Knowt
Chapter 4-Structure of the Atom
Charged Particles in Matter
By 1900, it was understood that although an atom was an indivisible particle, it also contained at least one subatomic particle, the electron, which J.J. Thomson had discovered.
Before the electron was named, E. Goldstein found novel radiations in a gas discharge in 1886 and named them canal rays.
These radiations, which were positively charged, eventually resulted in the identification of another subatomic particle.
This subatomic particle had a charge that was similar to the electron's in magnitude but different in sign.
Its mass was around 2000 times greater than that of an electron. It was given the name proton.
The mass of a proton is taken as one unit and its charge as plus one.
The mass of an electron is considered to be negligible and its charge is minus one
It appeared that protons and electrons made up an atom, with their charges being balanced by one another. Protons also appeared to be inside the atom, as they were more difficult to remove from an atom than electrons.
The Structure of an Atom
Dalton's atomic hypothesis, which we studied, claimed that the atom was unbreakable and indivisible. However, the failure of this component of Dalton's atomic theory was caused by the finding of two fundamental particles (protons and electrons) inside the atom.
It was then considered necessary to know how electrons and protons are arranged within an atom. For explaining this, many scientists proposed various atomic models
The existence of neutrons in an atom's nucleus was discovered by J. Chadwick. In other words, an atom is made up of three subatomic particles: electrons, protons, and neutrons. Protons are positively charged, neutrons have no charge, and electrons have a negative charge. About 1 /2000 times the mass of a proton, an electron has.
THOMSON’S ATOMIC MODEL
The atom's model, which Thomson proposed, resembled a Christmas pudding,The electrons, in a sphere of positive charge, were like currants (dry fruits) in a spherical Christmas pudding.
Thomson’s proposal
An atom is made up of a positively charged sphere with embedded electrons.
The magnitude of the negative and positive charges is the same. As a result, the atom as a whole has no electrical charge.
RUTHERFORD’S ALPHA-PARTICLE SCATTERING EXPERIMENT
Ernest Rutherford was curious about the arrangement of the electrons in an atom. For this, Rutherford created an experiment. Fast moving alpha particles were made to fall on a thin piece of gold foil in this experiment.
Because he wanted the layer to be as thin as possible, so he used gold foil. This gold foil had a thickness of roughly 1000 atoms.
Helium ions with two charges are called alpha particles. The fast travelling alpha particles carry a large quantity of energy due to their mass of 4 u.
The sub-atomic particles in the gold atoms were predicted to deflect alpha particles. He did not expect to observe significant deflections because the alpha particles were heavier than protons.
OBSERVATIONS
Most of the fast moving α-particles passed straight through the gold foil.
Some of the α-particles were deflected by the foil by small angles.
Surprisingly one out of every 12000 particles appeared to rebound.
RUTHERFORD’S CONCLUSION
Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.
The small amount of particles that were deflected from their intended path shows how little space the atom's positive charge actually takes up.
A very small fraction of α-particles were deflected by 1800 ,indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom
RUTHERFORD’S ATOMIC MODEL
The nucleus, which has a positive charge, is the centre of an atom. An atom's nucleus contains almost all of its mass.
The electrons follow circular pathways as they go around the nucleus.
When compared to the size of the atom, the nucleus is very small.
DRAWBACK OF RUTHERFORD’S ATOMIC MODEL
It is not expected that the electron's rotation in a circular orbit will be stable. Any particle would accelerate if it were in a circular orbit. Charged particles would emit energy as they accelerated. As a result, the spinning electron would start to lose energy and eventually crash into the nucleus. If this were the case, the atom should be extremely unstable, which would prevent matter from existing in the way that it does today. We are aware of how stable atoms are.
BOHR’S MODEL OF ATOM
Only a select few unique orbits of electrons are allowed inside the atom.
The electrons do not radiate energy while they are rotating in these orbits or shells which are called energy levels
The maximum number of electrons that can be accommodated in the outermost orbit is 8.
In a particular shell, electrons cannot fit until the inner shells are filled. In other words, the shells are filled gradually.
Valency
Valency is the number of electrons lost or gained when an atom enters into a chemical reaction.It is the combining capacity of an atom.
If number of outermost electrons in an element is less than 4 then it will lose its electrons to attain stability and are called electropositive elements.
If number of outermost electrons in an element is more than 4 then it will gain electrons to complete octet(8 electrons in the outermost shell) and attain stability and are called electronegative elements.
ATOMIC NUMBER
An atom's atomic number is determined by how many protons it has. Its symbol is "Z." The atomic number Z is shared by all the atoms of a certain element. In actuality, an element's number of protons defines it. Z = 1 for hydrogen because there is only one proton in the nucleus of a hydrogen atom. Similarly, Z = 6 for carbon. As a result, the total number of protons in an atom's nucleus is what is meant by the term "atomic number."
MASS NUMBER
Protons and neutrons essentially make up all of an atom's mass. These can be found in an atom's nucleus. As a result, nucleons also refer to protons and neutrons. Consequently, an atom's nucleus is where its mass is located.
The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom. It is denoted by ‘A’.
For instance, carbon has a mass of 12 u because it has 6 protons and 6 neutrons, or 6 + 6 = 12. Aluminum has a mass of 27 u (13 protons + 14 neutrons).
Isotopes
Two or more atoms of same element are called isotopes if they have the same atomic number but different mass number.
Isotopes of hydrogen namely protium (1 1 H), deuterium ( 2 1 H or D) and tritium ( 3 1H or T). The atomic number of each one is 1, but the mass number is 1, 2 and 3, respectively.
Other such examples are (i) carbon, 12 6 C and 14 6 C, (ii) chlorine, 35 17 Cl and 37 17 Cl
Applicatons
In nuclear reactors, an isotope of uranium serves as the fuel.
The treatment of goitre involves the use of an iodine
The therapy of cancer involves the use of a cobalt isotope.
Isobars
Two atoms of different element are called isobars if they have different atomic number but same mass number.
Eg Argon and calcium have atomic number 18 and 20 respectively but both have a mass number 40.
Chapter 4-Structure of the Atom
Charged Particles in Matter
By 1900, it was understood that although an atom was an indivisible particle, it also contained at least one subatomic particle, the electron, which J.J. Thomson had discovered.
Before the electron was named, E. Goldstein found novel radiations in a gas discharge in 1886 and named them canal rays.
These radiations, which were positively charged, eventually resulted in the identification of another subatomic particle.
This subatomic particle had a charge that was similar to the electron's in magnitude but different in sign.
Its mass was around 2000 times greater than that of an electron. It was given the name proton.
The mass of a proton is taken as one unit and its charge as plus one.
The mass of an electron is considered to be negligible and its charge is minus one
It appeared that protons and electrons made up an atom, with their charges being balanced by one another. Protons also appeared to be inside the atom, as they were more difficult to remove from an atom than electrons.
The Structure of an Atom
Dalton's atomic hypothesis, which we studied, claimed that the atom was unbreakable and indivisible. However, the failure of this component of Dalton's atomic theory was caused by the finding of two fundamental particles (protons and electrons) inside the atom.
It was then considered necessary to know how electrons and protons are arranged within an atom. For explaining this, many scientists proposed various atomic models
The existence of neutrons in an atom's nucleus was discovered by J. Chadwick. In other words, an atom is made up of three subatomic particles: electrons, protons, and neutrons. Protons are positively charged, neutrons have no charge, and electrons have a negative charge. About 1 /2000 times the mass of a proton, an electron has.
THOMSON’S ATOMIC MODEL
The atom's model, which Thomson proposed, resembled a Christmas pudding,The electrons, in a sphere of positive charge, were like currants (dry fruits) in a spherical Christmas pudding.
Thomson’s proposal
An atom is made up of a positively charged sphere with embedded electrons.
The magnitude of the negative and positive charges is the same. As a result, the atom as a whole has no electrical charge.
RUTHERFORD’S ALPHA-PARTICLE SCATTERING EXPERIMENT
Ernest Rutherford was curious about the arrangement of the electrons in an atom. For this, Rutherford created an experiment. Fast moving alpha particles were made to fall on a thin piece of gold foil in this experiment.
Because he wanted the layer to be as thin as possible, so he used gold foil. This gold foil had a thickness of roughly 1000 atoms.
Helium ions with two charges are called alpha particles. The fast travelling alpha particles carry a large quantity of energy due to their mass of 4 u.
The sub-atomic particles in the gold atoms were predicted to deflect alpha particles. He did not expect to observe significant deflections because the alpha particles were heavier than protons.
OBSERVATIONS
Most of the fast moving α-particles passed straight through the gold foil.
Some of the α-particles were deflected by the foil by small angles.
Surprisingly one out of every 12000 particles appeared to rebound.
RUTHERFORD’S CONCLUSION
Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.
The small amount of particles that were deflected from their intended path shows how little space the atom's positive charge actually takes up.
A very small fraction of α-particles were deflected by 1800 ,indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom
RUTHERFORD’S ATOMIC MODEL
The nucleus, which has a positive charge, is the centre of an atom. An atom's nucleus contains almost all of its mass.
The electrons follow circular pathways as they go around the nucleus.
When compared to the size of the atom, the nucleus is very small.
DRAWBACK OF RUTHERFORD’S ATOMIC MODEL
It is not expected that the electron's rotation in a circular orbit will be stable. Any particle would accelerate if it were in a circular orbit. Charged particles would emit energy as they accelerated. As a result, the spinning electron would start to lose energy and eventually crash into the nucleus. If this were the case, the atom should be extremely unstable, which would prevent matter from existing in the way that it does today. We are aware of how stable atoms are.
BOHR’S MODEL OF ATOM
Only a select few unique orbits of electrons are allowed inside the atom.
The electrons do not radiate energy while they are rotating in these orbits or shells which are called energy levels
The maximum number of electrons that can be accommodated in the outermost orbit is 8.
In a particular shell, electrons cannot fit until the inner shells are filled. In other words, the shells are filled gradually.
Valency
Valency is the number of electrons lost or gained when an atom enters into a chemical reaction.It is the combining capacity of an atom.
If number of outermost electrons in an element is less than 4 then it will lose its electrons to attain stability and are called electropositive elements.
If number of outermost electrons in an element is more than 4 then it will gain electrons to complete octet(8 electrons in the outermost shell) and attain stability and are called electronegative elements.
ATOMIC NUMBER
An atom's atomic number is determined by how many protons it has. Its symbol is "Z." The atomic number Z is shared by all the atoms of a certain element. In actuality, an element's number of protons defines it. Z = 1 for hydrogen because there is only one proton in the nucleus of a hydrogen atom. Similarly, Z = 6 for carbon. As a result, the total number of protons in an atom's nucleus is what is meant by the term "atomic number."
MASS NUMBER
Protons and neutrons essentially make up all of an atom's mass. These can be found in an atom's nucleus. As a result, nucleons also refer to protons and neutrons. Consequently, an atom's nucleus is where its mass is located.
The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom. It is denoted by ‘A’.
For instance, carbon has a mass of 12 u because it has 6 protons and 6 neutrons, or 6 + 6 = 12. Aluminum has a mass of 27 u (13 protons + 14 neutrons).
Isotopes
Two or more atoms of same element are called isotopes if they have the same atomic number but different mass number.
Isotopes of hydrogen namely protium (1 1 H), deuterium ( 2 1 H or D) and tritium ( 3 1H or T). The atomic number of each one is 1, but the mass number is 1, 2 and 3, respectively.
Other such examples are (i) carbon, 12 6 C and 14 6 C, (ii) chlorine, 35 17 Cl and 37 17 Cl
Applicatons
In nuclear reactors, an isotope of uranium serves as the fuel.
The treatment of goitre involves the use of an iodine
The therapy of cancer involves the use of a cobalt isotope.
Isobars
Two atoms of different element are called isobars if they have different atomic number but same mass number.
Eg Argon and calcium have atomic number 18 and 20 respectively but both have a mass number 40.
