ATOMS AND MOLECLUES in 1 Shot || FULL Chapter Coverage (Concepts+PYQs) || Class 9th Chemisty

Physics Wallah Foundation・2 minutes read

Atoms are the fundamental units of matter that cannot be created, destroyed, or further divided, with specific elements having identical mass and chemical properties. Various laws and theories, such as the Law of Conservation of Mass and Dalton's Atomic Theory, contribute to our understanding of atoms, their properties, and how they combine to form compounds.

Insights

Atoms are the smallest particles that make up matter and cannot be further divided or destroyed in chemical reactions, ensuring consistency in mass and properties within a specific element.
Dalton's Atomic Theory, though foundational, has been challenged by discoveries like isotopes and isobars, proving that atoms can be further divided and have varying masses even within the same element.
The modern symbol system for elements, derived from the first and another letter of an element's name, ensures consistency and clarity in representing elements, with symbols not necessarily reflecting English names.

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Recent questions

What are atoms?
Atoms are the smallest particles composing matter.
What is the Law of Conservation of Mass?
The Law of Conservation of Mass states mass remains constant in chemical reactions.
What is Dalton's Atomic Theory?
Dalton's Atomic Theory asserts matter is composed of indivisible atoms.
What is the concept of molecular mass?
Molecular mass involves adding the masses of atoms in a molecule.
What is valency in chemistry?
Valency is the combining capacity of atoms to form compounds.

Related videos

Summary

00:00

Laws of Atoms and Chemical Reactions

Atoms are the smallest indivisible particles that make up all matter and participate in chemical reactions.
The concept of atoms being indivisible means they cannot be created, destroyed, or further divided through chemical reactions.
Atoms of a specific element have identical mass and chemical properties, ensuring consistency within that element.
The Law of Conservation of Mass, established by Lavoisier in 1774, states that mass remains constant before and after a chemical reaction.
The total mass of reactants in a chemical reaction equals the total mass of products, showcasing the Law of Conservation of Mass.
The Law of Constant Proportions, introduced by Proust in 1779, highlights that compounds consist of elements in fixed proportions by mass.
Water, for example, always contains hydrogen and oxygen in a fixed ratio of 1:8, regardless of its source.
Dalton's Atomic Theory, presented in 1808, asserts that all matter is composed of tiny particles called atoms that are indivisible and participate in chemical reactions.
Atoms of the same element have identical mass and chemical properties, while atoms of different elements exhibit varying mass and properties.
Dalton's theory laid the foundation for modern chemistry, emphasizing the fundamental role of atoms in explaining the properties of matter.

15:02

"Atomic Theory and Element Symbols Explained"

Atoms have their own mass and chemical properties, with atoms of different elements having different masses and chemical properties.
Atoms combine in small whole number ratios to form compounds, meaning atoms cannot be divided into halves or combine in fractional amounts.
Atoms combine in compounds in the ratio of small whole numbers, with the relative number and kinds of atoms being constant in a given compound.
Dalton's atomic theory had drawbacks, including the belief that atoms were indivisible, which has been proven incorrect as atoms can be further divided into electrons, protons, and neutrons.
Dalton's theory stated that all atoms of an element have the same mass, but isotopes have shown that atoms of the same element can have slightly different masses.
Dalton's theory also suggested that atoms of different elements have different masses, but isobars have shown that atoms of different elements can have the same mass.
Atoms are the building blocks of matter, with each element having a symbol derived from its name, following the modern symbol system proposed by JJ Bergius.
JJ Bergius proposed using the first letter and another letter of an element's name as its symbol, leading to the modern symbol system approved by the International Union of Pure and Applied Chemistry (IUPAC).
Symbols of elements should have the first letter capitalized and the second letter in lowercase, following the guidelines set by IUPAC for consistency and clarity.
IUPAC approves the names of elements, their symbols, and units, ensuring uniformity and accuracy in the representation of elements in the scientific community.

29:20

Element Symbols and Relative Atomic Masses Explained

Helium's symbol is 'He', with the first letter capitalized and the second in lowercase.
The symbol for aluminum is 'Al', not necessarily derived from the first and second letters of its name.
Symbols of elements do not have to be based on English names; they can be from other languages like Latin.
Sodium's symbol can be 'SO' or 'SD', not necessarily following the English name.
Potassium's symbol is 'K', not necessarily based on English names.
The first 10 elements and their symbols are: hydrogen (H), helium (He), lithium (Li), beryllium (Be), boron (B), carbon (C), nitrogen (N), oxygen (O), fluorine (F), and neon (Ne).
The symbols for elements 11 to 20 are: sodium (Na), magnesium (Mg), aluminum (Al), silicon (Si), phosphorus (P), sulfur (S), chlorine (Cl), argon (Ar), potassium (K), and calcium (Ca).
The concept of relative mass is explained using the example of weighing tomatoes and a stone.
The relative atomic mass of elements is determined with respect to an atom of carbon 12.
A super trick is provided to learn the atomic mass of the first 20 elements based on their atomic numbers.

44:04

Understanding Atomic Mass Calculation and Molecules

The first 20 elements are discussed in relation to understanding atomic mass.
Atomic mass is calculated as 2 times the atomic number for even atomic numbers.
An example using oxygen with an atomic number of 8 is given to illustrate the calculation.
For odd atomic numbers, atomic mass is calculated as 2 times the atomic number plus 1.
Exceptions to the rule are noted for hydrogen, beryllium, nitrogen, and argon.
The atomic numbers and masses for the exceptions are specified.
The concept of atoms existing in combined states and forming molecules is explained.
Molecules are defined as groups of two or more atoms held together by attractive forces.
Different types of molecules, including monoatomic, diatomic, triatomic, tetraatomic, and polyatomic, are discussed.
The definition of a molecule as the smallest particle of an element or compound that exhibits all the properties of that substance is provided.

58:45

Understanding Molecular Mass and Ionic Compounds

Atoms of carbon 12 have a specific mass.
The molecular mass theory involves calculating the mass of molecules.
Molecules are groups of atoms that come together.
To calculate molecular mass, add the masses of all atoms in a molecule.
For example, the molecular mass of water is 18u.
Molecular mass calculations involve adding the masses of all atoms in a compound.
The formula unit mass is used for ionic compounds.
Ionic compounds are formed by the loss or gain of electrons.
Charged species formed by electron loss are called cations.
Valency is the combining capacity of atoms to achieve stability by losing, gaining, or sharing electrons.

01:14:07

Understanding Valency and Chemical Formulas

Valency is determined by the number of electrons lost, gained, or shared by an atom to achieve stability.
The combining capacity of an element is known as its valency, which helps in understanding how atoms of different elements combine to form chemical compounds.
Valency is crucial in determining how atoms of an element combine with atoms of other elements to form chemical compounds.
The charge on an ion is equal to the valency of the ion, which is determined by the number of electrons lost or gained.
Writing chemical formulas involves understanding the symbols of elements and their charges to create the correct formula.
The formula unit mass of a compound is calculated by adding the atomic masses of all atoms present in the formula unit.
The percentage composition of an element in a compound is calculated by dividing the mass of the element by the total mass of the compound and multiplying by 100.
Matching chemical names to formulas involves understanding the symbols and valencies of elements to create the correct compound formula.
Identifying incorrect symbols of elements and correcting them to their accurate representations is essential in chemistry.
Calculating the ratio by mass of combining elements in compounds involves understanding the masses of each element present in the compound.

01:29:34

"Chemical Formulas, Atoms, and Molecular Classification"

The formula for aluminum fluoride is AlF3, with aluminum having a valency of three and fluorine having a valency of one. The ratio by mass of aluminum to fluorine is 9:1, with aluminum weighing 27 and fluorine weighing 19 each.
When calculating the number of atoms in different chemical species, it's crucial to focus on the atoms present, not the charges. For example, in carbon monoxide, there is one carbon and one oxygen, totaling two atoms.
To find the fraction of the mass of water due to neutrons, the mass of water (H2O) is 18, with eight neutrons contributing to this mass. The fraction due to neutrons is calculated as 8/18, which simplifies to 4/9.
Molecules can be classified based on the number of atoms they contain. For instance, a molecule with two atoms is diatomic, while one with four atoms is tetraatomic. Understanding atomicity is essential for classifying molecules accurately.

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