Structure of the Atom Class 9 Notes CBSE Science Chapter 4 (Free PDF Download)

Summary of Structure of Atoms

• Atoms are the basic units of matter and the defining structure of elements.

• It consists of three basic particles, i.e., protons, electrons, and neutrons, that build the structure of an atom.

• Protons are positively charged particles and were founded by E. Goldstein.

• Electrons are negatively charged particles and were founded by J.J. Thomson.

• Neutrons have no charge and were founded by Chadwick.

• The nucleus is in the centre of the atom and contains protons and neutrons.

• The outer region of the atom which holds electrons in orbit around the nucleus is known as shell/energy level/orbits. 

• These shells are further divided into subshells.

• The electrons present in the outermost shell of an atom are known as the valence electrons.

• The electronic configuration of an element is the representation of the arrangement of electrons distributed among the orbital shells and subshells. The valence electrons are the determining factor for the unique chemistry of the element.

• The atomic number of an element is the same as the number of protons in the nucleus of its atom. As atoms are electrically neutral, an atom contains as many electrons as it has protons. The atomic number is denoted by Z. 

• The mass number of an atom is equal to the number of nucleons in its nucleus. Nucleons are the collective term for protons and neutrons. The mass number is denoted by A.

• In the notation of an atom, the atomic number is written as a subscript on the left of the element symbol and the mass number is written as a superscript on the left of the element symbol.

• Isotopes can be defined as elements that possess the same number of protons and electrons, but a different number of neutrons.

• For example, Protium, deuterium, and tritium are the isotopes of hydrogen. They each have one single proton Z=1 and a single electron but differ in the number of their neutrons. Hydrogen has no neutrons, deuterium has one, and tritium has two neutrons.

• Isobar is an element that differs in chemical property but has the same physical property which means isobars are those elements that have a different atomic number but the same mass number. 

• For example, Calcium and chlorine are isobars since both have a mass number of 40 but calcium has an atomic number of 20 and chlorine has an atomic number of 17.

• Isotones are elements that have the same number of neutrons but different atomic numbers.

• For example, Chlorine with atomic number 37 and potassium with atomic number 39 is isotones because both chlorine and potassium have the same number of neutrons i.e., 20.

Chapter 4 Science Class 9 Notes: Atomic Number and Mass Number

Chapter 4 Science Class 9 notes tell us that the total number of protons present in an element is called the atomic number while the mass number is both the number of protons plus the number of neutrons.

Atomic Number

According to Class 9 Chapter 4 Science notes, an atomic number is the number of the chemical elements present in the periodic system so that the elements are organised in order of the increasing number of protons present in the nucleus. Also, the total number of protons that are equal to the electrons in a neutral atom is called the atomic number. For example, iron has 26 protons in its nucleus; hence, the atomic number of the matter is 26.  An equal number of electrons and protons are presented in a neutral atom.

Mass Number

According to Class 9th Science Chapter 4 notes, the total number of protons and neutrons is combined together and forms an element’s mass number. The contribution of the mass from the electron is disdained and then calculated as the mass number. Hence, the approximation of the mass is used to calculate the number of neutrons that an element has by subtracting the total number of protons from the mass number. Both neutrons and protons weigh about 1 atomic mass unit or amu. Isotopes of this same element will have the same number of atoms but a different mass.

Class 9 Science Chapter 4 Notes: Details of Isotopes

Notes of Chapter 4 Science Class 9 also give us a brief description of isotopes. It is the variant of a specific chemical element that differs in its number of neutrons and also in the number of nucleons. The isotopes have the same number of protons and a different number of neutrons. Thus, they have the same atomic number but a different mass number. All elements are a mixture of isotopes. Each of them is a pure substance. They have the same chemical properties but different physical properties. If there is an absence of isotope in an element, then its mass would be the same as the total number of neutrons and protons. Chapter 4 Science Class 9 notes also tell us that if there is a presence of isotope in any element, then the percentage of all the isotope forms should be known and then the average mass should be calculated.

Science Class 9 Chapter 4 Notes: Details of Isobars

The nuclides of various chemical elements are called isobars. They have the same number of nucleons which means that they have the same mass number. But they differ in atomic number. Alfred Walter Stewart suggested the name “isobars”. It has been derived from the Greek word isos which means “equal” and bars, which means “weight”.

Class 9 Science Notes Chapter 4: A Summary of The Structure of An Atom

• An atom is made from the nucleus which is the centre of the atom and carries protons and neutrons and an outer region that holds the electron in its orbit around the nucleus. 

• All electrons have a negative charge which is equal to the positive charge of a proton.

• Neutrons are known as uncharged particles that are found in the nucleus.

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Conclusion

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