Chapter 4: 59 - 66
Atomic Structure
Sub-atomic particles
Proton
Neutron
Electron
Symbol : e-
p+
n
Charge : -1
Charge : +1
Charge : 0
mass(grams) : 9.1 * 10 ^ -28 g
mass(grams) : 1.6 * 10 ^ -24 g
mass(grams) : 1.6 * 10 ^ -24 g
Nucleus
Orbit
Imaginary paths around nucleus
protons + neutrons
located in centre
electrons revolve
Atomic Number
no. of protons distinguishes it from the other elements
denoted by the letter "Z"
no. of protons = no. of electrons
Mass Number
protons + neutrons = mass number
denoted by letter "A"
mass number = whole number approx. of atomic mass(amu)
Bohr-Bury Scheme
Distributing electrons in different shells(orbits)
Rules
Maximum possible number of electrons in a shell = 2n^21st
1st or K Shell = 2 e
2nd or L Shell = 8 e
3rd or M Shell = 18 e
4th or N Shell = 32 e
Shells are filled in stepwise manner
if inner shells are incomplete, then electrons don't accommodate in a given shells
The outermost shell can't accommodate more than 8 electrons
even if it has more capacity, it won't accommodate because the element becomes stable after acquiring 8 electrons
Except, hydrogen and helium, who can only acquire two electrons, other all atoms can accommodate 8 electrons in the outermost shell
Main features
except Hydrogen, all atoms of elements are made up of 3 sub-atomic particles
nucleus is at centre of the atom, which accounts for the major mass
Nucleus is positively charged
Electrons are outside nucleus, in their shells, and have negligible mass
Electrons revolve rapidly in energy levels or shells or orbits
Every atom of a different element has different number of sub-atomic particles
Valence Electrons
Outermost shells = valence shell
electrons in valence shell are valence electrons
they take part in chemical reactions
Elements who have valence electrons 1 or 2 or 3(except hydrogen and helium) are metals
they can lose electrons to form cations(positively charged)
elements with 4 or 5 or 6 or 7 electrons in valence shells are non-metals
they can gain electrons to form anions(negatively charged)
Octet Rule
Atoms of all noble gases(except helium) have eight electrons
Their combining capacity is zero
Helium follows duplet rule, though it is stable, keeping in mind the 2n^2 rule
chemically active atoms have an incomplete octet
Why do atoms combine?
Except the inert gases, other all atoms of elements combine because they have an incomplete outermost shell giving it an unstable configuration
to attain stable electronic configuration i.e. 8 electrons in valence shell ; this octet rule
What happens due to redistribution of atoms?
Force of attraction develops between atoms, which binds them together to form molecules. This is called Chemical bond
Isotopes
Atoms of same element have same no. of protons, but it may be possible that they have different no. of neutrons
Although the neutrons might differ, chemical properties are the same
This is known as ISOTOPY and its varieties are known as ISOTOPES
Properties
Have similar chemical properties
Equal number of electrons and protons, leading to same electronic configuration(EC). And chemical properties are dependant upon its EC
Isotopes have different physical properties like Density, boiling point, etc
Radioisotopes
This is a difference in its physical properties, which occurs because the atom is radioactive
Extra neutron in their nuclei cause them to be unstable so the nuclei breaks up spontaneously, emitting certain types of radiation
Its Uses
Co 60 27 is used in radiotherapy to treat cancer
C 14 6 is used to know the age of a historical material
U 235 92 is used as fuel in nuclear reactors
I 131 53 is used in treatment of goiter
Radioisotopes are used in industries to detect leakage in unground oil, water and gas pipes