the structure of an atom
TRANSCRIPT
The structure of the Atom
By Aditi Atmasidha
What is an atom?
O The word atom is a Greek word that
means Indivisible.
O It is the building block of a molecule.
O Atoms of same element are alike, but
atoms of different element are different.
Early Theories of Atoms and Discoveries
O Dalton’s Atomic Theory :
O All matter is composed of tiny particles called atoms.
O Atoms cannot be divided into smaller particles and cannot be destroyed.
O Atoms of an element have the same mass and properties.
O Atoms of different elements have different masses and different properties.
O Atoms combine in simple whole numbers when they form compounds.
Early Theories of Atoms and Discoveries
O Dalton’s Discoveries:
O Carbon- mono- oxide { CO }
O Carbon- di- oxide { CO2 }
O J.J.Thomson atomic model:
O He compared atom as a plum pudding.
O The positively charged protons are
compared as pudding, while negatively
charged electrons are raisins in the
pudding.
Early Theories of Atoms and Discoveries
O Thus according to this model- atom is a
homogenous sphere of positively charged
protons in which negatively charged
electrons are embedded.
O The number of positive and negative
charges are equal hence atom is
electrically neutral.
Early Theories of Atoms and Discoveries
Early Theories of Atoms and Discoveries
O Ernest Rutherford’s atomic model :
O The atomic model is compared with solar system.
O The sun is compared as positively charged nucleus while different planets orbiting the sun are negatively charged electrons.
O Alpha particle scattering experiment :
O A thin sheet of gold is hanged in the middle and scintillation screen was surrounded around the gold foil to detect alpha particles, { Helium Nuclei }.
Early Theories of Atoms and Discoveries
O He bombarded alpha particles on the gold foil
and eventually detected the marks or spots
made by alpha particles on the screen.
O He assumed that all the alpha particles would
pass straight through the gold atoms and
make a mark directly behind the gold sheet.
O Observations: most of the alpha particles
made a mark directly behind the metal sheet,
some made marks all around the screen.
Early Theories of Atoms and Discoveries
O Conclusion: the alpha particles passed
through the metal foil or gold foil means
there must be a large amount of empty
space in an atom which allows the alpha
particles to pass through them, scattering
of alpha particles all around the screen
means they must hit and bounce off
something inside the atom that repels
them.
O Rutherford’s atomic model
Early Theories of Atoms and Discoveries
O The atom has positively charged center which is named as nucleus.
O The negatively charged electrons are surrounded around the nucleus.
O There is a lot of empty space in an atom.
O He thought presence of other particles in the nucleus which are similar to proton but do not have any electrical charge.
O He called these particles as neutral doublet, which must be made up of protons and electrons together.
Early Theories of Atoms and Discoveries
Early Theories of Atoms and Discoveries
O James Chadwick’s alpha particle experiment :
O Alpha particles are bombarded on beryllium atoms and knocked out particles that had similar mass to protons but no electrical charge these particles were named as neutrons.
O But these particles are not composed of protons and electrons as suggested by Rutherford.
Structure of an Atom
O Diameter of an atom is about 0.1 to 0.5 nanometers.
O The subatomic particles are:
O Protons { p+ }
O Electrons { e- }
O Neutrons { n }
O The protons and neutrons make up the center called nucleus. It is positively charged.
Structure of an Atom
O Negatively charged electros orbit the positively charged nucleus at the speed of light.
O The number of positively charged protons = the number of negatively charged electrons. Also the charges are equal in magnitude { Magnitude means +1-1 for example } this balancing of charges makes the atom electrically neutral, therefore the overall charge of an atom is zero.
Structure of an Atom
Structure of an atom
O Electrons move around the nucleus in specific orbits. These orbits are called shells or energy levels, electrons in the same shell have same energy while different orbits have different energies.
O The distribution of electrons in different shells is given by formula 2nsquare. Where n is the shell number, according to this formula, maximum capacity of shell 1 is 2, shell 2 is 8 and so on.
Isotopes
O Atoms of same element that have different
number of neutrons are called isotopes.
O Example: Hydrogen shows 3 Isotopes.
O Hydrogen, Deuterium, Tritium
O Hydrogen also known as Protium - 1
proton, no neutron
O Deuterium- 1proton, 1 neutron
O Tritium- 1 proton, 2 neutrons
Isotopes of Hydrogen/ Protium
Isotopes of Carbon
Isotopes of Chlorine
Atomic Number & Atomic Mass Number
O Number of protons or electrons in an atom is called atomic number. It is represented by Z.
O Atomic number of carbon = Z = 6 , that is it contains 6 protons and 6 electrons.
O Sum of number of protons and neutrons together in nucleus is called atomic mass number. It is represented by A.
O Atomic mass number of chlorine = A = 35 , that is it contains 17 protons and 18 neutrons.
O A = Z + n { where n is number of neutrons }.
General representation of an element:
O A = atomic mass number
O X = symbol of an element
O Z = atomic numberA
X
Z
Sodium Atom
23
Na
11
Numerical on Atomic number and Atomic mass number
O Atomic mass of an element is 20 and it contains 9 protons, find number of neutrons?
O A = 20 , n{ P } = Z = 9 , n = ?
O A = Z + n
O 20 = 9 + n
O 20 – 9 = n
O n = 11
O There are 11 neutrons.
Brief Of Atoms
O http://www.youtube.com/watch?v=Djl1YaD
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