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8/7/2019 Chapter 2 - Part 2 -The Structure of the Atom
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THETHE ATOMICATOMIC
STRUCTURESTRUCTURE
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The existence of atoms was proposed as
early as in the 5th century by the Greek
philosophers Leucippus and his pupilDemocritus, for which they were called
atomists.
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Democritus, develop the idea of atoms. He
asked this question:
If you break a piece of matter in half, andthen break it in half again, how many
breaks will you have to make before you
can break it no further?
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Democritus thought that it ended at some
point, a smallest possible bit of matter. He
called these basic matter particles, atoms.
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The word "atom" is derived from the Greek
word "atomos", which means "indivisible".
The history of the development of atomic
models begins with John Daltons
discovery.
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J.J. Thompson (1856J.J. Thompson (1856--1940)1940)
An Englishman discovered electron. He
described the atom as a sphere of positive
charge which contains a few negatively-charged particles called electron
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ERNEST RUTHERFORDERNEST RUTHERFORD
(1871(1871--1937)1937)
Discovered proton, apositive charge in anatom. The positive charge
and most of the mass ofthe atom areconcentrated in a small,central region called thenucleus.
The electron move in aspace that is larger thanthe space occupied bythe nucleus.
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Neils BohrNeils Bohr
Proposed that the
electrons in an atom
move in shells aroundthe nucleus.
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James ChadwickJames Chadwick
James Chadwick proved the existence of
neutrons, the neutral particles in the
nucleus. Neutrons contribute
approximately to half the mass of an atom.
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Modern Atomic ModelModern Atomic Model
The atomic model in the present day is
based on the contributions of the abovescientists.
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According to the modern atomicAccording to the modern atomic
model :model :
1.The central nucleus consists of protons
and neutrons. It containing almost all themass of the atom.
2.The nucleus of an atom is very small
compared to the size of the atom
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3. the electrons are orbiting outside the
nucleus in the electron shells
4. the electrons are moving in electronshells at a very high speed and we
cannot determine the position of the
electrons at a particular time
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The subatomic particles of an atomThe subatomic particles of an atom
An atom contains three types of subatomic
particles:
proton,
neutron and
electron,
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The proton and neutron form the nucleus
at the centre of an atom.
The electron moves around the nucleus ata very high speed.
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The nucleus is positively charged because
of the presence of protons, which are
positively charged. The neutrons are
neutral.
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Proton number and nucleon numberProton number and nucleon number
Proton Number
The proton number represent the number
of protons found in the nucleus of an atom. Proton number = the number of protons
The proton number is also known as the
atomic number.
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In an atom of neutral charge, the number
of electrons also equals the atomic
number.
Hence, the proton number of an atom can
also represent the number of electrons.
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Nucleon Number
The nucleon number, also called atomicmass number or mass number, is the
number of protons plus the number of
neutrons in an atomic nucleus.
Nucleon number = Number of protons +
Number of neutrons
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The nucleon number of an atom is about
the same as the mass of the atom
because the mass of an electron is very
small and can be ignored.
Example
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Nucleon number = proton
number + number of neutrons
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Atoms of various elements differ from one
another because of the different number of
protons.
Each element has a specific proton
number.
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For example, sodium (Na) has a proton
number of 11 and oxygen has a proton
number of 8
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Symbol of ElementsSymbol of Elements
Each element is given
a name and a symbol.
Examples :
Element Symbol
Hydrogen H
Beryllium Be
Lithium Li
Argon Ar
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- Each symbol consists of one or two
letters ( H, Li)
- For elements with two-letter symbols, the
first letter is always a capital letter whilethe second letter is always a small lettter.
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The standard representation for an atom
of any element shows the proton number
and the nucleon number of the element.
A( nucleon number )
X( symbol of element )Z ( proton number )
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Sometimes an element is represented byusing only the nucleon number.
For example : 23
Na11
is reperesented by sodium - 23
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Isotopes and theirImportanceIsotopes and theirImportance
We have learnt that the number of protons
in atoms of the same element is always
the same.
However, chemists discovered that some
elements could have atoms with different
masses.
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Therefore, these atoms must have
different number of neutrons.
Such atoms are called isotopes.
Isotopes : are atoms of the same elements
with different numbers of neutrons
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Three important points to define isotopes.
1. Isotopes are different atoms of thesame element.
2. Isotopes have the same number ofprotons or same proton numbers.
3. Isotopes have different numbers ofneutrons or nucleon numbers.
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Uses of isotopes in ourdaily lives
1. There are two types of isotopes, namely
- the stable isotopes (non-radioactive)
- the non-stable isotopes (radioactive).
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2. Unstable isotopes go through
radioactive decay and emit radiation and
they are known as radioisotopes.
3. Radioisotopes have many applications
in daily life.
4. Several uses of radioisotopes in daily
life are shown as below :
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cobalt-60 is used to kill cancer cells
without surgery in patients. This treatment
is known as radiotherapy.
Gamma rays of cobalt-60 are used to
destroy bacteria in food without changing
the quality of food.
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Electron Configuration in Atom
- We have learnt that electrons occupy
orbits with definite energy level of an atom.
- These orbits with definite energy level areknown as the shell.
- Every single shell is capable of holding up
to certain amount of electrons.
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- The first shell can hold up to two
electrons. This is called a duplet.
- The second shell can hold up to eight
electrons. This is called an octet.
- The third shell can hold up to eighteen
electrons.
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- The electrons in the outermost occupied
shell are known as the VALENCE
ELECTRON.