ATOMS AND MOLECLUES in 1 Shot || FULL Chapter Coverage (Concepts+PYQs) || Class 9th Chemisty
Physics Wallah Foundation・2 minutes read
Atoms are the smallest indivisible particles of matter, combining in fixed ratios to form compounds. Dalton's Atomic Theory, while incorrect on some points, laid the foundation for our understanding of atomic structure and chemical reactions.
Insights
- Atoms are the smallest particles that make up matter and cannot be created, destroyed, or divided further by chemical reactions.
- The Law of Conservation of Mass states that the total mass of reactants equals the total mass of products in a chemical reaction, ensuring mass is conserved.
- Dalton's Atomic Theory, while groundbreaking, had limitations such as the indivisibility of atoms, which can be divided into subatomic particles like protons, neutrons, and electrons.
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Recent questions
What is Dalton's Atomic Theory?
Dalton's Atomic Theory posits that all matter is made up of indivisible atoms, with atoms of a given element having identical mass and chemical properties. Atoms of different elements exhibit varying mass and chemical properties, combining in small whole number ratios to form compounds. Dalton's theory emphasizes that atoms cannot be divided into halves and do not combine as fractions, with the relative number and kinds of atoms remaining constant in a given compound.
How are elements represented by symbols?
Elements are represented by symbols derived from their names, with the first letter capitalized and the second in lowercase. The symbols were proposed by JJ Bergius, using the first letter and another letter of the element's name. The International Union of Pure and Applied Chemistry approves the names of elements, their symbols, and units to standardize symbols. Symbols do not have to be based on English names and can be from other languages, like Latin. For example, the symbol for helium is "He," and the symbol for potassium is "K."
What is the Law of Conservation of Mass?
The Law of Conservation of Mass states that the total mass of reactants equals the total mass of products in a chemical reaction. This law asserts that mass is neither created nor destroyed during a chemical reaction, highlighting the importance of balancing chemical equations to ensure the conservation of mass.
How are molecules classified based on atomicity?
Molecules are classified based on atomicity, referring to the number of atoms present in a molecule. Molecules can be monoatomic, diatomic, triatomic, tetraatomic, or polyatomic, depending on the number of atoms they contain. The molecular mass of a molecule is the sum of the masses of all atoms in the molecule, calculated by adding the masses of each atom present in the compound.
What is valency in chemistry?
Valency is the combining capacity of atoms to achieve stability, determined by the number of electrons lost, gained, or shared by an atom. It remains the same for an atom regardless of how it achieves stability and helps determine how atoms combine to form molecules. Valency plays a crucial role in understanding how elements combine with each other to form chemical compounds, influencing the composition and properties of the resulting substances.
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Summary
00:00
Understanding Atoms and Chemical Reactions
- Atoms are the smallest indivisible particles that make up all matter and participate in chemical reactions.
- Atoms cannot be created, destroyed, or divided further by chemical reactions.
- Elements are composed of identical atoms with the same mass and chemical properties.
- Atoms of different elements have distinct mass and chemical properties.
- The Law of Conservation of Mass states that the total mass of reactants equals the total mass of products in a chemical reaction.
- The Law of Constant Proportions asserts that compounds consist of elements in fixed proportions by mass, regardless of their source.
- Water, for example, always contains hydrogen and oxygen in a fixed 1:8 proportion by mass.
- Dalton's Atomic Theory, presented in 1808, posits that all matter is made up of indivisible atoms.
- Dalton's theory emphasizes that atoms of a given element have identical mass and chemical properties.
- Atoms of different elements exhibit varying mass and chemical properties, distinguishing them from one another.
15:06
Atomic Theory and Element Symbols Standardization
- Atoms of different elements have different masses and chemical properties, combining in small whole number ratios to form compounds.
- Atoms cannot be divided into halves and do not combine as fractions, such as 3.5 atoms of hydrogen and 1.2 atoms of oxygen forming water.
- Atoms combine in the ratio of small whole numbers to create compounds, with the relative number and kinds of atoms remaining constant in a given compound.
- Dalton's atomic theory posited that atoms were indivisible, a major drawback as atoms can be further divided into electrons, protons, and neutrons.
- Dalton's theory also suggested that all atoms of an element have the same mass, which is inaccurate as isotopes can have slightly different masses.
- Dalton's theory claimed that atoms of different elements have different masses, but isobars show that atoms of different elements can have the same mass.
- The modern symbols of elements were proposed by JJ Bergius, using the first letter and another letter of the element's name as its symbol.
- The International Union of Pure and Applied Chemistry approves the names of elements, their symbols, and units to resolve conflicts and standardize symbols.
- Symbols of elements should have the first letter capitalized and the second letter in lowercase, following the naming conventions proposed by JJ Bergius.
- IU Peck is an international scientific organization that approves the names of elements, their symbols, and units to maintain consistency and avoid conflicts.
29:20
Element Symbols and Their Origins
- Helium's symbol is A, with the first letter capitalized and the second in lowercase.
- The symbol for helium is not written as "helium" but as "He."
- The symbol for aluminum is "Al," not necessarily derived from the first and second letters of the element.
- Symbols for elements do not have to be based on English names; they can be from other languages.
- Sodium's symbol can be "SO" or "SD," not necessarily following English name conventions.
- Symbols for elements can be derived from names in other languages, like Latin.
- The symbol for potassium 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).
- Relative atomic mass is determined by comparing the mass of an atom to a standard reference element, like carbon-12.
44:06
Atomic Mass and Molecule Classification Basics
- Atomic mass is equal to 2 * atomic number for elements with even atomic numbers
- Example: Oxygen with atomic number 8 has an atomic mass of 16
- Atomic mass is equal to 2 * atomic number for elements with odd atomic numbers
- Example: Fluorine with atomic number 9 has an atomic mass of 19
- Exceptions to the atomic mass trick are Hydrogen, Beryllium, Nitrogen, and Argon
- Exceptions have atomic numbers of 1, 4, 7, and 18 respectively
- Molecules are groups of two or more atoms held together by attractive forces
- Attractive forces holding atoms together in a molecule are called chemical bonds
- Molecules of elements consist of atoms of the same type, while molecules of compounds have different types of atoms
- Atomicity refers to the number of atoms present in a molecule, indicated by subscripts in chemical formulas
- Molecules can be classified as monoatomic, diatomic, triatomic, tetraatomic, or polyatomic based on the number of atoms they contain
58:52
Understanding Molecular Mass and Ionic Compounds
- A molecule is a group of atoms coming together, and the molecular mass is the sum of the masses of all atoms in the molecule.
- To calculate molecular mass, add the masses of all atoms in the molecule, such as in water (H2O) where 2 * atomic mass of Hydrogen + 1 * mass of Oxygen equals 18u.
- For compounds like Al2(SO4)3, calculate by multiplying the number of atoms of each element, like 2 * aluminum + 3 * 4 * oxygen, to find the molecular mass.
- When dealing with compounds like CuSO4.5H2O, do not multiply the dot (.) representing water molecules, but add the masses of each element present.
- Formula unit mass is calculated similarly to molecular mass, but for ionic compounds, where the mass of all atoms in the compound is added to find the formula mass.
- Ions are formed by the loss or gain of electrons, creating positively charged cations and negatively charged anions.
- An ion's charge is determined by the number of electrons lost or gained, with a negative charge indicating electron gain and a positive charge indicating electron loss.
- Simple ions are formed by single atoms, and a chemical formula can be understood using a periodic table to determine the elements present.
- Valency is the combining capacity of atoms to achieve stability, with stability in chemistry referring to having a complete octet or duet of electrons.
- Valency is the number of electrons lost, gained, or shared by an atom to achieve stability, determining how atoms combine to form molecules.
01:14:18
"Valency: Key to Chemical Element Stability"
- Atoms form elements by losing, gaining, or sharing electrons to achieve stability.
- Valency remains the same for an atom regardless of how it achieves stability.
- The combining capacity of an element is known as its valency.
- Valency helps determine how atoms of an element combine with atoms of other elements to form chemical compounds.
- The concept of combining capacity determines which elements combine and in what quantities.
- The charge on an ion is equal to its valency.
- Sodium loses one electron to achieve stability, resulting in a charge of +1.
- Chloride gains one electron, leading to a negative charge.
- Writing chemical formulas involves understanding the symbols of elements and their charges.
- Calculating the percentage composition of elements in a compound involves dividing the mass of the element by the total mass of the compound and multiplying by 100.
01:29:41
Aluminum Valency, Mass Ratios, and Atom Count
- Aluminum's valency is three, forming the formula AlF3 with one aluminum and three fluorine atoms.
- The mass ratio of aluminum to fluorine in AlF3 is 9:1, calculated based on the atomic masses of 27 for aluminum and 19 for fluorine.
- When determining the number of atoms in chemical species, the total atoms in a compound like CO3^2- is counted, excluding the charge.
- The fraction of the mass of water due to neutrons is calculated by dividing the mass of neutrons (8) by the total mass of water (18), resulting in a fraction of 4/9.
