what is atom and molecules class 9 notes? top most exam useful important facts

atom and molecules class 9 notes

• To understand electricity, it is important to understand the structure of matter.

• An object that occupies space and has weight is called matter.

• Basically, matter is divided into three parts- solid, liquid and gas. Apart from this, there are two other states which are known as plasma and Bose Einstein condensate. But these cannot be included in the basic form.

• The shape of solid substances does not change. For example- iron, stone.

• Liquids do not have any shape, they take the shape of the container in which they are filled but their volume remains fixed. For example- water, oil.

• Both the shape and volume of a gas are not fixed, and they are usually identified by their smell.

• Plasma is a gas made up of charged and neutral particles that are overall nearly neutral and exhibit a collective behavior. The study of plasma deepens our understanding of the universe.

• Bose Einstein condensation is a state of matter in which the matter formed by weak bonds is helpful in understanding the physics of super-cold (cooled to temperatures very close to absolute zero) atoms.

Structure of Material–

matter – particle – molecule – atom –stable atomic particle and unstable nuclear particles

Molecules-

• The smallest particle of a matter which can be divided and in which the physical and chemical properties of the matter remain present is called a molecule.

• The state of each molecule is different.

• The molecules of a solid substance move at a very fast speed and there is a lot of proximity between the molecules. That is, the force of attraction between the molecules is very high. The force of attraction between the particles in a liquid is less or normal. Whereas the force of attraction between gases is less than normal. Due to this reason, gaseous particles easily spread around in the medium. The decreasing order of the force of attraction is Solid >Liquid > Gas

Atom

• Atoms only participate in chemical reactions, they do not exist in free state.

• Atoms mainly contain three fundamental particles: electrons, protons and neutrons.

• An atom has a nucleus, which is called the compact part of the atom. The entire weight of the atom is concentrated in the nucleus.

• All the atoms of the same element may not have the same weight.

• Atoms of the same element having the same atomic number but different atomic weights are called isotopes.

• Atoms of different elements having the same atomic weight are called isobars.

• The diameter of the atomic orbit is in the range of 10-10 meters. Which is known as Angstrom.

• Hydrogen atom has the lowest weight, hence the atomic weight of other elements is determined by its ratio.

• Therefore, it can be said that “The atomic weight of an element is the ratio of the weight of one atom of that element to the weight of one atom of hydrogen.”

structure of atom class 11 notes

The structure of an atom is made up of three fundamental particles, electron, proton and neutron.

. Nucleus-

• The nucleus of an atom is made up of protons and neutrons, hence atomic weight is defined as the sum of protons and neutrons.

• The nucleus was discovered in 1911 by scientist Ernest Rutherford in his application.

• The electrons revolve around the nucleus in elliptical orbits.

• The radius of the nucleus is of the order of 10-15 meters.

Protons-

• It is a fundamental positively charged particle. Although the charge of a proton is half that of an electron The charge remains equal but the nature is opposite. It is a fundamental particle of nature which remains in a stable state in the nucleus.

• The charge of a proton is 1.602 × 10-19 coulomb.

• The mass of a proton is 1840 times the mass of an electron i.e. 1.673×10-27 kg.

• Remember – Protons in atoms were first seen by Goldstein in the form of hydrogen ions (H+ ions), but this particle was verified by Ernest Rutherford.

Neutrons-

• It is a neutral particle.

• This is a temporary particle.

• The heaviest particle among the particles present in an atom is the neutron.

• The mass of a neutron is almost the same as the mass of a proton, which is 1.6765 × 10-27 kg or 1.008625 amu.

Electron-

• It is a negatively charged particle. Its charge is (-) 1.602 × 10-19 coulomb.

• Its mass is 9.1×10-31 Kg or 0.00054 amu.

• It revolves around the atom in an elliptical orbit.

• The electron revolves around the nucleus due to centripetal force.

• The lightest particle among the three particles of atoms is the electron.

Law of structure of atom class 11 notes

• Every atom is neutral in normal state because it has equal number of protons and electrons.

• The orbitals present in the atoms are represented by the letters K, L, M, N and it is in these orbitals that the electrons are completed.

• The distribution of electrons in these orbitals is done according to the (2n²) rule.

Where n = represents the number of orbitals.

Number of electrons in orbit K = 2(1)² = 2

Number of electrons in orbit L = 2(2)² = 8

Number of electrons in orbit M = 2(3)² = 18

Number of electrons in orbit N = 2(4)² = 32

• There cannot be more than 8 electrons in any last orbit. Because the octet of orbitals is complete with only eight electrons.

Atomic Number :-

• The number of electrons or protons present in an atom of an element represents its atomic number.

• The atomic number is denoted by “Z”.

Atomic Weight:-

• The sum of the total numbers of protons and neutrons present in the nucleus of an atom is called its atomic weight or mass number.

• Atomic weight is denoted by “A”.

Valence Electrons :-

The number of electrons present in the last orbit of an atom are called valence electrons.

Therefore, in metals, insulators and semiconductors, the order of number of electrons in the outermost shell is given below, which is also known as their valency.

Metals have valency of – 1, 2 or 3. Semiconductors have valency of 4.

The valency of OPPNs is more than 8.

Conclusion

The atom consists of a central nucleus with protons and neutrons, surrounded by electrons in orbitals. This structure explains how atoms form bonds and interact in chemical reactions. and exam useful important fact of structure of atom class 11 notes

exam useful important facts

• An atom is the smallest component or unit of any simple substance. It is the smallest recognized division of a chemical element.

• When two or more atoms combine a molecule is formed.

• The atomic number in one molecule is equal to 6.022 × 1023 atoms.

• Element is the purest form of matter in which same kind of atoms are present like Ca, fe etc.

• Two or more elements combine in a definite proportion to form a new substance whose properties are completely different from the two elements are called compounds.
• The atomic weight of an element is a number that shows how much heavier an atom of the element is than (1/12th) of an atom of carbon-12. Or in other words how many times heavier it is than 1.008th of the mass of hydrogen.

• Atomic weight is actually the ratio of the mass of the element to one-twelfth of the carbon atom.

• Atomic weight is approximately the sum of the number of protons and neutrons. Atomic number is equal to the number of protons or equivalent number of electrons present in the nucleus.

• The full form of “ISO” is “International Organization for Standardization”. Its headquarters is located in Geneva (Switzerland).
  

calculation

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