NEET Study Notes for Structure of Atom: In 1808, John Dalton proposed that atoms are the smallest particle of matter. Atoms consist of three subatomic particles that are proton, neutron, and electron. The structure of an atom comprises a nucleus which is the center of the atom in which the positively charged protons and neutral charge neutrons are present. Electrons (negatively charged) revolve around the center of the nucleus.

• Structure of Atoms is a part of Physical Chemistry for NEET 2022 Syllabus. With a weightage of 32% from Physical chemistry, at least 1 question is expected in NEET 2022 from Structure of Atom.
• Atoms are the building block of matter and the basis of everything that surrounds us. This makes it essential to understand how the concept of an atom gives rise to topics like isotopes, isobars, dual nature of matter and light, de Broglie's relationship, Heisenberg principle.

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NEET Study Notes: Important Topics in Structure of Atom

NEET Study Notes for Structure of Atom

Amongst the several theories around Atom, the first theory of Atom was John Daltons Atomic Theory which suggested that all the matter is made up of indivisible and indestructible atoms. The following points come into existence from John Dalton's theory:

• All matter is made up of atoms.
• Atoms of an element were the same. However, the atoms of different elements differ from each other in terms of size and mass.
• Atoms rearrange themselves during a chemical reaction.
• Atoms cannot be created or destroyed but transformed from one form to another.

Sub-Atomic Particles

There are three subatomic particles of Atoms- Protons, Neutrons, and Electrons

Protons

• Protons are positively charged subatomic particles. Proton carries positive charge 1e which is equal to approximately 1.602 × 10^–19 coulombs or 4.803 × 10^–10 esu.
• Mass of Proton is approximately e 1.672 × 10^–24 g or 1.672 × 10^–27 kg or 1.00728 amu.
• Protons are over 1800 times heavier than an electron.
• The total number of protons in the atom of an element is always equal to the atomic number of the element.
• The volume of Proton is approximately 1.5 × 10^–38 cm³.

Neutrons

• Neutrons are neutral particles that carry no charge.
• Different isotopes of an element carry the same number of protons but the number of neutrons present in the respective nuclei varies.
• Mass of neutron is 1.675 × 10^–24 gm or 1.675 × 10^–27 kg or 1.00866 amu. It is heavier by proton by 0.18%.

Electrons

• Electrons were discovered through the study of Cathode Rays. The name of electrons was proposed by Stoney.
• The charge of an electron is -1e which is approximately close to –1.602 × 10^–19 coulombs or 4.803 × 10^–10 e.s.u.
• Mass of an electron is approximately 9.11 × 10^–28 g.
• The mass of an electron is relatively negligible, this is why it is ignored for calculating the mass of atom

Isotopes and Isobars

Isotopes are the atoms of an element that have the same atomic number but a different mass number. Eg- ₈O¹⁶, ₈O¹⁷ and ₈O¹⁸.

Isobars- When atoms of different elements have the same mass number but different atomic number, these are called isobars. Eg- ₁₉K⁴⁰, ₂₀Ca⁴⁰.

Read NEET Study Notes for Chemical Bonding

What is Atomic Number and Mass Number?

Atomic Number

The number of protons present in nucleus is called atomic number of an element. Atomic number is also known as a nuclear charge. For neutral atom- Number pf Proton= Number of electrosn , For a charged atom Number of Protons- Charge on Atom

Mass Number

The sum of neutrons and protons is known as the mass number of an element. Mass number of an atom is always a whole number.

Bohr’s Atomic Theory

Neil Boh’s Atomic theory was put forth in 1915. The postulates of Bohr’s Atomic Theory are:

• Electrons revolve around the nucleus in defined circular orbits. These orbits are called energy shells. These shells can be represented by quantum numbers.
• The energy of Electrons in a particular orbit is constant.
• Electrons can jump to a higher state by absorbing energy This is called excited state whereas electrons move to a lower energy state by losing energy. This is called the ground state.
• No energy is absorbed or lost when electrons stay in their own stationery. The energy of stationary orbits is quantized

Check:

• NEET Study Notes for Chemical Equilibrium

• NEET Study Notes for Dual Nature of Matter and Radiation

NEET Study Notes for Electronic Configuration

The way elements are distributed in atomic orbitals of an element is known as electronic configuration. The electronic configuration follows a standard method in which all electron that contains atomic subshells are placed in a sequence.

How does Electronic Configuration help?

Electronic configuration help in the following ways:

• Determination of valency of an element.
• Predicting properties of elements with similar electronic configurations.
• Interpretation of atomic spectra

Shells

Maximum number of electrons that can be accommodated in a shell is based on n (principal quantum number). It can be represented by the formula 2n² where ‘n’ is the shell number. Shells, values of ‘n’ and the total number of electrons are tabulated below:

Subshells

• The subshells into which electrons can be distributed is based on the azimuthal quantum number (I).
• Quantum number is dependent on the value of principal quantum number (n). When the value of n is 4, four subshells are possible.
• A maximum number of electrons that can be accommodated in a subshell can be determined by 2I+1

What are Orbitals?

Orbitals are defined as the space around the nucleus where the probability of the existence of an electron is maximum.

• Even at infinity, Probability does not become zero. It is given by .
• Atomic orbitals are electron orbitals in atoms while those in molecules are called molecular orbitals.
• Orbitals have definite energy and momentum is quantifiable.

Read NEET Study Notes for Basic Concepts of Chemistry

Quantum Numbers

Quantum numbers help in providing information about an electron in an atom. There are four quantum numbers (n.I.m,s)

• Principal Quantum Number- Principle Quantum Number, denoted by n determine the energy and average distance of electron. Whole number values are denoted by K, L, M, N. When Principle Quantum Number (n) increases, distance from electron and energy also increases.
• Azimuthal Quantum number- Azimuthal Quantum Number (I). The electrons which can be distributed in subshells are also determined by I. It determines the angular momentum of electrons as well as the shape of orbitals.

Pauli’s Exclusion Principle

Pauli’s Exclusion Number states that no two electrons in an atom have the same values for all four quantum numbers.

• It also states that a maximum of two electrons can be accommodated in an orbital. The maximum number of electrons in a shell can be 2n2
• Maximum number of electrons in a subshell s,p,d,f can be 2,6,10,14 respectively whereas the maximum number of electrons in an orbital is 2.

Aufbau’s Rule

As per Aufbau’s rule electrons is added to an orbital in increasing order of energies. This order of increasing energies is 1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,5p,6s,4f,5d,6p,7s.

• The energy of an orbital is calculated by the sum of a principal number and azimuthal quantum number (n+I). The lower n value is filled first.
• The energy of atomic orbitals for H- atoms depend only on the value of n.

1s < 2s = 2p < 3s = 3p = 3d < 4s = 4p = 4d = 4f

Hund's Rule of Maximum Multiplicity

As per Hun’s Rule of Maximum Multiplicity, electrons do not pair in a subshell until all orbitals of subshell are singly occupied.

• This arrangement gives rise to lower energy levels.
• To maximize the total spin that gives rise to lower energies, electrons in an orbital that have one electron should have the same spins.

Read NEET Study Notes for Carbonyl Compounds

Heisenberg's Uncertainty Principle

As per Heisenberg's Uncertainty Principle, it is not possible to determine the position and momentum of small moving subatomic particles simultaneously

Mathematically, 

Where, X= uncertainty in position

p = uncertainty in momentum,

h = Planck’s constant

The uncertainty decreases, when the mass of particle increases.

Check NEET Study Notes for Dual Nature of Matter and Radiation

Broglie Principle

Broglie Principle proposes that radiations have a particle nature and the material particles also have a wave nature.

De Broglie wavelength is h=

Planck’s constant m = mass of object

; v= velocity . This is known as the de Broglie Equation.

NEET Sample Questions for Structure of Atom

Question: Who proposed the first atomic theory?

1. Rutherford
2. John Dalton
3. Neils Bohr
4. J.J. Thompson

Answer: John Dalton was the first to propose the Atomic Theory. John Dalton proposed that all matter is composed of small particles called atoms. These atoms cannot be created or destroyed. Atoms of different elements vary in mass.

Question: Which element has the largest atoms?

1. Li
2. H
3. O
4. F

Answer: Li( Lithium) element has the largest atom. The largest atom is determined by atomic radius or the distance between the nucleus to the outermost orbit. The atomic radius decreases as we move from left to right in the periodic table. However, the atomic radius increases as we go down in the periodic table. Lithium has the largest atom greater than Hydrogen, Oxygen, and Fluorine.

Question: What is the element with atomic numbers 35, 53, and 85?

1. Alkaline
2. Noble gases
3. Halides
4. Halogens

Answer: Halogens

Halogens belong to Group 17 in the periodic table. The element name, symbol, and the atomic number of elements is tabulated below:

Question: What charge do electrons carry?

1. Variable
2. Positive
3. Negative
4. Zero

Answer: Negative charge

Electrons carry a negative charge Electrons revolve around the nucleus of atoms in shells. The properties of an atom largely depend on the number of electrons in an atom. The charge of an electron –1.602×10-19 Coulumb. Number of filled electron shells increases as we move down in the periodic table.

Question: How can we determine the mass number of an atom?

1. Number of protons+ Number of electrons
2. Number of neutrons+ Number of electrons
3. Number of protons+number of neutrons
4. None of these

Answer: Number of Protons+ Number of neutrons

Question: What determines the atomic number of an atom?

1. Number of Protons
2. Number of electrons
3. Number of protons and neutrons
4. Number of electrons and protons

Answer: Number of protons present in an atom determine the atomic number.

Question: What are atoms with same mass number and different atomic number called/

1. Isotopes
2. Isotones
3. Isobars
4. Isomers

Answer: Isotopes are atoms with same mass number and different atomic number.

Question: Electronic configuration for oxygen is written as 1s2 2s2 2p4. Which of the following rule is violated in this configuration?

1. Aufbau’s Principle
2. Hund’s Rule
3. Pauli’s exclusion principle
4. None of the above

Answer: Pauli’s exclusion rule is violated here which states that no two electrons in the same atom can have the same values for all four quantum numbers. In other words, it is not possible to accommodate more than two electrons in an orbital.

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