucleus.

Mass number =  Number of protons + Number of neutrons

Let's understand this using a few examples.

Atom of Hydrogen: It is written as \(\large_1^1 H\). An atom of hydrogen has 1 proton, 1 electron, and 0 neutrons.

The Atomic number of Hydrogen atom is = p = e = 1

The Mass number of Hydrogen atom is = p + n = 1

Atom of Oxygen: It is written as  \(\large_{8}^{16} O\). It has 8 protons, 8 electrons, and 8 neutrons.
 

The Atomic number of Oxygen atom is = p = e = 8

The Mass number of Hydrogen atom is = p + n = 8 + 8 = 16
 

How electrons are distributed in these orbits?
Electrons revolve around the nucleus in an imaginary path called orbits or shells. The first shell is K (energy level 1, n = 1), the second shell is L (energy level 2, n= 2) and then M shell (n = 3) and so on. The number of electrons in each shell is determined using the below rule:

Maximum number of electrons in each shell = 2 × n²

• K shell = 2n² = 2×1 = 2 electrons
• L shell = 2n² = 2×2² = 2×4 = 8 electrons
• M shell = 2n² = 2×3² = 2×9 = 18 electrons
• Outer most shell cannot have more than 8 electrons.

Example:

1) Sodium Atom: The number of protons and electrons is 11 and the number of neutrons is 12. p = 11, e = 11, n = 12
Electronic configuration for Na atom is:

 
2) Nitrogen Atom: p = 7, e = 7, n = 7

Electronic configuration for Nitrogen atom is:
 

Atomic Weight [Relative Atomic Mass]

Relative Atomic mass or atomic weight of an atom is defined as the number of times one atom of an element is heavier than the \(^1/_{12}\) of an atom of carbon.

Isotopes

Isotopes are atoms of same element having same atomic number but different mass number. Example : Three naturally existing isotopes of hydrogen are Tritium \(\large_1^3 H\) : p = e = 1, n = 2
Deuterium \(\large_1^2 H\): p = e= 1, n = 1
Protium \(\large_1^1 H\) : p = e = 1, n = 0

Stable Electronic Configuration and Unstable Electronic configuration

An atom is said to have an unstable electronic configuration when

• Their valence/outermost shell is incomplete.
• They tend to attain stable electronic configuration by sharing or gaining/losing electrons.
  So all the above examples Sodium(\(Na\)), Nitrogen(N), Oxygen(O), Hydrogen(H) have an unstable electronic configuration.

Noble gases have a stable electronic configuration as their outer shell is complete. Example:
Helium(He)- Electronic configuration:  2
Neon(Ne) - Electronic configuration 2, 8

How does the unstable electronic configuration atom attain stability? 
They combine with other element atoms. Combining atoms redistribute their electrons so that each combining atom attains a stable configuration of the nearest inert gas ( check the nearest inert gas for Na and Cl). Let us  understand this using an example:

Na atom: Electronic Configuration: 2,8,1                               
(nearest inert gas is Ne, atomic number 10)                              
Cl atom: Electronic configuration : 2, 8, 7
(nearest inert gas is Ar, atomic number 18) 

The Sodium(Na) and Chlorine(Cl) atom combine to form Sodium Chloride(NaCl) compound:
Na atom loses one electron from the outer shell to attain stability[2, 8] and the Cl atom takes in this electron to complete its outer shell to gain stability[2,8,8]
 

Please note that it is very difficult to show the exact location of an electron as an electron has almost no mass and revolves around it at incredible speed. For this reason, electrons are often shown as negatively charged clouds around the nucleus. Orbitals show electrons in different energy states surrounding the nucleus. As we move further away from the nucleus, the energy level increases. The only electron in the highest energy state or outermost orbitals takes part in the chemical reaction, they are called valence electrons and they are involved in chemical bonding between atoms. 

Various theories exist to explain the nature of the atom.

Dalton's Atomic Theory (1808)	-Matter consists of small indivisible particles called atoms. -Atoms can neither be created nor destroyed. -Atoms combine with other atoms in whole-number ratios forming compounds or molecules.	Indivisible
Modern Atomic Theory (20thCentury)	- Atoms are divisible into sub-atomic particles called protons, electrons, and neutrons. - Atoms of the same element may not be alike in all respects. - Isotopes have been discovered which are atoms of the same element differing in properties.	Atom is divisible into protons, electrons, and neutrons