Editing Content :: Admin Scraper

Chapter

Content type

Position

Value

Journey Inside the Atom 153 8.8 Combining Capacity of an Atom: Valency You have learnt that atoms of the same or different elements can combine Grade 8 to form molecules. The number of atoms of hydrogen or chlorine with Curiosity which one atom of an element can combine to form a compound is Chapter 7 called its combining capacity. It is expressed in terms of hydrogen and chlorine because both possess a combining capacity of one. For example, in H 2 (water), oxygen combines with two hydrogen atoms, so the combining capacity of oxygen is two. In NH (ammonia) and MgCl 3 2 (magnesium chloride), what will be the combining capacities of nitrogen and magnesium respectively? These examples show how the combining capacity of elements is determined by the number of hydrogen or chlorine atoms they combine with. Let us learn how the combining capacity of an atom is decided by its electronic configuration. The outermost shell containing electrons of an atom is known as its valence shell. The electrons present in it are known as valence electrons. If the outermost shell of an atom has 8 electrons, it is called an octet. It has been observed that elements with complete octet of electrons (8 electrons), or 2 electrons in the case of helium in their valence shell are largely unreactive and more stable. On the other hand, atoms with incomplete valence shells are usually more reactive. Such elements lose, gain, or share electrons to complete their octet. The number of electrons gained, lost, or shared to complete the octet is called the valency of the element. Generally, if the element has fewer than four electrons in its valence shell, it tends to lose electrons to complete its octet and become stable. On the other hand, if the number of valence electronsismorethanfour,ittendstogainelectronstocompleteitsoctet.For example, the electronic configuration of sodium is 2, 8, 1. It can get an octet by losing one electron. Therefore, its valency or combining capacity is 1. On the other hand, oxygen has an electronic configuration of 2, 6. It has six valence electrons. Therefore, it can gain two electrons to attain an octet, its valency is 2. The electronic configuration of carbon is 2, 4. Carbon has four valence electrons and cannot easily gain or lose them. Therefore, it can share four electrons with other atoms to complete its octet. Thus, the valencyofcarbonis4.Somecompoundsappeartoviolatetheusualvalency Next rule, about which you will learn in higher grades. LUpel Can you predict what happens to the atoms that already have eight electrons in their outermost shell (except the elements with one shell only, where only two electrons are possible)? Will they still try to lose or gain electrons? Examine Table 8.4. Add one more column to it, and write down the common valency of each element. You have learnt that all the atoms of an element have the same number of electrons and protons, which is equal to its atomic number. Can you say the same regarding the number of neutrons too? Scientists have observed that atoms of the same element can have different numbers of neutrons. What effect does this difference have on the properties of the atom? Let us try to find out!