Atomic Foundations of Matter
NCERT Class 9 Science • Chapter 9
Quick Answer
This Class 9 chemistry chapter builds on atoms and molecules: laws of chemical combination, Dalton's atomic theory, atomic and molecular mass, the mole concept, and how atoms combine to form molecules and ions. Students learn to write chemical formulae and use the mole to relate mass, particles, and amounts of substance.
↓ Free PDF, NCERT solutions, summary, key terms, FAQs below.
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Key Terms
- Atom
- The smallest particle of an element that takes part in a chemical reaction and retains the element's properties.
- Molecule
- A group of two or more atoms chemically bonded together, representing the smallest unit of an element or compound that can exist independently.
- Ion
- An atom or group of atoms that carries an electric charge because it has gained or lost electrons.
- Mole
- The SI unit for the amount of substance, containing Avogadro's number (about 6.022 x 10^23) of particles.
- Atomic Mass
- The mass of an atom expressed in atomic mass units, compared to one-twelfth the mass of a carbon-12 atom.
- Law of Conservation of Mass
- The principle that mass is neither created nor destroyed in a chemical reaction; total mass of reactants equals total mass of products.
- Chemical Formula
- A symbolic representation of a compound showing the types and numbers of atoms present in one molecule or formula unit.
Frequently Asked Questions
What is the difference between an atom and a molecule?▾
An atom is the smallest particle of an element, while a molecule is two or more atoms chemically bonded together. A molecule can be made of the same element, like oxygen, or different elements, like water.
What is the mole concept?▾
The mole is a unit for counting particles, where one mole contains about 6.022 x 10^23 particles, known as Avogadro's number. It lets chemists relate the mass of a substance to the number of atoms or molecules it contains.
What is the law of conservation of mass?▾
The law of conservation of mass states that in a chemical reaction, the total mass of the reactants equals the total mass of the products. Matter is neither created nor destroyed, only rearranged.
What is an ion?▾
An ion is a charged particle formed when an atom or group of atoms gains or loses electrons. Losing electrons creates a positive ion (cation), while gaining electrons creates a negative ion (anion).
What was Dalton's atomic theory?▾
Dalton's atomic theory proposed that all matter is made of tiny indivisible atoms, that atoms of an element are identical, and that atoms combine in fixed whole-number ratios to form compounds. It explained the laws of chemical combination.
How do you write a chemical formula?▾
A chemical formula uses element symbols and subscripts to show the number of each kind of atom in a compound. It is written by combining the symbols according to the valencies of the elements involved.
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