the presentation for students

1. ATOMS AND MOLECULES
MS. S. ANGEL BAKYAVATHI
M.SC NEUROSCIENCE

2. ATOMS
• All matter is composed of atoms.
Atoms are composed of
-protons – positively charged particles
-neutrons – neutral particles
-electrons – negatively charged particles
Protons and neutrons are located in the
nucleus. Electrons are found in orbits
surrounding the nucleus.

3. STRUCTURS OF AN ATOM

4. J.J. Thomson Plum Pudding Model
• The first person who discovered
electrons, Sir J.J. Thompson, put
forth his 'Plum Pudding' Model of
an atom. He believed that the
atom was a sphere with a positive
charge and had electrons
embedded in it.

5. J J THOMSON MODEL OF ATOM

6. Rutherford's Atomic Model
• Ernest Rutherford through his
experiment of bombardment of
alpha particles through a foil of
gold atoms, he observed that alpha
particles passed straight through
the gold foil without deflection. This
showed that there is a lot of empty
space in an atom.

7. The Gold Foil Experiment
Rutherford’s Atomic Theory
Beam of alpha
particles
Source of alpha
particles
Slit
Deflected
particle
Undeflected
particle
Screen
Gold atoms
Alpha particles
Alpha particles
Nucleus

8. • Next, he observed that some alpha
particles were deflected through small
and large angles. This proved that
there was a 'center of positive charge'
in an atom. Only few alpha particles
rebounded after hitting the gold foil
and this proved that the nucleus is
very dense and hard.

10. Bohr's Model of the Atom
• In 1913, Neil Bohr proposed a theory to
explain the stability of an atom, where
electrons revolved around the nucleus at
certain energy levels. Each orbital had a
different radius and electrons revolving
in a particular orbit did not radiate
energy. This happened even if the
electron had accelerated motion around
the nucleus. Bohr put forth some
postulates that supported his atom
diagram.

11. • Bohr's Postulates:
1.The electrons are in orbit around the
nucleus. The electrostatic forces hold the
electrons in an orbit.
2.An electron can be only in a particular orbit
and when it is present in that particular
orbital, it does not radiate energy.
3.When an electron falls from a higher energy
state to a lower state, only then it emits
radiation.

12. 4.The radii of the orbitals are quantized and
each energy state has a specific radius that
is proportional to h/2n.
5. The angular momentum of electron will
be mvr = nh/2n, where n is the principle
quantum number.

13. JAMES CHADWICK DISCOVERY OF
NEUTRONS
• In 1932, James Chadwick discovered
the neutron with his experiment.
• Chadwick's experiment included
projecting alpha particles toward
Beryllium "the light element". The
emitted particles after enduring the
Beryllium target (becomes neutrons)
and fall on the paraffin wax. The
paraffin wax in turn releases another
type of particle (protons).

15. PROPERTIES OF AN ATOM:
Every different atom has a characteristic
number of protons in the nucleus.
atomic number = number of protons
Atoms with the same atomic number have the
same chemical properties and belong to the
same element.

16. ISOTOPES
Each proton and neutron has a mass of
approximately 1 dalton.
The sum of protons and neutrons is the
atom’s atomic mass.
Isotopes – atoms of the same element that
have different atomic mass numbers due to
different numbers of neutrons.

17. ISOTOPES

18. NEUTRAL ATOMS AND IONS
Neutral atoms have the same number of
protons and electrons.
Ions are charged atoms.
-cations – have more protons than
electrons and are positively charged
-anions – have more electrons than protons
and are negatively charged

19. Electrons can be transferred
from one atom to another, while still retaining
the energy of their position in the atom.
-oxidation = loss of an electron
-reduction = gain of an electron

20. VALENCE ELECTRONS
Valence electrons are the electrons in the
outermost energy level of an atom.
An element’s chemical properties depend on
interactions between valence electrons of
different atoms.

21. IONIC BONDS
• Ionic bonds are formed by the attraction of
oppositely charged ions.

22. IONIC BONDS
• ELECTRONEGATIVE ATOM:
If an atom gains electron
from another atom is called electronegative
atom.
• ELECTROPOSITIVE ATOM:
If an atom loses electron to
another atom it is called electropositive
atom.

23. COVALENT BONDS
• Covalent bonds form when atoms share 2
or more valence electrons.
• Covalent bond strength depends on the
number of electron pairs shared by the
atoms.
single < Double < Triple
bond bond bond

25. DOT AND CROSS DIAGRAM

26. • Sodium ions have the formula Na+, while chloride
ions have the formula Cl-. You need to show one
sodium ion and one chloride ion. In the exam, make
sure the dots and crosses are clear, but do not worry
about colouring them.

27. QUANTUM NUMBERS
• An integer or half integer that denotes the
value of a physical quantity is known as a
quantum numbers.
• The energy levels of electrons can be
explained by 4 quantum numbers:
Principal quantum number
Azimuthal quantum number
Magnetic quantum number
Spin quantum number

28. PRINCIPAL QUANTUM
NUMBER
• The quantum number which gives the
distance of the orbit from the nucleus is
called principal quantum number.
• It is denoted by 1,2,3,4,5,6 and 7 or by the
letters K,L,M,N,O,P and Q.
• Each orbit can accommodate a particular
number of electrons.
• It is given by the formula 2n2 ,where n
refers to the orbits.

29. • The number of electrons that can
accommodate in different shells are
K Shell -2*12 =2 electrons
L Shelll-2*22 = 4 electrons
M Shell-2*32 = 18 electrons
etc….

31. AZIMUTHAL QUANTUM NUMBERS
• Azimuthal quantum number and also known
as orbital angular momentum, is denoted by l.
• It describes the shape and the type of orbital.
• There are 4 types of orbitals i.e. s, p d and f.
• The value of l ranges from 0 to n-1.
• The shape of the s-orbital is spherical, p-orbital is
dumb-bell shaped. The shapes of d and f orbital are
very complex and cannot be determined easily. It has
been observed that d-orbital are pear shaped.

32. AZIMUTHAL QUANTUM NUMBERS

33. MAGNETIC QUANTUM NUMBERS
• The magnetic quantum number which is denoted by
m . This describes the spatial orientation of the
orbital .
• The s-orbital has only 1 possible position, p-orbital
has 3 possible positions, d-orbital has 5 possible
positions and f-orbital has 7 possible positions.
• The number of possible positions in an orbital is
given as 2l+1.

35. SPIN QUANTUM NUMBER
• The spin quantum number is denoted by (ms). It
deals with the spin of an electron in a sub-shell about
an axis.
• Spin quantum number describes the intrinsic
electronic spin in which +12 denotes the counter-
clockwise spin of an electron and −12 represents the
clockwise spin.
• The maximum number of electrons in an orbital is
two and these two electrons should have opposite
spins.

36. SPIN QUANTUM NUMBER