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# NCERT Class 9 Science Solution, Notes, Atoms and molecules

## NCERT Class 9  Atoms and molecules

NCERT Class 9 science subject

25.Rules for writing formula of compound:

• Formula of compound is given by writing side by side the symbols of constituent elements
• Symbol of the more metallic element is written first in formula
• Number of atoms of each of the constituent element present in the molecule is indicated by subscript
• When either of the ions or both the ions are polyatomic and their valency is more than 1, we enclose the polyatomic ions in brackets. No brackets are necessary if the valency(ies) of polyatomic ion(s) is(are) 1
• While writing formula of compound if the valency numbers have a highest common factor (H.C.F), divide the valency numbers by

H.C.F to get the simple ratio between the combining elements

NCERT Class 9

26.The simplest compounds, which are made up of two different elements are called binary compounds

27.Formula of a binary compound is written by criss crossing the valencies of elements present in a molecule of the compound

28.A chemical compound is always electrically neutral, so the positive and negative valencies or charges of the ions in the compound must add up to zero

1. The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is therefore the relative mass of molecule expressed in atomic mass units (u)

30.The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound

1. 1mole of any substance = 6.022 x 1023units
2. The mole is the amount of substance that contains the same number of particles (atoms/ ions/ molecules/ formula units etc.) as there are atoms in exactly 12 g of carbon-12
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