Class 7 Science The Structure of the Atom Chapter Notes

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The Structure of the Atom

Ancient view of matter

The smallest particle of matter is the atom. One of the question which puzzled Indian and Greek thinkers alike was the nature of matter. Slowly, an idea developed among the natural philosophers in Greece–that all matter must be made of some basic elements. They thought these elements were water, earth, fire and sky. Some view was also maintained by Indian philosophers. According to Kanada if you cut a pure substance into smaller and smaller bits, you would ultimately reach these indestructible particles which could not be broken further. The atomic view of matter was developed to quite an extent by other Indian philosophers. We can equate the ‘anu’ with molecule and the ‘paramanu’ with atom. The greek philosopher Democritus called the atoms as building blocks of matter. He thought matter was made of tiny, invisible, eternal and immutable particles or atoms.

Dalton’s atomic theory
Based on the scientific study of Lavoiser and others on chemical reactions, the following conclusions were made–
1. Mass of reactants is always equal to the mass of products, matter is neither created nor destroyed.
2. A compound always contains same elements combined in the same proportion by mass.

Guided by these observations, John Dalton put forward the atomic theory, which says:
1. Atoms of a particular element are alike, but differ from the atoms of other elements.
2. Atoms are indestructible.
3. Atoms combine in simplest whole number ratio to form molecules.
4. Atom of an element has characteristic mass.

Atom is electrically neutral, but actually it contains positive and negative charges which normally balance each other.

Discovery of electron
Some experiments conducted involve passage of electricity through a gas in a sealed tube called gas discharge tube. Scientists discovered that negatively charged particles are produced when an electric charge is passed through a gas in this tube. Sir J.J Thomson measured the mass and
charge of the particles and named them electrons.

Discovery of proton
These positively charged particles were discovered by E. Goldstein during experiments with discharge tube. Protons, carry same amount, but of opposite charge as carried by electrons.

Thomson’s model of atom
Thomson suggested that the positive charge in an atom is spread evenly over entire atom which, he supposed, was spherical. The negative charges are embedded in the positively charged sphere. This is called as the plum pudding model. This model failed to explain many experimental observations.

Rutherford’s model
He put forward the following model of atom:
1. Entire positive charge in an atom is concentrated in a small area at its centre, called nucleus
2. Size of nucleus is extremely small compared to the atom.
3. Electrons move around the nucleus in circular orbitals.
4. Most of the mass of atom is concentrated in its nucleus.
5. The number of electrons orbiting around the nucleus is equal to the number of protons present in the nucleus.
6. Most of the space in atoms is empty.

Atomic number
The number of protons in the nucleus of the atom of an element is called its atomic number. It is denoted by Z.

Atomic mass
The atomic mass of an element is the number of times by which the mass of its atom is greater than 1/12 th the mass of carbon atom.

Mass number
The mass number of an element is the sum of the number of protons and neutrons in its nucleus. It is denoted by A.
Mass number (A) = number of protons + number of neutrons
Atomic number (Z) = number of protons
Number of neutrons = mass number (A) – atomic number (Z)

Isotopes
They can be defined as atoms of the same element that have the same number of protons but different number of neutrons. E.g.: The ordinary hydrogen atom is Protium (¹ ₃ H)  while the other two isotopes are Deuterium ( ² ₁ D) and tritium ( ³ ₁ T) 

Bohr’s model of atom
Bohr suggested that electrons revolve around the nucleus in certain definite or fixed paths called orbits or shells. These orbits are called energy levels or shells. The energy associated with an orbit increases with its distance from the nucleus. The shells are numbered 1, 2, 3.. or denoted as K, L, M, N....

How electrons are arranged?
1. Each shell can accommodate 2n2 electrons where ‘n’ stands for the number of shell.
2. The maximum number of electrons that an outermost shell of an atom can have is 8, except the first shell, which has only 2. This distribution of electrons in various orbits of the atom of an element is called its electronic configuration of element.

How atom combine?
Atoms of elements combine, in accordance with their valencies, to form molecules of compounds during a chemical reactions.

Noble gases – Helium, neon, argon, Kr, Xe and Rn are noble gases because they are stable and do not react with other elements to form compounds.

Valence electrons – Electrons in the outermost shell are called valence electrons. They determine the valency of an element and its combining capacity.

Covalent compound – A compound formed by sharing of electrons between two or more atoms.

Electrovalent compound – The electrostatic force of attraction that holds ions together is called electrovalent bond. The compounds formed this way are called electrovalent compounds.

Ion – An atom which becomes charged by losing or gaining electrons is called ion. Positively charged ion is cation and negatively charged ion is anion.

Valency – The valency of an element is the number of electrons lost, gained or shared by its atom in a chemical reaction. Elements like iron, copper, tin and sulphur have variable valency. Simply it may be defined as combining capacity of an atom of an element.

Mole Concept
Concept of Limiting Reagent: In the reactions involving more than one reactant, the amount of product formed depends on the limiting reactant i.e. the reactant which is completely consumed. All calculations have to be worked on the basis of the limiting reagent only.

Chemical Bonding

Ionic Bond: An ionic bond is formed at a result of the transference of electrons from one atom to the other. Elements having the tendency to lose electrons are called electropositive (e.g. alkali metals) while those having a tendency to gain electrons are called electronegative (e.g. halogens) elements.

Covalent Bond: Atoms can also acquire eight electrons in their valence shell by sharing electrons. When two atoms get bounded by sharing of electrons, the bond formed is called a covalent bond.

Co-ordinate Bond or Dative Bond: A bond formed by sharing of an electron pair, where one of the bonding atoms provide both the electrons to be shared, is called a co-ordinate bond or dative bond. The atom which supplies the electron pair (a lone pair) is called donor and the other atom accepting the share in this lone pair is called the acceptor.
A coordinate bond is indicated by an arrow (®) pointing towards the acceptor atom.