Atom | Chemistry, Definition, History, Structure, Types & Example |

The structure of an atom, theoretically consisting of a positively charged nucleus surrounded and neutralised by negatively charged electrons revolving in orbits at varying distances from the nucleus, the formation of the nucleus and the arrangement of different electrons with different chemical elements.

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Atomic Theory

The first definite theory about the structure of matter was put forward by John Dalton in 1890.

Main Postulates of Dalton's Atomic Theory

• All matter is made up of very small particles known as atoms.

• Atoms can neither be created nor destroyed, they are the smallest particles that take part in a chemical reaction.

• All the atoms of an element are the same and different elements have different ones.

• Atoms combine with each other in the ratio of smaller whole numbers.

Drawbacks of Dalton Atomic Theory

• It does not explain why atoms of different elements differs in their mass, valencies etc.

• It does not explain the existence of isotopes and isobars.

• It does not explain the reason for difference in atoms of different elements.

• Discovery of electron, proton and neutron discarded the individual nature of atom proposed by Dalton.

Electron (-1e⁰)

• Electron was discovered by JJ Thomson in 1897.

• Magnitude of charge on electron = 1.6 × 10-¹⁹ coulomb.

• Mass of electron = 9.1 × 10-²⁸ g or 9.1 × 10-³¹ kg.

• Which 1/1837 so of the mass of H-atom. It is represented by e- or -1e⁰

• When an electrical discharge at high voltage is passed through a gas at low pressure, a stream of rays emerge from the cathode surface. These are called cathode rays.

Proton (1P¹or 1H¹)

• The existence of positively charged particles in an atom was shown by E Goldstein in 1886.

• The magnitude of charge on proton is 1.6 × 10-¹⁹ coulomb.

• Mass of proton = 1.6 x 10-²⁷ kg.

• Which is equal to the mass of H atom.

• It is represented by 'P' or 1P¹.

Neutron (0n¹)

• The neutrons were discovered by James Chadwick.

• These particles are neutral having charge 0.

• Their mass is nearly same as that of a hydrogen atom.

• So, its mass = 1.675 x 10-²⁷ kg.

• It is represented by 'n' or 0n¹.

Atomic Number

The concept of atomic number was given by Moseley. He also observed that when a metal is bombarded with high-speed electrons, X-rays are emitted.

Atomic number = Number of protons in the nucleus

                 = Number of electrons

Mass Number

The sum of number of protons and neutrons present in the nucleus is called mass number and is a whole number.

Mass number (A)

= Number of protons + Number of neutrons

= Number of electrons + Number of neutrons

= Atomic number + Number of neutrons

Isotopes

• Isotopes were discovered by F Soddy.

• Isotopes have the same atomic number but differ in their mass number.

• Isotopes have similar chemical properties but differ in physical properties, Example, 1H¹ (Protium), 1H² (Deuterium) and 1H³ (Tritium), 8O¹⁶, 8O¹⁷, and 8O¹⁸.

Isobars

• Isobars are atoms with the same mass number but different atomic numbers.

• Isobars are the atoms of different elements and they possess different physical and chemical properties, e.g., 1H¹ and 2H³, 180Ar⁴⁰, 19K⁴⁰ and 20Ca⁴⁰, 52Te¹³⁰ and 56Ba¹³⁰.

Isotones

• Isotones are the atoms of different elements possessing the same number of neutrons and have different mass numbers, e.g., 1H³ and 2He⁴, 15P³¹ and 16S³², 19K³⁹ and 20Ca⁴⁰.

Arrangements of Electron and Protons in an Atom

Thomson's Atomic Model

The first simple model of the atom was proposed by JJ Thomson. According to this model, an atom has a uniform region (radius approximately 10-¹⁰ m) of positive charge with electrons embedded in such a way as to give the most stable electronic arrangement. This model had failed.

Rutherford's Atomic Model (1912)

The α-particle scattering experiments conducted by Rutherford are related to the discovery of nuclei.

The following conclusions were drawn from these experiments

A major portion of atom is empty space because most ot α-particles passed straight through the atom.

An atom has a very small, rigid, positively charged body called a nucleus.  Due to this positively charged part α-particles are repelled from the metal foil.

The entire mass of an atom is concentrated in the nucleus, so it is very heavy and hard.

Main Postulates

• An atom mainly or largely consists of free space. Each atom contain a heavy positively charged body at its centre called the nucleus.

• The negatively charged electrons are revolving around the nucleus and are called planetary electrons.

• Rutherford atomic model failed because he could not explain the existence ot revolving electrons.

Bohr's Atomic Model (1913)

Niels Bohr proposed this atomic model which is based upon the Planck's quantum theory of radiations.

The electrons in an atom move around the nucleus in only selected circular paths called orbits. Each orbit has a certain amount of energy.

Electron in the particular orbit, does not lose or gain energy.

Only those orbits are permitted in which the angular momentum ot the electron is a whole number multiple of h/2π ⇒ ('h' is Planck's constant).

Energy is emitted when an electron jumps from a higher energy level to a lower energy level and energy is absorbed when an electron jumps from a lower energy level to a higher energy level.

Failures of Bohr's Atomic Theory

• He could not explain the effect of magnetic fields in the spectrum of atoms or ions.

• He could not explain the line spectra of atoms with more than one electron, called a multi-electron atom.

• He could not explain the effect of electric field on the spectra of atoms.

• Bohr's theory failed due to the dual nature of matter and uncertainly principle.

Electronic Configuration of Elements

The arrangement of electrons in different shells of an atom is known as electronic configuration of the element.

Following rules are used to write the electronic configuration

Aufbau's Principle

According to this principle, "the sub-shells are filled with electrons in increasing order of their energy." That is, the lower energy subshells will be filled with electrons first.

• The order of increasing energies is summed as 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5d, 6d, 7p

• Note that Cr²⁴ and Cu²⁹ do not obey this law

Cr = 1s², 2s², 2p⁶, 3s², 3p⁶, 3d⁵, 4s¹

Cu = 1s², 2s², 2p⁶, 3s², 3p⁶, 3d¹⁰, 4s¹ 

Hund's Rule of Maximum Multiplicity

According to this rule, "Pairing of electrons in a sub-shell starts after all the available atomic orbitals or the sub-shell are singly filled (half filled)"

Pauli's Exclusion Principle

It states that, "No two electrons can have the same value of the four quantum numbers".

Knowledge Update

• Exactly half filled subshells have less energy and thus assume greater stability than any other arrangement. Thus, p³ is more stable arrangement than p², p⁴ or p⁵.

Iso Electronic Species

Species having same number of electrons are known as iso-electronic, e.g, NH3 or H3O^+ are iso-electronic.

Radioactivity

Radioactivity can be defined as the spontaneous emission of suitable particle and electromagnetic radiations by unstable nuclei.

It was discovered by Prof. Henre Becquerel in 1896.

The three rays emitted from radioactive substance are

Alpha Rays (α-rays)

• They are deflected towards negative plate in electric field.

• Each particle carry two units positive charge and four unit, mass i.e., it is doubly ionised Helium atom.

• They produce maximum ionisation in gases because of maximum kinetic energy.
• Their penetration power of very small because of large mass.

• Velocity of these rays is 1/10 of the velocity of light.

Beta Rays (β-rays)

• They are deflected towards positive plate in electric field.

• β-particles carry one unit negative charge and negligible mass that is electron.

• They produce lesser ionisation in gases than α-particle.

• Emission of one β-particle, from unstable nucleus results in the increase in the number of protons by one unit.

Gamma Rays (γ-rays)

• γ-rays are electromagnetic radiations of very high frequencies even more than X-rays. Thus, the γ-rays have no charge and no mass.

• They are not deflected from their path in electric or magnetic field.

• They produce minimum ionisation in gases.

• They possess maximum penetration power.

Nuclear Stability

Due to the very small size of the nucleus, there are very high repulsive forces between protons. The presence of neutrons in the nucleus reduces these forces. Therefore, the stability of the nucleus depends on the ratio of n and p, i.e., n/p. For stable nucleus, n/p should be 1.0 - 1.6.

Theory of Radioactive Disintegration

During disintegration of an atom, either an α-particle or a β-particle is emitted. 

α-Emission  During α-emission from a nucleus, its mass number decreases by 4 and charge number by 2, i.e., 

β-Emission  During β-Emission from a nucleus, its mass number remains same while atomic number increases by 1, e.g.,

γ-Emission  During γ--emission, mass number and atomic number remain same.

Half-Life Period (t1/2)  The time during which half the number of atoms present initially in the element decay is known as half life period.

N = N0 × (1/2)^n  and  T = t1/2 × n

Where,       N0 = Initial amount

                    N = Remaining amount

                    T = Total time

                    n = Number of half lives

                    t1/2 = 0.693/k

Where,  k = decay constant

Average Life Period = T = 1/k = t1/2/0.693 = 1.44 t1/2 

Artificial Transmutation

The transformation of a stable nucleus into another nucleus by pre-bombing it with suitable high energy particles is called artificial transformation, e.g., -------

Methods of Obtaining Nuclear Energy

Nuclear Fission

It is the phenomenon of the splitting of a heavy nucleus into two or more lighter nuclei with the evolution of a large amount of energy, for example,

92U²³⁵ + 0n¹  →  56Ba¹³⁹ + 36Kr⁹⁴ + 30n¹ + E

Two types of chain reactions occur in nuclear fission.

1. Un-controlled Chain Reaction  Atom bomb is based on it. 
2. Controlled Chain Reaction  These reactions occur in nuclear reactors.

In nuclear reactors

• U-235 and Pu²³⁹, are used as fuels.

• D2O or graphite is used as moderator.

• Cd or B rods absorb neutrons. These are control rods.

• Liquid sodium is used as coolant.

Nuclear Fusion

It is the phenomenon of fusion of two or more lighter nuclei to form a heavier nucleus that contains a large amount of energy, Example, the energy of the hydrogen bomb and sum.

1H² + 1H²  →   2He⁴ + 17.6 MeV

Uses of Radio Isotopes

1. CO⁶⁰ is used to cure brain tumours and cancer.
2. C¹⁴ is used in carbon dating.
3. I¹³¹ is used in the treatment of thyroid gland.
4. Na²⁴  is used to measure the speed of blood circulation.

Important TIPS

• Isotopes have same number of protons but different number of neutrons 

• Isotones have same number of neutrons. 

• Radioactivity does not change with temperature, pressure, etc. 

• Extranuclear part of atom contains electrons.

Half life period, t1/2 = 0.693/k (k = decay constant)

Average life = 1/k 1.44 × t1/2 

• Nuclear fusion is source of sun's energy. 

• The principle of the atomic bomb is based on nuclear fission and the hydrogen bomb is nuclear fusion.

• β-particle is strongest reducing agent. 

• Cu and Cr are exceptions to Aufbau's principle.

Energy of an electron in 

nth orbit (En) = -13.6Z²/n²  eV/atom

Radius of nth orbit (rn) = 5.53n²/Z Å

Velocity of e- in nth orbit = 2.18 × 108Z cm/s 

Z = Atomic number

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