Summary -Class 9- Chemistry-CH 4 – Structure of the Atom

Summary -Class 9- Chemistry-Chapter 4 – Structure of the Atom

Structure of an Atom:

◆Atoms are the basic building blocks of matter and are composed of three main subatomic particles: protons, neutrons, and electrons.

◆Protons are positively charged particles found in the nucleus of the atom. They contribute to the atomic number of the element.

◆Neutrons are neutral particles also located in the nucleus. They add mass to the atom without affecting its charge.

◆Electrons are negatively charged particles that orbit the nucleus in specific energy levels called electron shells or orbitals. They are much smaller and lighter than protons and neutrons.

Cathode Ray Experiment:

◆J.J. Thomson’s cathode ray experiment provided evidence for the existence of electrons.

◆He observed that when high voltage was applied to electrodes placed in a vacuum tube, a stream of particles (cathode rays) moved from the negatively charged cathode to the positively charged anode.

◆Thomson concluded that these cathode rays were composed of negatively charged particles (electrons) based on their behavior in electric and magnetic fields.

Electrons:

◆Electrons are fundamental particles with a negative electrical charge (-1 elementary charge).

◆They have a much smaller mass compared to protons and neutrons, approximately 1/1836 times the mass of a proton.

◆Electrons are located outside the nucleus and are arranged in energy levels or orbitals around the nucleus.

◆The number of electrons in an atom is equal to the number of protons in a neutral atom, ensuring overall charge neutrality.

Thomson’s Model of an Atom:

◆Thomson proposed a model of the atom known as the “plum pudding” or “raisin bun” model.

◆According to this model, atoms consist of a positively charged sphere with negatively charged electrons embedded within it, resembling raisins in a pudding.

◆The positive charge of the atom is spread out uniformly, and electrons are dispersed throughout, similar to raisins distributed in a pudding.

Radioactivity:

◆Radioactivity is the spontaneous emission of radiation from the nucleus of an atom.

◆It occurs in certain isotopes of elements that have unstable nuclei, which decay over time to become more stable.

◆The types of radiation emitted during radioactive decay include alpha particles (helium nuclei), beta particles (electrons or positrons), and gamma rays (high-energy electromagnetic radiation).

Rutherford Model:

◆Ernest Rutherford’s gold foil experiment provided evidence for the nuclear model of the atom.

◆He bombarded a thin gold foil with alpha particles and observed their scattering patterns.

◆Most alpha particles passed through the foil with little deflection, but some were deflected at large angles or even reflected back.

◆Rutherford concluded that atoms have a small, dense, positively charged nucleus surrounded by mostly empty space, with electrons orbiting around the nucleus.

Bohr Model:

◆Niels Bohr proposed a modification to the Rutherford model to explain the stability of atoms.

◆In the Bohr model, electrons orbit the nucleus in fixed energy levels or shells, similar to planets orbiting the sun.

◆Electrons can transition between energy levels by absorbing or emitting energy, but they cannot exist in between these levels.

◆The Bohr model successfully explained the discrete line spectra observed for hydrogen and other elements.

Orbits:

◆Orbits, also known as energy levels or shells, are regions surrounding the nucleus where electrons are most likely to be found.

◆Each orbit corresponds to a specific energy level, and electrons occupy these orbits based on their energy.

◆Electrons fill the lowest energy levels first before occupying higher energy levels, according to the aufbau principle, Hund’s rule, and the Pauli exclusion principle.

Valency:

◆Valency refers to the combining capacity of an atom, which is determined by the number of electrons in its outermost shell.

◆Atoms tend to gain, lose, or share electrons to achieve a stable electron configuration, corresponding to their valency.

◆Elements with full outer shells (noble gases) have a valency of zero and are chemically inert, while other elements exhibit valencies determined by the number of electrons needed to achieve a noble gas configuration.

Atomic Number and Mass Number:

◆The atomic number of an element is equal to the number of protons in the nucleus of its atoms. It determines the identity of the element and is denoted by the symbol “Z.”

◆The mass number of an atom is the total number of protons and neutrons in its nucleus. It is denoted by the symbol “A.”

Isotopes and Isobars:

◆Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

◆Isobars are atoms of different elements with the same mass number but different atomic numbers.

Calculation of Mass Number for Isotopic Elements:

◆The mass number for isotopic elements is calculated based on the relative abundances of each isotope and their respective masses.

◆It represents the weighted average of all isotopes present in a sample, taking into account their proportional contributions to the overall mass