Atoms and Molecules Class 9 Notes

Atoms and Molecules Class 9 Notes – The Laws of Chemical Combination, also known as the Laws of Definite Proportions and Multiple Proportions, describe the relationships between the elements in a chemical compound. There are three laws:

  1. Law of Constant Composition (Law of Definite Proportions): A given compound always contains the same elements combined in the same proportions by mass. For example, water always contains hydrogen and oxygen in a ratio of 2:1, regardless of the source of the water.
  2. Law of Multiple Proportions: When two elements combine to form different compounds, the ratio of their masses is always in small, whole-number multiples. This law was proposed by John Dalton and is based on the idea that atoms combine in fixed ratios to form compounds. For example, carbon and oxygen can form two compounds: CO and CO2. The ratio of the mass of oxygen in CO to that in CO2 is 1:2.
  3. Law of Conservation of Mass: The mass of a system before a chemical or physical change is equal to the mass of the system after the change, assuming that there are no external factors such as the addition or removal of mass.

Atoms and Molecules Class 9 Notes

What is Atom?

Atoms are the basic building blocks of matter and the defining structure of elements. They consist of a nucleus made up of protons and neutrons, surrounded by electrons in shells. The number of protons in the nucleus determines an element’s atomic number and therefore its identity. Atoms can bond with other atoms to form molecules, which are the building blocks of compounds. The properties of a substance are determined by the type and arrangement of its atoms, and chemical reactions involve changes in the bonds between atoms.

Dalton’s Atomic Theory

Dalton’s Atomic Theory is a set of postulates proposed by John Dalton in the early 19th century that describe the behavior of atoms and their role in chemical reactions. The main postulates of Dalton’s Atomic Theory are:

  1. All matter is made up of atoms: Atoms are the basic building blocks of matter and cannot be divided into smaller units.
  2. Atoms of a given element are identical: Atoms of the same element have the same mass and chemical properties.
  3. Atoms of different elements have different masses and chemical properties: The mass and chemical properties of an atom determine the element to which it belongs.
  4. Atoms are indivisible: Atoms cannot be divided into smaller parts.
  5. Atoms are indestructible: Atoms cannot be created or destroyed in chemical reactions, only rearranged.
  6. Atoms combine in simple whole-number ratios to form compounds: Compounds are formed by the combination of atoms in whole-number ratios, such as 1:1, 2:1, or 3:2.
daltonss atomic theory
Modern Day Symbols of Atoms of Different Elements 
Symbols of atoms
Modern Day Symbols of Atoms of Different Elements
symbol of element
Different Elements Symbol
Atomic Mass

An indicator of how heavy an atom of a particular element is is its atomic mass. It combines the masses of all the minuscule atom-forming particles (protons, neutrons, and electrons). Atomic mass is used by scientists to compare the masses of various elements and to comprehend the properties of matter. It is an average of the weights of all the various versions (isotopes) of an element and is typically stated in quantities called atomic mass units (amu).

Atomic masses of a few elements

ElementAtomic Mass (u)
Atomic masses of a few elements

What is Molecule?

A molecule is the smallest unit of a substance that retains the chemical properties of that substance, made up of two or more atoms held together by chemical bonds. The type of atoms and their bonding arrangement determine the properties of a molecule.

Molecules can be simple, for example molecule of hydrogen gas (H2) consists of two hydrogen atoms, or molecule of glucose (C6H12O6), which consists of six carbon atoms, twelve hydrogen atoms, and six oxygen atoms. The molecule of oxygen is O2 whereas the molecule of Ozone is O3 although both are made of oxygen.

Molecules of Elements

When a group of atoms of the same element that are chemically bonded together is known as molecules. For example, a molecule of oxygen (O2) contains two oxygen atoms bonded together, while a molecule of nitrogen (N2) contains two nitrogen atoms bonded together.

Elements can also form molecules with other elements to create compounds. For example, a molecule of water (H2O) contains two hydrogen atoms and one oxygen atom chemically bonded together.

What is Atomicity

Atomicity is a property of chemical substances that refers to the smallest particle of the substance that still retains all of its chemical properties. It is defined as the number of atoms in a molecule of a chemical substance.

For example, the atomicity of a molecule of hydrogen (H2) is 2, as it contains two hydrogen atoms. The atomicity of a molecule of oxygen (O2) is also 2, as it contains two oxygen atoms.

What is an ION?

When unequal number of protons and electrons are present in the atom or molecule and giving it a net positive or negative charge is known as ion. Ions can be either positively charged (called cations) or negatively charged (called anions).

The formation of ions occurs when an atom gains or loses one or more electrons. This can happen through chemical reactions or through interactions with external sources of energy, such as heat or radiation.

Some ionic compounds
Some ionic compounds

Writing Chemical Formulae

Chemical compounds are created by combining different elements that can be represented by their unique symbols. The valency of an element shows how many atoms of other elements it can connect with to create a compound, much like how arms can be used to grasp and connect with other objects.

To illustrate this, let’s say an octopus with eight arms wants to hold onto a group of people, each with two arms. Based on the number of arms available, the octopus can hold onto up to four people at once. We can use the symbols O and H to represent the octopus and humans, respectively, and the formula OH4 to show that the octopus is holding onto four humans.

To write a chemical formula, you need to follow these rules:

  1. Balance the charges: The charges of the ions in the compound must balance each other out. The total positive charge of the cations (positively charged ions) must be equal to the total negative charge of the anions (negatively charged ions).
  2. Metal first: When writing a formula for a compound that consists of a metal and a non-metal, write the symbol or name of the metal first.
  3. Polyatomic ions: If a compound contains a polyatomic ion, enclose the ion’s formula in brackets and write the number of ions outside the bracket. If there is only one polyatomic ion, the bracket is not required.

For example, to write the chemical formula for magnesium hydroxide, which contains the polyatomic ion hydroxide (OH)-:

  • Magnesium is a metal, so its symbol goes first: Mg
  • Hydroxide is a polyatomic ion, so its formula goes in brackets: (OH)-
  • To balance the charges, you need two hydroxide ions: Mg(OH)2

Another example is sodium sulfate, which contains the polyatomic ion sulfate (SO4)2-:

  • Sodium is a metal, so its symbol goes first: Na
  • Sulfate is a polyatomic ion, so its formula goes in brackets: (SO4)2-
  • To balance the charges, you need two sodium ions: Na2SO4

What is Valency?

The capacity of an element to combine with other elements to create chemical compounds is referred to as valency. It’s comparable to how many “hands” or “arms” one element has to extend and connect with other elements.

How many bonds an element may create with other elements depends on its valency. For instance, if an element has a valency of 4, it can construct a chemical complex by forming 4 bonds with other elements.

The elements show more than on valency are –

Iron shows valency II – Fe2+
and valency III – Fe3+
Copper shows valency I – Cu+
and valency II – Cu2+

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