Atoms and Molecules

Atoms and Molecules - Class 9 Science

  • Laws of Chemical Combination
  • Dalton's Atomic Theory
  • Atom
  • Molecules
  • Atomicity
  • Molecular Mass
  • Ions
  • Valency
  • Mole Concept
  • Solved Questions on Atoms and Molecules
  • Laws of Chemical Combination

    The two fundamental laws of chemical combination are the Law of Conservation of Mass and the Law of Constant Proportion. These laws were formulated to describe the behaviour of matter in chemical reactions and the composition of compounds.
    These laws help us understand how tiny particles like atoms and molecules interact with each other. They give us important information about how these particles work together in the world of chemistry.

    Law of Conservation of Mass

    1. This law, also known as the Law of Conservation of Matter, states that mass cannot be created or destroyed in a chemical reaction. In other words, the total mass of the reactants in a chemical reaction is equal to the total mass of the products formed.
    2. This law is based on the principle that atoms are neither created nor destroyed during a chemical reaction; they are merely rearranged to form new compounds.
    3. The Law of Conservation of Mass is attributed to Antoine Lavoisier, a French chemist, and is considered one of the foundational principles of modern chemistry.

    Mathematically, the law can be expressed as:
    Mass of reactants = Mass of products

    For example, consider the reaction between hydrogen gas (H2) and oxygen gas (O2) to form water (H2O):
    2H2 + O2 → 2H2O

    According to the Law of Conservation of Mass, the total mass of hydrogen and oxygen on the left side of the equation must be equal to the total mass of water on the right side of the equation.

    Law of Constant Proportion (Law of Definite Proportion)

    1. This law states that in a chemical compound, the elements are always combined in fixed and definite proportions by mass. Regardless of the source or method of preparation, a pure compound will always contain the same elements in the same proportions by mass.
    2. This law was formulated based on observations made during experiments involving the analysis of various compounds.
    3. For example, consider water (H2O). No matter how water is produced, it will always have a fixed proportion of hydrogen and oxygen by mass (approximately 1:8).

    Dalton's Atomic Theory

    Dalton's Atomic Theory, formulated by the British chemist John Dalton in the early 19th century, is a fundamental concept in the field of chemistry. It consists of several key postulates that provide a framework for understanding the nature of matter and chemical reactions.

    Postulate of Dalton's Atomic Theory

    a) Matter is composed of indivisible particles called atoms: According to Dalton, all matter, whether it's a solid, liquid, gas, or compound, is ultimately made up of extremely small particles called atoms. These atoms are the building blocks of matter and cannot be further divided by chemical means.

    b) Atoms of the same element are identical: Dalton proposed that atoms of a given chemical element are identical in terms of their properties, such as size, mass, and chemical behaviour. This postulate suggests that each element has its own distinct type of atom.

    c) Atoms of different elements are different: Dalton's theory asserts that atoms of different elements vary in terms of their properties. This variation in properties is what distinguishes one element from another.

    d) Atoms combine in fixed ratios to form compounds: One of the key ideas of Dalton's theory is that atoms combine with each other in specific and predictable ratios to create compounds. For example, water (H2O) is composed of two hydrogen atoms and one oxygen atom, and this ratio is consistent for all water molecules.

    e) Conservation of mass in chemical reactions: Dalton's theory emphasizes that in a chemical reaction, the total mass of the reactants (the substances being combined) is equal to the total mass of the products (the new substances formed). This principle reflects the idea that atoms are neither created nor destroyed during a chemical reaction; they are rearranged to form new combinations.

    Drawbacks of Dalton's Atomic Theory

    a) Atom is no longer the smallest particle: With advancements in scientific research and the discovery of subatomic particles like electrons, protons, and neutrons, it became evident that atoms are not indivisible. They are composed of even smaller particles, which are the building blocks of atoms themselves.

    b) Masses of atoms can have variations: Atoms of the same element can have different masses due to variations in their composition, contrary to Dalton's theory. Additionally, atoms of different elements can sometimes have similar masses, challenging Dalton's idea that distinct elements have distinct masses.

    c) Substances made up of the same kind of atoms may have different properties: Dalton's theory suggested that substances made up of the same type of atoms should have identical properties. However, this notion was contradicted by the discovery of substances of the same element that exhibit distinct physical and chemical properties. For example, diamond and graphite are both composed of carbon atoms but have vastly different properties.


    1. Atoms are the fundamental units that make up all forms of matter. They serve as the basic building blocks from which everything around us is composed.
    2. The size of atoms is quantified using a measure known as the atomic radius. This radius is expressed in nanometers (nm), where 1 nm is equal to one billionth of a meter (10-9 m). This unit allows us to grasp the extremely small dimensions of atoms, highlighting their minuscule nature.
    3. Each atom consists of a nucleus at its centre, which is made up of positively charged protons and electrically neutral neutrons. Orbiting the nucleus are negatively charged electrons. The number of protons in the nucleus determines an atom's identity as a specific element. For example, an atom with 6 protons is always carbon, and an atom with 1 proton is always hydrogen.

    Symbols of Atoms

    1. Symbols of atoms serve as a universal language in the field of chemistry, enabling scientists and researchers from around the world to quickly recognise and refer to different elements without ambiguity.
    2. In the past, John Dalton proposed the following symbols for atoms.
    3. Symbols of Atoms - Symbols of Elements as Proposed by Dalton

    4. Nowadays, the International Union of Pure and Applied Chemistry (IUPAC) is responsible for approving element names and symbols to ensure consistency and accuracy in scientific communication.
    5. Symbols of atoms are shorthand representations used to denote elements in a concise and standardised manner. These symbols play a crucial role in scientific communication, making it easier to refer to elements without having to write out their full names. The symbols are derived from the element's name, and there are specific rules for their formatting:
    • First Letter Uppercase: The first letter of the symbol is always written in uppercase (capital letter). This helps to quickly identify the element.
    • Second Letter Lowercase: If the element's name consists of two letters, the second letter is written in lowercase. This convention differentiates elements with similar initial letters.
    • Examples: For example, the symbol for hydrogen is "H," for helium is "He," for carbon is "C," and so on. Many elements have symbols derived from their Latin names or their common names in various languages. Some examples are Sodium: Na (from the Latin word "natrium"), Tungsten: W (from the German word "wolfram"), Mercury: Hg (from the Latin word "hydrargyrum"), etc.
    • Symbols of Atoms - Science Grade 9

    Atomic Mass and Atomic Mass Unit

    1. Atomic mass refers to the mass of an individual atom of a specific chemical element. It is usually expressed in atomic mass units (amu) or unified atomic mass units (u).
    2. An atomic mass unit (amu) is a standardized unit of measurement used to express the relative masses of atoms and molecules.
    3. The atomic mass of an element is a measure of how heavy its atoms are compared to the mass of a single atom of carbon-12, which is assigned a mass of exactly 12 atomic mass units (amu). Other elements' atomic masses are measured in comparison to the mass of carbon-12. In other words, 1 amu is defined as exactly 1/12th of the mass of a carbon-12 atom.
    4. Formula of Atoms Mass Unit - Science Grade 9

    5. The concept of the atomic mass unit provides a convenient way to compare the masses of different atoms. For example, if the atomic mass of oxygen is 16 amu, it means that an oxygen atom is 16 times heavier than 1/12th of a carbon-12 atom.


    1. A molecule is the tiniest unit of a substance – whether it's an element or a compound – that can stand on its own and retain all the unique characteristics of that substance. In other words, a molecule embodies the distinct properties of the substance it represents.
    2. In compounds, which are substances made up of two or more different elements, molecules form as atoms of these various elements combine. This combination occurs through chemical bonds, which are strong forces holding the atoms together within the molecule. These bonds are responsible for the stability and properties of the molecule.
    3. Additionally, the arrangement of atoms in a molecule adheres to a precise ratio by mass, a fundamental principle known as the law of constant proportion. This law states that the elements within a compound always combine in fixed, unchanging proportions by mass. This adherence to a fixed ratio by mass is a defining characteristic of molecules, particularly those found in compounds.
    4. These bonds can be strong, like in water where two hydrogen atoms are bonded to one oxygen atom, or they can be weak, like in gases where atoms are loosely connected.
    5. Molecules are fascinating because they determine the characteristics and behaviour of matter. For example, the water molecule (H2O) is what makes water wet and able to dissolve other substances. The oxygen molecule (O2) is what we breathe to survive. Different arrangements of atoms create different molecules, and that's why we have such a wide variety of materials and chemicals around us.


    Atomicity refers to the number of atoms that are present in a single molecule of an element. In other words, it tells us how many atoms are tightly bound together to form the smallest unit of that element.



    Argon (Ar)

    Monoatomic, meaning that a single atom of argon makes up one molecule. It exists as individual atoms that don't bond together.

    Helium (He)

    Monoatomic, with each molecule consisting of a single helium atom.

    Oxygen (O2)

    Diatomic, which means that two oxygen atoms are chemically bonded together to form one molecule of oxygen. This is the form of oxygen that we breathe.

    Hydrogen (H2)

    Diatomic, exists as molecules with two hydrogen atoms bonded together. This is the most common form of hydrogen.

    Nitrogen (N2)

    Diatomic, two chlorine atoms bonded together.

    Ozone (O3)

    Triatomic, molecule is made up of three atoms

    Phosphorus (P4)

    Tetra-atomic, molecule with four phosphorus atoms bonded together.

    Sulphur (S8)

    Polyatomic, eight sulphur atoms bonded together.

    Molecular Mass

    Molecular mass refers to the combined mass of all the atoms that make up a single molecule of a compound. This includes the atoms of different elements that are chemically bonded together. Molecular mass is expressed in atomic mass units (amu) or grams per mole (g/mol).

    Calculation of Molecular Mass

    To calculate the molecular mass of a compound, you follow these steps:

    1. Identify the Elements: Determine the elements present in the molecule and their corresponding atomic masses. Atomic masses can be found on the periodic table.
    2. Count the Atoms: Count the number of atoms of each element in the molecule. This is usually represented by the subscripts in the chemical formula.
    3. Multiply and Sum: Multiply the atomic mass of each element by the number of atoms of that element in the molecule. Then, add up the products for all elements to get the molecular mass.

    Example: Let's calculate the molecular mass of water (H2O)

    Hydrogen (H) atomic mass = 1 amu
    Oxygen (O) atomic mass = 16 amu
    In one molecule of water (H2O), there are 2 hydrogen atoms and 1 oxygen atom.
    Molecular mass of water = (2 × 1 amu) + (1 × 16 amu) = 18 amu

    Similarly, the molecular mass of any compound can be calculated using the above steps.


    An ion is a particle that has an electric charge due to the gain or loss of electrons. Ions are formed when atoms or molecules undergo a process called ionization, which involves the transfer of electrons. The charge on an ion is either positive or negative.

    Types of Ions

    1. Anions: Anions are negatively charged ions formed when an atom or molecule gains one or more electrons. This addition of electrons gives the anion more negative charges than positive charges, resulting in an overall negative charge. Anions are usually formed by nonmetals. For example, chlorine (Cl) gains one electron to become the chloride ion (Cl-).

    2. Cations: Cations are positively charged ions formed when an atom or molecule loses one or more electrons. This loss of electrons leaves the cation with more positive charges than negative charges, resulting in an overall positive charge. Cations are typically formed by metals. For example, sodium (Na) loses one electron to become the sodium ion (Na+).

     Explore more about Metals and Non-Metals

    Ionic Bonds and Ionic Compounds

    Ions play a crucial role in the formation of ionic compounds through ionic bonding. Ionic compounds are formed when positively charged cations and negatively charged anions are attracted to each other due to their opposite charges. This attraction results in the formation of a stable compound with a balanced charge.

    For example, sodium chloride (NaCl) is an ionic compound formed by the combination of sodium cations (Na+) and chloride anions (Cl-). The strong electrostatic attraction between these oppositely charged ions holds the compound together.

    Significance of Ions

    Ions have important implications in various fields of science and technology:

    1. In chemistry, they contribute to the properties and behaviour of substances in reactions and solutions.
    2. In biology, ions are crucial for nerve impulses, muscle contractions, and maintaining the body's pH balance.
    3. In technology, ions are involved in processes like electrolysis, batteries, and fuel cells.


    Valency refers to the combining capacity or power of an element, indicating the number of bonds an atom of that element can form when it interacts with other atoms. It helps determine how many atoms of another element an atom can combine with to form a molecule. Valency is a fundamental concept in understanding chemical reactions and the formation of compounds.

    Valency for Elements

    The valency of an element is determined by the number of electrons in its outermost energy level (valence shell). Elements strive to achieve a stable electron configuration, often by gaining, losing, or sharing electrons with other atoms. The valency of an element corresponds to the number of electrons it needs to lose, gain, or share to attain a stable configuration.

     Explore more about Periodic Classification of Elements

    Valency for Ions

    For ions, the valency is indicated by the charge they carry. Positive ions (cations) have a valency equal to the number of electrons they have lost. Negative ions (anions) have a valency equal to the number of electrons they have gained. The magnitude of the charge indicates the number of electrons involved in the ionization process.


    1. Hydrogen (H) has a valency of 1, as it can bond with one other element to achieve a stable electron configuration.
    2. Oxygen (O) has a valency of 2, as it can form two bonds by gaining two electrons or sharing two electrons.
    3. Sodium (Na) has a valency of 1, as it loses one electron to form the sodium cation (Na+).
    4. Chlorine (Cl) has a valency of 1 or 7, depending on whether it gains one electron to form the chloride anion (Cl-) or achieves a stable configuration by sharing one electron in a covalent bond.

    Importance of Valency

    Understanding valency is crucial for predicting and explaining chemical reactions and the formation of compounds. It helps chemists determine how elements interact to create new substances with distinct properties. Valency also plays a key role in the development of materials, pharmaceuticals, and various industrial processes.

    Mole Concept

    The mole concept is a fundamental idea in chemistry that helps us understand and quantify the amount of matter in a substance, whether it's made up of atoms, molecules, ions, or particles. It provides a bridge between the microscopic world of individual particles and the macroscopic world that we can measure and observe.

    Definition of a Mole

    A mole is a unit used to count entities like atoms, molecules, or ions. One mole of any substance contains Avogadro's number (approximately 6.022 x 1023) of those entities. Just as a dozen is 12 items, a mole is a specific number of entities.

    Avogadro's Number

    Avogadro's number (6.022 x 1023) is the number of atoms, molecules, ions, or particles in one mole of any substance. It's a constant that helps relate the microscopic scale of individual particles to the macroscopic scale of measurable amounts.

    Gram Atomic Mass or Molar Mass

    Molar mass is the mass of one mole of a substance and is expressed in grams per mole (g/mol). For atoms, it's the atomic mass from the periodic table, and for molecules, it's the sum of the atomic masses of its constituent atoms.

    Relation to Gram Atomic Mass or Molar Mass

    The mass of a substance (in grams) is directly related to the number of moles of that substance. You can calculate the number of moles using the formula:

    Relation to Gram Atomic Mass or Molar Mass - Science Grade 9

    For instance, let's consider oxygen gas (O2) with a molar mass of approximately 32 g/mol. If you have 64 grams of oxygen gas, you can use the equation to find the number of moles:

    Conversely, if you have 3 moles of oxygen gas, you can calculate the corresponding mass using the same equation:

    Mass of Substance = Number of Moles x Molar Mass
    Mass of Substance = 3 moles x 32 g/mol = 96 grams

    Relation to Number of Particles

    The number of moles of a substance is equal to the count of particles divided by Avogadro's number.

    Relation to Number of Particles - Formula

    For example, if you have 1.204 x 1024 water molecules (H2O), the number of moles would be:

    Number of Moles = 1.204 x 1024 / 6.022 x 1023 ≈ 2 moles

    Relation to Molecular Mass

    By dividing the mass of the substance by its molar mass, you get the number of moles of that substance. Multiplying this by Avogadro's number gives you the total number of molecules in that mass.

    Relation to Molecular Mass - Formula

    For example, consider 32 g of oxygen gas (O2), with a molar mass of approximately 32 g/mol.

    Using the formula: Number of Molecules = (32 g / 32 g/mol) x 6.022 x 1023 ≈ 6.022 x 1023 molecules

    This result aligns with Avogadro's number, showing that in one mole of oxygen gas, there are approximately 6.022 x 1023 molecules.


    The mole concept allows chemists to work with practical quantities of substances. It's used to measure, compare, and predict reactions in chemical processes. It helps determine stoichiometry (the ratios of substances in a reaction), make conversions between mass and moles, and much more.

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