metas adventist school class: viii subject: chemistry...
TRANSCRIPT
METAS ADVENTIST SCHOOL
CLASS: VIII
SUBJECT: CHEMISTRY
CHAPTER 4-ATOMIC STRUCTURE
INTRODUCTION
An atom is the basic unit of the matter. There are
many different types of atom each with its own
name, mass and size .The different types of atoms
are called chemical elements .Chemical elements are
organised on the periodic table .Atom themselves
are made up of smaller particles called protons,
neutrons and electrons .Protons and electrons are in
the middle of atom they are called the nucleus.
ANCIENT VIEWS ON THE ATOMIC STRUCTURE
Leucippus and Democritus were first to develop the
concept of the atom in the 5th century BC.
Democritus proposed that the different types of
combination of this particles were responsible for
various form of matter. However this ideas were
largely ignored.
In in the 6th century Indian philosopher Maharishi
Kanada suggested that matter is made up of very
small particles .He called the smallest particle of
matter as anu. He also believe that the anu may be
made of particles which we called as paramanu.
The first scientific theory about the structure of
matter was given by JOHN DALTON.
DALTONS ATOMIC THEORY
The main points call postulates proposed by John Dalton
are:
1. Matter is made up of very small particles called
atoms.
2. All atoms of element are chemically alike.
3. Atoms of one element different from the atoms of
other element in all respects.
4. Atoms cannot be broken down by ordinary chemical
means.
5. Atoms of different elements combine in simple
whole number ratios.
6. Atoms can neither be created nor destroyed.
LIMITATION OF DALTONS THEORY
It cannot explain certain things like:
1. The cause of chemical combination.
2. The law of gaseous volumes.
3.It does not distinguish between ultimate particle
of an element and that of compound.
Later on DALTON'S theory was modified by
many scientists to explain two things:
1. Divisibility of atom.
2. Existence of negative and positive charges.
https://youtu.be/syi3pXJNe58
SUBATOMIC PARTICALES.
Discoveries in late nineteenth and twentieth century
prove that atoms are divisible. There are three main
particles present in atom- electrons, protons and
neutrons. These particles are known as a subatomic
particles or fundamental particles.
DISCOVERY OF ELECTRONS
DISCOVERY OF PROTONS
DISCOVERY OF NEUTRONS
In1932, British scientist James Chadwick performed the
experiment .He demonstrated that the rays emerging
from beryllium were composed of particles with no
charge but with the mass similar to that of photons this
particles was called neutrons.
https://youtu.be/XPqoGlKP5IY
DISCOVERY OF NUCLEUS
The nucleus was discovered by Rutherford in 1911
through the scattering experiment. The experiment was
conducted by Hans Geiger and Earnest Marsden under
the guidance of Rutherford .In this experiment a thin
gold foil was bombarded with a very small positively
charged particle called alpha particles .Most of the alpha
particles are undeflected and passed right through the
foil ,but a few of them be deflected by small angles.
The following are the conclusion drawn by Rutherford :
1.Most of the alpha particles fast straight through the
gold foil indicating that there is a lot of empty space in
the atom.
2.Some of the alpha particles deflected at small angles
to their path indicating that a positively charged body
is present at the centre of the atom.
3.Few of the alpha particles rebounded, indicating that
the centrally located body or the nucleus is dense and
hard.
https://youtu.be/7l6y3iwErM4
Modern ARRANGEMENT OF PARTICLES WITHIN ATOM
According to the modern atomic theory the arrangement
of particles with the atom are:
1. Atom is divisible into subatomic particles called
electrons, protons and neutrons.
2. There are two structural parts of atom nucleus and
orbits or shells
3. Nucleus is a central part of an atom .It contains
protons and neutrons are collectively known as
nucleons.
4. The entire mass of an atom lies in its nucleus.
5. Orbits are imaginary path traced by the electrons in
the empty space surrounding the nucleus .Each orbit
is associated with definite amount of energy called
energy level.
6. This energy levels are designated as K,L,M,N.....
starting from the innermost shell.
7. The shell lying closest to the nucleus carries the
lowest amount of energy and that lies farthest from
it and carries the highest amount of energy.
8. An atom is electrically neutral because the number
of protons and electrons present in it are same .Thus
balancing the total charge of the atom.
ARRANGEMENT OF ELECTRONS IN THE ATOM
In general the maximum number of electrons that a shell
can have is 2n²,where n is the number of shell .For
example ,the first shell can have2×1²=2electrons.The
second shell can have2×2²=8 electrons. The outermost
shell of an atom cannot have more than 8 electrons .The
arrangement of electrons in the shell is called electronic
configuration.
https://youtu.be/-jrYmggqQBk
Ko
ATOMIC NUMBER
The number of protons present in the nucleus of an atom
is called atomic number it is denoted by letter z.
Z=p=e
MASS NUMBER
The sum of the number of protons and neutrons present
in the nucleus of an atom is called its mass number. It is
denoted by letter A.
A=p + n
If we know mass number and atomic number, number of
neutrons can be calculated .
Number of neutrons=A-Z.
https://youtu.be/_S7ov25y3_M
RELATIVE ATOMIC MASS OR ATOMIC MASS
The relative atomic mass of an element is the number of
times one atom of that element is heavier than one
twelfth of the mass of an atom of carbon -12.
https://youtu.be/SdhLTfma_Eg
CHEMICALCAL BEHAVIOUR OF ATOMS
TENDENCY TO LOSE ELECTRONS:
Those atoms which have 1,2or3 electrons in their
outermost shell do not hold this electrons tightly they
lose this extra electrons and become positively charged
ions. The atom with a positive charge is called positive
Ion or cation.
TENDENCY TO GAIN ELECTRONS.
Those atoms which have 1,2or3 less electrons in the
outermost shell then a stable number, hold their
outermost electrons tightly .They gain this number of
electrons from the other atom and become negatively
charged ions. The atoms with the negative charge is
called negative Ion or anion.
TENDENCY TO SHARE ELECTRONS
If two or more atoms with the same number of electrons
in the outermost shell chemically combines, then the
atoms share the electrons.
https://youtu.be/ITNJDdDt9JQ
CHEMICAL BOND
When atoms combined chemically to become stable by
gaining losing sharing electrons ,a force is developed that
holds the atoms together chemical combination of atom
is called a chemical bond . Chemical bonding between
atoms take place mainly in two ways ionic bond and
covalent bond.
IONIC BOND
An ionic bond also called electrovalent bond, is formed
when an atom gives one or more of extra electron to
other atom which lacks one or two electrons to become
stable .The compound formed by the ionic bond are
called ionic compounds.
1. Ionic compounds are hard and rigid due to strong
force of attraction between oppositely charged ions.
2. Generally ionic compounds are solids.
3. They are generally soluble in water.
4. They have high melting and boiling points.
5. They are good conductors of heat and electricity.
COVALENT BONDS
The bond formed by sharing of electrons between two or
more atoms is called covalent bond or a molecular bond.
The compound formed by covalent bond is called
covalent compounds.
1. Covalent bonds consist of molecules held by weak
forces
2. They are generally liquids or gases
3. They have low melting and boiling points
4. They are bad conductors of heat and electricity
5. Covalent compounds are usually insoluble or in non
polar solvents like water.
https://youtu.be/kTf0TF0gJaI
VALENCY
It is defined as the number of electrons donated to
accepted by an item when it forms a chemical
compound
The items that donates electron is said to have
electropositive Valency whereas atom that accept the
electron is said to electronegative Valency.
Variable Valencies
https://youtu.be/xXO65qUv6LM
WORKSHEET 1
ANSWER THE FOLLOWING QUESTION IN ONE
SENTENCE.
Que.1 Which atomic particle is neutral?
Que.2 Write the electronic configuration of an atom that
has atomic number 15.
Que.3 Name the subatomic particles.
Que4.The atom of an element has 25 electrons and 30
neutrons write down its atomic number and mass
number.
Que.5 What is the maximum number of electrons in each
of the first two shell of an atom?
FILL IN THE BLANKS
1. The circular path in which electrons revolve around
the nucleus of an atom are called the _________
2. The neutron was discovered by________
3. The electronic configuration of sulphur
is__________
4. An isotope of an element has different___________
5. Valence electrons sodium atoms are__________
CORRECT THE FALSE STATEMENT
1.Deuterium is an isotope of chlorine.
2.Neil Bohr suggested that an atom has nucleus.
3. An atom always has an equal number of protons and
neutrons.
4. Potassium has 2 electrons in its M shell.
5 . Elements having same number of neutrons are called
isotopes.
CHOOSE THE ODD ONE OUT.GIVE SCIENTIFIC REASON
FOR YOUR ANSWER.
1. Electron, proton ,neutron ,molecule
2. Dalton, Lavoisier, Bohr, Rutherford.
3. Carbon, Protium , Deuterium ,Tritium.
4. Uranium ,barium ,thorium ,radium
WORKSHEET 2
COMPLETE THE FOLLOWING TABLE
Sr. no Element Ion 1. Nitrogen (a)______ 2. (b)________ S²- 3. Calcium (c)_______ 4. (d)_______ Cr³+
Element Atomic number
Mass number
Proton Electron Neutron
Hydrogen 1 1 ___ ___ ___ Oxygen
___ 16 ____ 8 _____
Sodium
____ _____ 17 ___ 18
MATCH THE FOLLOWING
COLUMN A. COLUMN B
1.Protons A)equal to number of protons and neutrons
2. Atomic number. B) formed by sharing of electrons
3. Ionic bond. C) positive charged particle
4.Covalent bond. D)equal to the number of protons
5.mass number. E)formed by transfer of electrons
DRAW ORBITAL STRUCTURE
1.Argon
2.Fluorine
3.carbon
4.Silicon
Electronic configuration of elements A And B are given below .Identify
the elements .
1. A – 2,8,8,1
2. B- 2,8,4
Answer the following questions in short.
Que1. Write the postulate of Dalton’s atomic theory.
Ans. 1.Matter is made up of small particles called atoms
2.All atoms of an element are chemically alike.
3. Atoms can neither be created nor destroyed.
Que2. Describe Thomson's atomic model.
Ans. 1. An atom consists of a sphere of positive charge
with negative charge electrons embedded in it.
2.An atom is electrically neutral. It has no negative
or positive charge.
Que3. State the location of electrons protons and
neutrons in an atom
Ans. Protons and neutrons are present in the nucleus of
an atom and electrons revolves around the nucleus in the
orbits.
Que4. What are isotopes ?Name the isotope of carbon.
Ans. Atoms of an element having same atomic number
but different mass number is called isotopes.
Isotope s of carbon are Carbon-12,carbon-13,carbon-14.
Que5.How are atomic number and mass number related
to the number of protons and neutrons?
Ans. Atomic number (Z) is the number of protons and
mass number(A) is number of protons and neutrons
present in the nucleus. Thus the number of
neutrons =A-Z.
Que6. Describe the nucleus of an atom.
Ans. The nucleus is the core of each atom .It contains
protons and neutrons .The nucleus has positive charge
because it contains a positively charged protons.
Que7 .An atom has an atomic number 19 and mass
number 39.How many electrons protons and neutrons
does it have?
Ans. Z=19
A=39
Z=e=p
e =19
p=19
n =A-Z
= 30-19
n =20
Que8. What is meant by term Valency ?Explain it in
terms of electrons donation and acceptance.
Ans. Valency of an element it is defined as the number
of electrons donated or accepted by atom when it forms
a chemical compound.
The atom that donate electrons is said to have
electropositive valency , whereas the atom that accept
electron has a electronegative valency..
Que9.what are the various ways in which an atom of an
element can achieve the noble gas configuration?
Ans.1.By complete transfer of electrons.
By sharing of electrons.
Que10. write the nearest noble gas of each of the
following.
(a) Oxygen
Ans. Neon
(b) Potassium
Ans. Argon
(c) Magnesium
Ans. Neon