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Atomic Number & Mass Number, Isotopes & Isobars | Chemistry Class 11 - NEET PDF Download

Atomic Number (Z)

The atomic number is the number of protons found in the nucleus of an atom of a given element. Since protons carry a positive charge, the atomic number also determines the positive charge of the nucleus. 

Atomic Number & Mass Number, Isotopes & Isobars | Chemistry Class 11 - NEET

  • It is represented with the letter ‘Z.’
  • All the atoms of a particular element have the same number of protons, and hence the same atomic number.
  • Atoms of different elements have different atomic numbers.

Example

  • Carbon: Atomic number Z=6. This means every carbon atom has 6 protons in its nucleus.
  • Oxygen: Atomic number Z=8. This means every oxygen atom has 8 protons in its nucleus.

Mass Number (A)

In chemistry, the mass number, also known as the atomic mass number, is a key concept that represents the total number of protons and neutrons in the nucleus of an atom.

  • It is represented using the letter ‘A.’
  • As both protons and neutrons are present in the nucleus of an atom, they are together called nucleons.
  • For example, an atom of carbon has 6 protons and 6 neutrons. Thus, its mass number is 12.
  • While the number of protons remains the same in all atoms of an element, the number of neutrons can vary. Thus, atoms of the same element can have different mass numbers, and these are called isotopes.
  • The weight of an electron is almost negligible. Thus, the atomic mass of an atom is almost the same as its mass number.

Representation of an Atom

Representation of an AtomRepresentation of an Atom

Example:

Example of Carbon AtomExample of Carbon Atom

Difference Between Atomic Number and Mass Number

Question for Atomic Number & Mass Number, Isotopes & Isobars
Try yourself:
What is the atomic number of an atom that has 11 protons in its nucleus?
View Solution

Different Types of Atomic Species

(i) Isotopes

Species with the same atomic number but a different mass number are called isotopes, e.g. 1H1, 1H2, 1H3.

Isotopes of HydrogenIsotopes of Hydrogen


Uses of Isotopes:

  1. It is used as uranium for the nuclear reactor
  2. It is used as cobalt for treating blood cancer.
  3. Some isotopes are used in the process of labelling for various studies. Like, isotopes of hydrogen, carbon, nitrogen etc. are used for the labelling process.
  4. Used for nuclear imaging of various tissues for detection of diseases. fluorine-18, gallium-67, etc. are used for this purpose.

(ii) Isobars

 Species with the same mass number but a different atomic number are called isobars, e.g. 18Ar4019K40, 20Ca40 all of them contain the same mass number 40 but different atomic number i.e 18, 19, and 20.

Example of IsobarsExample of Isobars

Differences between Isotopes and Isobars

Differences between Isobars and IsotopesDifferences between Isobars and Isotopes

Question for Atomic Number & Mass Number, Isotopes & Isobars
Try yourself:Which of the following best describes the difference between isotopes and isobars?
View Solution

(iii) Isotones 

Isotones are atoms or nuclei that have the same number of neutrons but different numbers of protons. This means that isotones belong to different elements but share a common neutron count. 

Atomic Number & Mass Number, Isotopes & Isobars | Chemistry Class 11 - NEET

Key Points about Isotones:

  1. Same Neutron Number: Isotones have the same neutron number (N).
  2. Different Proton Number: Isotones have different atomic numbers (ZZZ), meaning they are different elements.
  3. Example: Carbon-14 (14C6) and Nitrogen-15 (15N7) are isotones because both have 8 neutrons (14 - 6 = 8 and 15 - 7 = 8).

(iv) Isodiaphers 

Species with the same isotopic number are called isodiaphers, e.g. 19K39, 19F9.
Isotopic number = mass number − × [2 atomic number]

(v) Isoelectronic 

Isoelectronic species are atoms, ions, or molecules that have the same number of electrons. Despite having the same electron count, these species can have different atomic or molecular structures and compositions.

Key Points about Isoelectronic Species:

  1. Same Electron Number: Isoelectronic species have the same total number of electrons.
  2. Different Atomic or Molecular Identity: Isoelectronic species can be different elements, ions, or molecules.

Example: The fluoride ion (F -), the neon atom (Ne), and the sodium ion (Na^+Na+) are isoelectronic because each has 10 electrons.

Question for Atomic Number & Mass Number, Isotopes & Isobars
Try yourself:Which of the following pairs of atoms are isotones?
View Solution

Solved Questions


Q1: Isobars have the same number of ______?
(A) Protons
(B) Electrons
(C) Neutrons
(D) Nucleons

Solution: The answer is option 4 (Nucleons). The number of protons and neutrons alone will vary in isobars but the number of nucleons or the sum of protons and neutrons will always be the same.

Q2: Which of the following statements about Isobars is false?
(A) Their atomic numbers vary from each other.
(B) Chemically they are the same element but their forms are different.
(C) Physical properties can be similar to each other.
(D) They are different chemical elements having the same atomic mass.

Solution: Statement 2 is false. Isobars are different elements altogether.

Q3: If the atomic number of sodium is 11. Find out how many electrons and protons are present in a sodium atom.

Solution: We know that “Atomic Number = No. of Protons = No. of Electrons”
Thus, number of electrons=11 and number of protons =11

Q4: What is the atomic number of chlorine?
(A) 18
(B) 19
(C) 17
(D) 16

Solution: The correct answer is “C”. The atomic number of chlorine is 17 because the number of protons in a chlorine atom is 17.

Q5: Isotopes of a single element vary in the number of
(A) Neutrons
(B) Protons
(C) Electrons
(D) All of the above

Solution: The answer is "A" (Neutrons).
The number of protons will always constant in a single element. However, the number of neutrons can change. The number of neutrons varies in isotopes of an element but the number of protons always remains the same.

The document Atomic Number & Mass Number, Isotopes & Isobars | Chemistry Class 11 - NEET is a part of the NEET Course Chemistry Class 11.
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FAQs on Atomic Number & Mass Number, Isotopes & Isobars - Chemistry Class 11 - NEET

1. What is the difference between atomic number and mass number?
Ans. The atomic number (Z) of an atom represents the number of protons in its nucleus, while the mass number (A) represents the total number of protons and neutrons in the nucleus.
2. How is an atom represented in terms of its atomic number and mass number?
Ans. An atom is represented by its chemical symbol, with the atomic number written as a subscript on the left side of the symbol and the mass number written as a superscript on the right side of the symbol.
3. What are the different types of atomic species based on their atomic number and mass number?
Ans. The different types of atomic species include isotopes, which have the same atomic number but different mass numbers, and isobars, which have the same mass number but different atomic numbers.
4. Can two atoms have the same atomic number but different mass numbers?
Ans. Yes, two atoms can have the same atomic number but different mass numbers, leading to the formation of isotopes.
5. How are isotopes and isobars important in the study of atomic structure?
Ans. Isotopes and isobars play a crucial role in understanding the variations in atomic structure, such as differences in stability, chemical properties, and behavior in nuclear reactions.
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