Download COURSE NAME Chemistry for B.Sc. 1st Year PAPER TITLE

COURSE NAME
Chemistry for B.Sc. 1st Year
PAPER TITLE
Physical chemistry
Paper III, CH-103
TOPIC
Basics of Atom
OBJECTIVE:
After going through this e-content the student will be able to:

Gain an understanding about atom, its structure & subatomic
particles like Proton, Neutron & Electron.

Know about Atomic Number, Mass Number & Atomic Mass.

Know about the forces that hold an atom together.

Have an idea about Isotopes, Ions and the formation of
molecules.
Introduction
Fear, Anxiety, Happiness, sorrow.......there are so many emotions that we feel in our
day to day lives. Have you ever thought why do we feel so many emotions?
Actually these emotions are nothing much but chemial reactions that happen within
our bodies and they give rise to various hormones that make us feel the way we do.
So many such mystries and phenomenas ocurring within our bodies and around us
can be explained through the subject of chemistry.
But understanding chemistry would not be possible without understanding the
most basic unit of this science i.e. the atom. And in this discussion we would be
learning about the atom, its structure, its fundamental particles like the proton,
neutron and the electron,
weight.
the Atomic mass, the mass number and the atomic
But before we go any further in this discussion let us first define the subject of
chemistry. So formally speaking, chemistry can be understood as a science
concerned with the composition, structure, and properties of matter, as well as the
changes that matter undergoes during the chemical reactions. In other words we can
call it is a physical science for studies of various atoms, molecules, crystals and other
aggregates of matter whether in isolation or combination, which incorporates the
concepts of energy and entropy in relation to the spontaneity of chemical processes.
Atom & its structure
So coming back to the atom, Everything that we see, hear, feel, or touch is composed
of atoms. Yet an individual atom is so small that it takes millions of millions of
millions of them to form the fingernails or to form the sound that we hear when a
pin drops. So an atom can be understood as the most basic building block of matter,
and the smallest unit of an element that retains the properties of that element. So
what exactly do we mean by matter? Actually matter is anything that has both mass
and volume or anything that occupies space. For example, a car would be said to be
made of matter, as it occupies space, and has mass.
Let us understand one more term here that is element. When a material is formed
from only one type of atom then it is called as an element. If a material is made from
a combination of more than one kind of atom then it is called a Compound.
We can have a look here at few familiar materials to understand the difference
between element and compound.
Common Name
What Atoms is it made Element or Compound?
of?
Water
Hydrogen(H)
and Compound
Oxygen(O)
Salt
Sodium(Na)
and Compound
Chlorine(Cl)
Diamond
Carbon(C)
Element
Oxygen
Oxygen(O)
Element
So we can say that all matter is made up of elements which are fundamental
substances which cannot be broken down by chemical means. There are 92 such
elements that occur naturally. The elements hydrogen, carbon, nitrogen and oxygen
are the elements that make up most living organisms. Some other elements that can
be found in living organisms are: magnesium, calcium, phosphorus, sodium,
potassium.
Although we will find that atoms are divisible into smaller particles, still it is the
smallest piece of matter because we cannot divide it any further without losing the
chemical properties of the matter. For example, we may divide the graphite from the
ashes in a barbecue into smaller and smaller pieces, until we eventually have only
single carbon atoms. These atoms still have all the properties of carbon, and are
distinct from other types of atoms. But if we still further break these atoms apart, we
will loose the chemical properties of carbon, and the pieces which we will get would
be indistinguishable from the particles that we would get by smashing any other
material. So thus we can understand that though atoms are divisible into smaller
particles still they are the smallest piece of matter.
Let me add here that understanding the atom can be one of the most powerful tools
in the understanding of chemistry. But the concept of atom is a difficult one to
understand, why so? Simply because we are unable to resolve the world into
individual atoms in our daily lives. So unless we work in a specialized laboratory,
we will never see, touch or feel individual atoms. So understanding the structure of
the atom can be very difficult, because there are no simple experiments that can be
performed to make it clear what atoms look like or how they behave.
So if we want to understand the structure of an atom we can think of an atom like a
tiny solar system. In the center of the atom is the nucleus which is a cluster of
protons and neutrons. The protons have a positive electric charge and the neutrons
are electrically neutral. The nucleus makes up of almost all of the atomic mass or
weight. And around this
nucleus we have in orbitals we have the negatively
charged electrons.
So Nucleus can be defined as the core of the atom, which is made up of protons and
neutrons. The nucleus accounts for nearly all of the mass of the atom, but only a tiny
fraction of its volume.
So an atom consists of three main parts: protons, neutrons, and electrons. These
parts are known as subatomic particles or fundamental particles. So an atom would
always have the same number of protons having the positive charge and the same
number of electrons having the negative charge to make itself electrically neutral.
Let us here see proper definitions of these subatomic particles one by one.
Proton was the first subatomic particle to be discovered, in the year 1886, by E.
Goldstein and can be defined as “A subatomic particle which has one unit mass and
one unit positive charge”.
The second subatomic particle that is electron can be defined as “A subatomic
particle which bears charge -1.60x10-19 coulomb and has mass 9.1x10-28 g”. In other
words an electron can be defined as “A particle bearing one unit negative charge
and mass 1/1835th of a hydrogen atom”. The knowledge about the electron was
derived by J. J. Thomson in the year 1896.
In the year 1932 Sir James Chadwick discovered the third subatomic particle, the
neutron, and it was defined as “A subatomic particle which has a mass almost equal
to that of a proton and has no charge”.
Now let us summarized these three basic particles that is the fundamental particles
of an atom:
Particle
Symbol
Electron
Mass
Charge
amu
grams
Units
Coloumbs
e-
1
1835
9.1x10-28
-1
-1.60x10-19
Proton
P+
1
1.672 x 10-24
+1
+1.60x10-19
Neutron
N or no
1
1.674 x 10-24
0
0
Protons and neutrons are responsible for most of the atomic mass let us take an
example here of an 150 kg man, so in a 150 kg man 149 pounds (lbs) and 15 ounce
(oz) would be due to the protons and neutrons respectively while only 1 ounce (oz).
would be due to the electrons.
So it is the number of protons that determines the atomic number, e.g., H = 1.
Though the number of protons in an element is constant like H=1, Ur=92, the
neutron number may vary, and so would the mass number.
Atomic number, Mass number & Atomic Mass
Here let us understand few important terms before we move any further in the
disucssion, these terms are the Atomic number, Mass number and Atomic mass:
Atomic number of an element is equal to the number of protons in the nucleus of the
atom of that element.
Now-a-days the atomic number is also refered to as the Proton number.
Now what is the Mass Number? The total number of protons and neutrons present
in the nucleus of an atom is referred to as the Mass Number, A, of an atom. In
situations where we do not need to differentiate between protons and neutrons, we
call these particles as nucleons. So let us take an example of Sodium here, Sodium
has 11 protons and 12 neutrons. The mass number would be the total number of
protons plus the total number of neutrons which would be equal to 23. In other
words we can say that the mass number is the total number of particles present
within the nucleus. The mass number and the atomic number would be written on
the top left and the bottom of the left side of the chemical symbol of any element.
Here, we can take the example of Sodium again, the chemical symbol or sodium is
Na and 23 is the mass number would be written on the top left of Sodium and 11
that is atomic number would be written on the left bottom of Sodium.
23
11
Na
Obviously, the mass number of an atom would be a whole number. Since electrons
have practically no mass, the entire atomic mass is due to the protons and neutrons,
each of which has a mass almost exactly equal to one unit. Therefore, the mass
number of an atom can be obtained by rounding off the experimental value of
atomic mass to the nearest whole number. For e.g. the atomic mass of sodium and
fluorine obtained by experiment is 22.9898 and 26.9815 atomic mass unit (amu)
respectively. Thus their mass numbers would be 23 for sodium and 27 for fluorine if
we round off the experimental values in case of sodium and fluorine.
Knowing the atomic number (Z) and mass number (A) of an atom, we can tell the
number of protons and the number of neutrons present inside the nucleus. Let us
see how: By definition:
Atomic Number, Z = Number of protons
Mass Number, A = Number of protons + Number of neutrons
So if we want to find out the number of neutrons, we can do it by this expression.
N=A–Z
So we get the total number of nuetrons present within the nucleus. Here let us
understand one more term that the atomic mass. The atomic mass may be
considered as the total mass of protons, neutrons and electrons in a single atom.
Since atomic number is the number of protons in an atom and atomic mass is the
mass of protons, neutrons, and electrons in an atom, it seems obvious that
increasing the number of protons would increase the atomic mass. However, if you
look at the atomic masses in a periodic table you will see that Cobalt, atomic number
of 27, is more massive than Nickel, atomic number of 28. Uranium, having an atomic
number 92, is more massive than Neptunium, having an atomic number 93. And
why is it so?
This is because many atoms don't have the same number of neutrons as they have
the number of protons. So they do not have the equal number of neutrons and
protons within the nucleus. We can understand that ‘increasing atomic number
doesn't always equate to increasing mass’. In other words, several isotopes of an
element may exist. Now what do we mean by isotopes. By definition, the atoms of
an element which have the same number of protons and different number of
neutrons are called Isotopes. If a sizeable portion of an element of lower atomic
number exists in the form of heavy isotopes, then the mass of that element may be
heavier than that of the next element. If there were no isotopes and all elements had
a equal number of protons and neutrons, then atomic mass would be approximately
twice the atomic number because the protons and neutrons don't have exactly the
same mass... the mass of electrons is so small that it is negligible. Thus, the atoms of
each chemical element have a different nucleus. Let us take the example of
hydrogen which has
one proton and no neutrons. An atom of nitrogen has 7
protons and 7 neutrons. Heavy elements have a large number of protons and
neutrons. For example, the most common isotope of uranium, uranium-238 has 92
protons and 146 neutrons in its nucleus.
Another example is Oxygen, with atomic number of 8 and it can have 8, 9, or 10
neutrons in it nucleus.
Here let me tell you about the periodic table? The periodic table is a collection of
information about every element in the universe. It is a tabular arrangement of the
chemical element according to the atomic number as based on the periodic law. It
gives information about the chemical symbol of each element, the atomic
weight/mass, also the name and the charge. The layout of the table has been refined
and extended over time, as new elements have been discovered and new theoretical
models has been developed to explain chemical behaviour. Different periodic tables
even list different numbers for atomic masses.
Now let us come back to the atom and understand how electrons are organized
around the nucleus?
All atoms would like to attain electron configurations like that of noble gases. That
is, they would love to have completed outer shells. This makes the element
completely inert. Helium is an example of a noble (inert) gas. It is not present in
organisms because it is not chemically reactive.
Atoms can form stable electron configurations like noble gases by:
1. losing electrons
2. gaining electrons
3. sharing electrons.
For a stable configuration each atom must fill its outermost energy level. In the case
of noble gases that means eight electrons in the last shell have to be filled up, with
the exception of Helium which has only two electrons.
Atoms that have 1, 2 or 3 electrons in their outer levels will tend to lose them in
interactions with atoms that have 5, 6 or 7 electrons in their outermost levels. Or
putting it the other way, atoms that have 5, 6 or 7 electrons in their outer levels will
tend to gain electrons from atoms with 1, 2 or 3 electrons in their outer levels. Atoms
that have 4 electrons in the outer most energy level will neither tend to totally lose
nor totally gain electrons during the interactions.
The mass of an atom is primarily due to the mass of the nucleus and the mass of a
single electron is almost 2,000 times less than the mass of a single proton or a single
neutron. However, the nucleus occupies less than 1% of the total volume of the
atom. Most of the atom is actually empty space. Let us imagine that the atom was
the size of your classroom, then the nucleus would be the size of a grain of sand in
the entire class.
What holds an atom together?
Here let me ask you a question. Have you ever wondered what holds an atom
together? Let us try to answer this one.
Actually the atom is held together by two fundamental forces that exist in the
universe: first is the electromagnetic force and second is the strong nuclear force.
The strong electromagnetic force is one that we encounter in everyday experiences.
For example, the attraction between the north pole and the south pole of a bar
magnet is due to the electromagnetic force. Electrical charges behave similarly, due
to the same force i.e. like charges repel and unlike charges attract. The
electromagnetic force is responsible for keeping the electrons attached to the nucleus
in an atom. Since the nucleus is made of protons and neutrons, it is positively
charged because protons have a positive charge and neutrons are electrically
neutral, while the surrounding electrons are negatively charged. We have all heard
the phrase “opposites attract” and since the nucleus and electrons have opposite
charges they attract each other and it is this attraction which binds the atoms
together. The strong nuclear force is one that we do not encounter in daily lives and
is somewhat more abstract. A logical question is: “If we cannot make tangible
observations about this force, then how do we know that it exists?” Actually, it
comes from the thinking about the nature of the elctromagnetic force. The
elctromagnetic force is responsible for opposite charges attracting, such as occurs
between the positively-charged nucleus and the negatively –charged electrons in an
atom. However, the elctromagnetic force would then also be responsible for like
charges repelling each other and the question would then arise that how do multiple
protons live together inside the nucleus? For eg: helium which we use to fill the
balloons to make them ligheter than air, is an element with two protons in the
nucleus. So only if the electromagnetic force existed, then we could argue that
helium could not exist, because the protons in the nucleus would repel each other
and the nucleus would fly apart. But then, however, since we know that helium
does exist, we know that there must be some another force which makes the nucleus
stable and this force is the strong nuclear force.
Atoms can be in the ground state, which means that all the electrons are in their
lowest energy levels, or they can be in the excited state. When atoms make
transitions from excited states to the states with lower energy levels, they emit
photons. Now what are these photons? A Photon is an elementary particle, the
quantum of the electromagnetic field and the basic unit of light and all other forms
of electromagnetic radiation. The study of the photons is called Spectroscopy. Lasers
work because of the photons emitted when excited atoms de-excite to the ground
state.
Here let us discuss one more thing i.e. the making of molecules or how are
molecules formed?
Actually if two or more atoms share electrons then they form a bond. When atoms
form bonds, they form molecules. So let us take up an example here of water. What
is the formula for Water? The formula is H2O which means that it is made up of two
hydrogen atoms (H) and one oxygen atom (O). So when atoms bond together they
can make larger building blocks of matter called molecules.
And let us see what happens when chemical reactions occur? Chemical reactions
involve interactions between the electrons of one atom and the electrons of another
atom. Atoms which have different amounts of electrons and protons have a positive
or negative electrical charge and are called Ions.
Conclusion
Before concluding this discussion let us go through some useful facts about atoms:

Atoms cannot be divided using chemicals. They do consist of parts, which include
protons, neutrons, and electrons, but an atom is a basic chemical building block of
matter.

Each proton has a positive electrical charge. The charge of a proton and an
electron are equal in magnitude, yet opposite in nature. So we can say that
electrons and protons are electrically attracted to each other.

Each neutron is electrically neutral. In other words, neutrons do not carry any
charge and are not electrically attracted to either the proton or the electron.

Protons and neutrons are about the same size as each other and are much larger
than electrons.

The mass of a proton is essentially the same as that of a neutron but the mass of a
proton is many times greater than the mass of an electron.

The nucleus of an atom contsists protons and neutrons. The nucleus carries a
positive electrical charge.

Electrons move around outside the nucleus.

Almost all of the mass of an atom is in the nucleus; almost all of the volume of an
atom is occupied by electrons.

The number of protons, also known as the atomic number, determines the
element. Varying the number of neutrons results in Isotopes. Varying the number
of electrons results in Ions. Isotopes and Ions of an atom with a constant number
of protons are all variations of a single element.

The particles within an atom are bound together by powerful forces. In general,
electrons are easier to add or remove from an atom rather than a proton or
neutron. Chemical reactions largely involve atoms or groups of atoms and the
interactions occur between their electrons.
With this we come to the conclusion of this e-content module on atoms. I hope you
found this e-content informative and interesting. Actually this was a basic
discussion on chemistry. Or should I say it was just the tip of the iceberg. As you
will explore this iceberg in detail or dive deeper into the vast sea of chemistry, I am
sure that you will find this subject interesting and enjoyable.