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Valency, Isotopes - Structure of Atom : Notes, Class 9 Science PDF Download

Structure of The Atom

Valency

You have read that noble gases have fully filled outermost shell. Due to this, they are stable and they do not react with other elements. Other elements also tend to attain stable configuration by completing the octet in their outermost orbit. This is important to note that, the number of electrons in the outermost orbit of an element is closer to octet. An element can lose or gain electron in order to complete the octet. This tendency of losing or gaining electrons imparts valency to an element.

Let us take example of hydrogen. Hydrogen can readily lose or gain an electron. So, its valency is one. Now, let us take example of Hydrochloric Acid (HCl). One atom of chlorine combines with one atom of hydrogen to form hydrochloric acid. In this case, hydrogen loses one electron and thus gets +1 charge. On the other hand, chlorine gains an electron and thus gets – 1 charge. So, valency of hydrogen and chlorine are one.

Valency can be defined as combining capacity of an atom.

Example : –

Hydrogen molecule - Hydrogen has only one electron in its outermost orbit, thus it requires one more electrons to complete its outermost orbit. Therefore, in order to complete outermost orbit, hydrogen shares one electron with another hydrogen atom and form H(hydrogen molecule).

In the case of LiCl (Lithium chloride) - Lithium has three electrons in its outermost orbit and chlorine has seven electrons in its outermost orbit. Thus in order to make outermost orbit completely filled lithium loses one electrons and chlorine gains one electron. After losing one electron, lithium has two electrons in its outermost orbit and after gaining one electron, chlorine has eight electrons in its outermost orbit. And they form LiCl (Lithium chloride)

Name, Symbol, Atomic number, Number of electrons, Distribution of electrons in shells (electronic configuration) and Valency of some elements (From Hydrogen to Calcium)

Valency, Isotopes - Structure of Atom : Notes, Class 9 Science

Isotopes

Elements having same atomic number but different atomic masses are known as Isotopes.

Example –

Carbon-12, Carbon-13, Carbon-14 are thee isotopes of carbon atom. Here 12, 13 and 14 are the atomic masses of isotopes of carbon respectively. Since, atomic number is the unique property of an atom, thus the atomic number of carbon is 6 even in the case of three types of carbon (isotopes)

Hydrogen -1 , Deuterium – 2, Tritium -3 are three isotopes of hydrogen.

The isotopes of hydrogen are written as:

Valency, Isotopes - Structure of Atom : Notes, Class 9 Science

Use of Isotopes:

Carbon – 14 is used in carbon dating.

An isotope of uranium is used as fuel in nuclear reactor.

An isotope of cobalt is used in treatment of cancer.

An isotope of iodine is used in treatment of goitre.

Isobars:

Atoms having same atomic mass and different atomic numbers are known as Isobars.

Example –

Valency, Isotopes - Structure of Atom : Notes, Class 9 Science

Both the elements have same atomic mass equal to 40 but different atomic numbers, i.e. argon has atomic number equal to 18 and calcium has atomic number equal to 20.

 
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FAQs on Valency, Isotopes - Structure of Atom : Notes, Class 9 Science

1. What is valency in chemistry?
Ans. Valency is a measure of an atom's ability to form chemical bonds with other atoms. It refers to the number of electrons that an atom can gain, lose, or share in order to achieve a stable electron configuration. Valency determines the type and number of chemical bonds that an atom can form.
2. How is valency determined?
Ans. Valency is determined by the number of valence electrons in an atom. Valence electrons are the electrons present in the outermost shell of an atom. The valency of an element can be found by looking at its position in the periodic table. For main group elements, the valency is usually equal to the number of electrons needed to complete the octet (8 electrons) in the outermost shell.
3. What are isotopes?
Ans. Isotopes are variants of an element that have the same number of protons but different numbers of neutrons in their atomic nuclei. This means that isotopes of the same element have the same atomic number but different mass numbers. Due to the different number of neutrons, isotopes may have slightly different physical properties, but their chemical properties remain the same.
4. How are isotopes represented?
Ans. Isotopes are represented by the element's symbol along with the mass number. The mass number is the sum of the number of protons and neutrons in the nucleus of an atom. For example, carbon has three isotopes: carbon-12, carbon-13, and carbon-14. Carbon-12 has 6 protons and 6 neutrons, carbon-13 has 6 protons and 7 neutrons, and carbon-14 has 6 protons and 8 neutrons.
5. Why are isotopes important?
Ans. Isotopes have several important applications in various fields. In nuclear medicine, radioactive isotopes are used for diagnostic purposes and in cancer treatment. Isotopes also play a crucial role in radiocarbon dating, which is used to determine the age of ancient artifacts and fossils. Additionally, isotopes are used in scientific research, industrial processes, and even in agriculture for tracing nutrient uptake in plants.
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