TLDR;
This video is a chemistry lesson focused on the structure of the atom, atomic theory, subatomic particles, and chemical bonding. It covers Dalton's atomic theory, the discovery of subatomic particles (protons, neutrons, and electrons), atomic models (Thomson's plum pudding model, Rutherford's model, and Bohr's model), and basic concepts of chemical bonding, including ionic and covalent bonds. The lesson also touches on electronic configuration, valency, and the octet rule.
- Dalton's atomic theory and its postulates.
- Discovery and properties of subatomic particles.
- Atomic models: Plum pudding, Rutherford, and Bohr.
- Basics of chemical bonding: ionic and covalent bonds.
- Electronic configuration, valency, and octet rule.
Introduction [0:01]
The session starts with a welcome and an overview of the topics to be discussed, emphasizing the importance of chemistry for both NEET and JEE exams. The instructor outlines the syllabus, which includes the structure of the atom, properties of matter, redox reactions, and acid-base chemistry. The focus of the day is on the structure of the atom, covering atomic theory, subatomic particles, and atomic models.
Exam Pattern [3:24]
The instructor briefly discusses the exam pattern for the APO Next Exam, noting that it includes questions from chemistry, biology, and mathematics. Specifically, the chemistry section consists of 25 questions.
What is an Atom? [4:10]
An atom is defined as the most basic unit of a chemical element and the fundamental particle of all things in the universe. It's the smallest unit of matter that retains the properties of an element. The instructor poses the question of whether an atom is divisible.
Dalton's Atomic Theory [6:18]
John Dalton's atomic theory is introduced, which includes the following key postulates:
- Atoms are indivisible and cannot be divided.
- All matter is made up of extremely small, indivisible particles called atoms.
- Atoms of the same element are identical, having the same properties and mass.
- Atoms of different elements are different and have different masses and properties.
- Atoms combine in fixed ratios to form compounds.
- Atoms are conserved in chemical reactions and are neither created nor destroyed, only rearranged.
Subatomic Particles [13:55]
The discussion shifts to subatomic particles, which are components smaller than an atom. The atom is divisible into three main subatomic particles:
- Protons: Positively charged particles located in the nucleus.
- Neutrons: Neutral (chargeless) particles also located in the nucleus.
- Electrons: Negatively charged particles that orbit the nucleus.
The nucleus is the center of the atom, containing protons and neutrons, collectively referred to as nucleons. The electrons reside outside the nucleus in shells or orbits.
Discovery of Subatomic Particles [22:13]
The scientists who discovered the subatomic particles are identified:
- Ernest Rutherford: Discovered protons and the nucleus.
- J.J. Thomson: Discovered electrons.
- James Chadwick: Discovered neutrons.
- Eugene Golstein: Experiments led to discovery of positively charged anode race.
Charge and Mass of Subatomic Particles [25:27]
The charges and masses of the subatomic particles are detailed:
- Proton: Positive charge (+1.6 x 10^-19 Coulombs), relative charge of +1, mass is almost similar to neutrons.
- Electron: Negative charge (-1.6 x 10^-19 Coulombs), relative charge of -1, negligible mass (approximately zero for practical purposes).
- Neutron: No charge (neutral), mass is almost similar to protons.
The mass of the atom is concentrated in the nucleus due to the presence of protons and neutrons. The unit used to measure the mass of atoms and subatomic particles is the unified mass unit (U).
Atomic Number and Mass Number [31:43]
The concepts of atomic number (Z) and mass number (A) are explained:
- Atomic Number (Z): The number of protons in an atom.
- Mass Number (A): The sum of protons and neutrons in an atom.
The representation of an element includes the symbol of the element, with the atomic number written below and the mass number above. For a neutral atom, the number of protons equals the number of electrons. The number of neutrons can be calculated by subtracting the atomic number from the mass number (A - Z).
Atomic Models - Thomson's Plum Pudding Model [38:38]
The lesson transitions to atomic models, starting with J.J. Thomson's plum pudding model, proposed in 1904. According to this model:
- An atom is a sphere of positive charge with electrons embedded within it, similar to seeds in a watermelon or raisins in a plum cake.
- The positive charge is uniformly distributed.
- The total positive charge is equal to the total negative charge, making the atom electrically neutral.
Atomic Models - Rutherford's Gold Foil Experiment [46:11]
Rutherford's gold foil experiment is discussed, which led to a new understanding of atomic structure. In this experiment, alpha particles (positively charged) were directed at a thin gold foil. The observations were:
- Most alpha particles passed through the gold foil undeflected.
- A few alpha particles were deflected by small angles.
- Very few alpha particles bounced back at large angles (approximately 180 degrees).
From these observations, Rutherford made the following inferences:
- Most of the space in an atom is empty.
- There is a positively charged part inside the atom.
- The positive charge and mass of the atom are concentrated in a very small space at the center, which he called the nucleus.
Atomic Models - Rutherford's Planetary Model [55:43]
Based on his experiment, Rutherford proposed the planetary model of the atom:
- The atom has a central nucleus where the positive charge and most of the mass are concentrated.
- Electrons revolve around the nucleus in circular paths (orbits).
- The size of the nucleus is very small compared to the size of the atom.
However, Rutherford's model had limitations, as it could not explain the stability of the atom. According to electromagnetic theory, an electron revolving around the nucleus should continuously emit energy and eventually fall into the nucleus, which does not happen.
Atomic Models - Bohr's Model [1:04:39]
Niels Bohr proposed an improved atomic model in 1913 to address the limitations of Rutherford's model:
- Electrons revolve around the nucleus in fixed orbits or shells, each with a definite energy level.
- These orbits are also called energy levels or stationary energy levels.
- As long as an electron revolves in a particular orbit, its energy does not change.
- Electrons can only change their energy by jumping from one orbit to another.
- When an electron jumps from a lower energy level to a higher energy level, it absorbs energy.
- When an electron jumps from a higher energy level to a lower energy level, it emits energy.
The energy of the shells increases as the distance from the nucleus increases. The shells are designated by letters K, L, M, N, etc., with K being the closest to the nucleus and having the lowest energy.
Basics of Chemical Bonding [1:13:33]
The basics of chemical bonding are introduced, defining a chemical bond as the force of attraction that holds atoms together in a compound. The two main types of chemical bonds are:
- Ionic Bond: Formed by the transfer of electrons from one atom to another.
- Covalent Bond: Formed by the sharing of electrons between two atoms.
Electronic Configuration [1:14:32]
The lesson covers how to write the electronic configuration of atoms, which is the arrangement of electrons in the different shells. Each shell can hold a maximum number of electrons:
- K shell: 2 electrons
- L shell: 8 electrons
- M shell: 18 electrons
- N shell: 32 electrons
The electrons are filled in the shells starting from the lowest energy level (K shell) to the higher energy levels.
Valency and the Octet Rule [1:21:30]
The concept of valency and the octet rule are explained. Atoms react chemically to achieve stability, which usually means having eight electrons in their outermost shell (valence shell), known as the octet configuration. Atoms may gain, lose, or share electrons to achieve this stable configuration.
Noble gases (inert gases) are already stable with eight electrons in their valence shell (except for helium, which has two), and therefore do not participate in chemical reactions.
Valency is the number of electrons an atom needs to gain, lose, or share to complete its octet. If an atom loses electrons, it becomes positively charged (cation), and if it gains electrons, it becomes negatively charged (anion).
Exceptions - Helium and Duplet Rule [1:30:57]
Helium is an exception to the octet rule, as it has only two electrons and only one shell (K shell). Helium is stable with two electrons in its valence shell, following the duplet rule. Hydrogen, with one electron, also tends to follow the duplet rule.