ICSE Chemistry Class 8 Notes Chapter 4 Atomic Structure
→ Atom in Hindi is called Paramanu. Kanada an Indian philosopher conceived that matter is made up of small particles called Paramanus (atoms).
→ Atom is a neutral particle made up of two parts solid, massive, +vely charged very small inner part called nucleus and hollow outer bigger part called extra nucleus.
→ As atom has no charge, number of protons (+ve charge) equal to number of electrons (-ve charge).
→ Electrons are revolving around the nucleus in different orbits, each electron carries, negative charge, has negligible mass.
→ Protons are present in nucleus along with neutrons. Each proton carries 1 positive charge and neutron has no charge. Mass of 1 pron and 1 neutron is same as mass of 1 hydrogen atom.
→ Dalton’s atomic theory :
(a) Matter is made up of small particles – atoms.
(b) Atom are indivisible and cannot be created nor destroyed.
(c) Atoms of same element are alike in all respect of their size, mass etc. i.e. atoms of sodium are all similar, atoms of H2 are all similar but atoms of H2 are different from atoms of sodium.
(d) Atoms combine with other atoms in simple whole number ratio to give compounds or molecules.
(e) Atom is the smallest unit of matter which takes part in a chemical reaction.
→ Modern atomic theory : Nearly all the postulates of Dalton Atomic Theory are contradicted.
Now,
(a) Atoms are destructible.
(b) Atom is made up of protons, electrons and neutrons and even more sub-atomic particles have been discovered.
(c) Isotopes show that atoms of the same element may not be allike.
→ Cathode rays are nothing but beam of -vely charged particles – electrons.
→ Electrons were discovered by J.J. Thomson. Protons were discovered by – Goldstein. Neutrons were discovered by – Chadwick.
→ Electrons revolved around the nucleus in fixed ‘orbits’ called energy levels.
→ Energy levels are represented by 1, 2, 3 or K, L, M. Starting from nucleus to outwards.
→ Electronic configuration or distribution of electrons is given by the formula 2n2 where n is the number of shell or orbit there cannot be more than 2 electrons in first orbit 2(1)2 = 2.
In 2nd orbit max. of electrons can be 2(2)2 = 8.
In 3rd orbit 2(3)2 = 18 and so on but in the outer most orbit cannot be more than 8 electrons and 18 in the penultimate orbit. A new shell starts only when the previous shell is filled completely.
→Atomic number is the number of protons in the atom Z = p but number of protons is equal to number of electrons as atom is electrically neutral.
∴ Z = p = e
→ Mass number : Total number of protons and neutrons in the nucleus of atom.
A = p + n
→ Isotopes are the atoms of the same element having same atomic number [Z] but different mass number [A].
→ Isotopes of hydrogen 22H, 21H, 31H, Mass number
→ Valency is the number of – hydrogen atoms which can combine with [or displace] – one atom of the element or radical forming a compound.
OR
Is also the number of electrons – donated or accepted by an atom.
→ Atom which loses electrons becomes electro-positive ion and is attracted by cathode and are called cations.
→ Metals lose 1,2 or 3 electrons to achieve the stable configuration of nearest noble gas.
→ Non-metals can accept 1, 2, 3 electrons to achieve stable configuration of nearest noble gas. By accepting the atom becomes electro-negative ion and are attracted by anode and are called anions.
→ Metal – after losing electrons Na+, Ca++, Al+++.
Non-metal – after accepting electrons Cl–, O––, N–––