CHEMISTRY FORM FIVE: ATOMIC STRUCTURE

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ATOMIC STRUCTURE

Atomic structure deals with structure and component of an atom. The first scientist discovered that matter is made up of small particles called atoms. The term atom means indivisible particles. But later, different scientists put forward atomic models. These atomic models account for atomic structure. There are several atomic models which include the following:-

• Dalton’s atomic theory.
• Thompson’s atomic theory.
• Bohr’s atomic model.
• Rutherford atomic model.
• Wave particles duality nature of matter.
• Heisenberg uncertainity principle.

ATOMIC STRUCTURE

Atomic structure deals with structure and component of an atom. The first scientist discovered that matter is made up of small particles called atoms. The term atom means indivisible particles. But later, different scientists put forward atomic models. These atomic models account for atomic structure. There are several atomic models which include the following:-

• Dalton’s atomic theory.
• Thompson’s atomic theory.
• Bohr’s atomic model.
• Rutherford atomic model.
• Wave particles duality nature of matter.
• Heisenberg uncertainity principle.

       DALTON’S ATOMIC THEORY

Dalton’s atomic theory includes the following main points:-

i.    Matter is made up of small indivisible particles called atoms.

ii.   Atom is neither created nor destroyed.

iii.  Atoms of the same elements are similar especially in mass.

iv.  Atoms of different elements are different especially in mass.

v.   Atoms of different elements when combine they do so in small ratio whole numbers.

 

RECENT MODIFICATION OF DALTON’S ATOMIC THEORY

Dalton’s atomic theory was modified because all points were not valid. This resulted into discover of modern atomic theory. The following include point of modern atomic theory:-

i.  Matter is made up of small indivisible particles called atoms was not valid due to the existence of three particles in atom. Matter is made up of a small divisible particle called atoms.

 

Particle Nature of change symbol mass Position
Proton +1 11P 1.00 Nucleus
Neutron   0 01n 1.00 Nucleus
Electron  -1 -10e 1/1840 Shell

 

ii.   Atom is neither created nor destroyed was not valid due to the existence of radioactivity therefore Atom can be created or destroyed by either nuclear fission or fusion.

iii.  Atoms of the same elements are similar especially in mass was not valid due to the existence of isotopes. Atoms of the same elements have either same or different mass.

 

iv.  Atoms of elements are different especially in mass was not valid. Atoms of different elements have either same or different mass.

 

v.  Atoms of different elements when combine they do so in small ratio of whole number was not valid because different elements combine by using variable ratio of whole number.

THOMPSON’s EXPERIMENT (DISCOVERY OF ELECTRONS)

 

Thompson’s conducted an experiment to investigate if air conducts electricity. The following circuit was used during the experiment.

• The emission tube have electrode at each end which is connected to the external circuit. The emission tube is connected to the vacuum pump in order to maintain the low pressure in         the emission tube.

The circuit is switched  on which results into the following observation:-

• The bulb emitted light which indicate that the gas conduct electricity.

• There is glowing of emission tube or emission of light.

• There is fluorescence of emission tube.

• The stream of rays running from cathode to the anode. Through investigation of properties of cathode rays by using magnetic field, electric field and gold electroscope results into
discover of electron. The cathode rays were the electrons.

EXPLANATION OF THOMPSON’S EXPERIMENT IN TERMS OF ATOMIC STRUCTURE

Ground state: Is lowest energetic state of an atom. Is a state when an electron filled in the lowest energy level before filling the highest energy level available. The electron filled in atom in order of increasing energy level. This state make atom to be stable.

 

Excited state: Is a state of an atom when electron filled in the highest energy level before filling lowest energy level available. If the electron excited jumps to the extent that the nuclear attractive force act upon it result pulling back of electrons. When return back to the ground state release all amount of energy which was absorbed inform of radiation.

 

RADIATION: This energy causes glowing of emission tube. When the radiation strikes the emission tube causes florescence of emission tube.

 

Convergent limit: Is a state of atom when an electron is removed completely from ground state to the infinite. The convergent limit occurs if atom gain high energy which result electrons to jump to the highest energy level where the nuclear attractive force cannot act up on it. This electron cannot return back to the ground state it result the atom left to positively charged. The convergent limit is a factor which causes some electrons to move from cathode to anode. These were stream of rays called cathode rays which later was electrons.

 

SIGNIFICANCE OF CONVERGENT LIMIT

These include the following:-

• It resulted into discover of the ionization energy. This ionization energy used in the inorganic section.

• It resulted into formation of ion particles. The ionic particle is more reactive when take part during chemical reaction.

• It resulted into production of rays. These rays are known as atomic spectrum.

THOMSON MODEL OF THE ATOM

After the discovery of electrons and protons, the next question was to know how these particles are arranged in an atom. The first simple model of the atom was proposed by J.J.Thomson in 1898.The Thomson atomic model is popularly known as the Thomson’s “plum-pudding” model of the atom.

Thomson considered an atom to be a sphere (radius = 10-10 m or 10-8 cm) of uniform positive charge into which the negatively charged electrons were embedded. This model is like plum-pudding dotted with raisins

 

This model of an atom could not explain many experimental facts. So, it was abandoned.

 

RUTHERFORD’S SCATTERING EXPERIMENT

In 1911 Rutherford performed an experiment which is now known as Rutherford’s scattering experiment. In this experiment, he bombarded a thin sheet (0.00006 cm thickness) of gold with alpha α particles. The α-particles were obtained from a radioactive substance. The -particles are doubly ionized helium atoms (He2*).

The scattered α -particles produced tiny flashes on striking with the zinc sulphide screen. These tiny flashes were observed with a movable microscope. The experimental set up used in the famous α-scattering experiment is shown in figure below. The following observations were made from the scattering experiment.

Structure of atom

 

The α -scattering from metal foils. The -particles are produced by a radioactive source.Since lead absorbs α -particles a lead plate with a hole is used to obtain a beam of a particles.The α -particles scattered from the metal foil strike the fluorescent (zinc sulphide) screen and produce tiny flashes. A movable microscope is used to view the flashes.

i.        Most of the a-particles passed through the metal foil without any change in their path i.e. they remained undeflected.

ii.        Some of the α -particles   deflected through small angles.

iii.        Only a few of them (1 in 10,000) were actually deflected by as much as 90°, or even larger angles. One in 20,000 particles returned back suffering a deflection of 180°.

Explanation. The results of the scattering experiment could not be explained by the Thomson’s atomic. Calculations showed that a charge spread over a sphere of radius 10-8 cm could deflect α-particles only through    small angles. The deflections of onlyα -particles through larger angels as observed would be possible If the positive charge in the atom is spread over a sphere of radius of about 10-13 cm thus, the α-particles scattering result could not be explained by Thomson’s atomic model.

RUTHERFORD’S NUCLEAR MODEL OF THE ATOM

On the basis of α-particle scattering experiment, Rutherford put forwarded in 1912, his nuclei model of the atom. According to this assumptions.

i.    An atom consists of a positively charged nucleus surrounded by a system of electronics. Electrons are moving around the nucleus. The positive charge of the nucleus is due to the
protons present in it.

ii.   Electrons and the nucleus are held together by coulombic force of attraction,

iii.  The effective volume of the nucleus is extremely small as compared to the effective volume of the atom. From the experiments, it was found that, Approximate radius of the
nucleus of an atom  10-14– 10-15 m (10-12 – 10-13 cm), Approximate radius of sphere of electrons (or the radius of an atom)  10-10 m (10-8 cm).Since, the volume varies as r3,
hence the volume occupied by the nucleus is about 1012 times the volume of the atom.

iv.   Almost the entire mass of the atom is concentrated in the nucleus.

v.     The positive charge on the nuclei of different elements are always integral multiples of the electron charge, but opposite in sign. Since, each atom is electrically neutral, hence in
an atom, the number of positive charges on the nucleus of an atom is equal to the number of electrons in it. The Rutherford’s model of an atom is shown in the figure below

 

Rutherford, in 1912, put forwarded an atomic model. According to this model an atom consists of positively charged nucleus which is surrounded by a system of electrons. The electrons and I nucleus are held together by electrostatic forces. Because of the electrostatic attraction between the nucleus and the electrons, the electrons should ultimately fall into the nucleus. But, it does not happen. In order to explain why electrons not fall into the nucleus, Rutherford postulated that the electrons are not stationary, but are revolving about the nucleus in orbits. But, this explanation did not solve the problem completely. The physicists had observed that a revolving electric charge must emit radiation and thus lose energy. Thus, an electron revolving around a nucleus in an orbit should emit radiation and lose energy

If so, the continuous loss of energy should slow down electrons. As a result, the electron will not be able to stand the attraction of the nucleus and gradually move towards the nucleus. The electron should, therefore, follow a spiral path and ultimately fall into the nucleus within 10-8s. The atom should thus, collapse. But, it does not happen, as the atoms are stable. Thus, there must be something wrong in the Rutherford’s nuclear model of the atom.


RUTHERFORD ATOMIC MODEL

(i) The atom consists of extreme denser region at center called nucleus which have positive charged particles.
(ii) The negative particles (electron(s) of atoms revolve around the nucleus in a path called orbit.
(iii) The large area of atom is empty space.

SHORT COMING OF RUTHERFORD ATOMIC MODEL.

Rutherford atomic model accounted that electron revolve around the nucleus in the path called orbit. This is an imagination because no any scientist proved occurrence but discovered that when charged particle revolve around the opposite charge reach a point when they collapse due to the attraction force occurs between the nuclear and electron(s) hence electron capture. But this proved the truth of Rutherford because no any atom discovered to collapse.

BOHR’S ATOMIC MODEL The electrons revolve around the nucleus in a path called orbits which have certain energy.
The electron when revolve in stationery state does not radiate energy.
The electrons emit or absorb energy when shift from one energy level to another.
The electronic motions are those which its angular momentum is integral multiple of  where n = 1, 2, 3…

SHORT COMING OF BOHR’S ATOMIC MODEL

Bohr’s accounted that electrons revolve in a single plane. But discovered that movement of electrons is not restricted in only on plane.
The Bohr’s model did not explain spectral of multi-electrons atom but accounted only spectral line of uni-electron hydrogen atom.
The Bohr’s model did not account for electrons which were found in a chemical bond.
Bohr viewed an electron as being placed at a certain distance from the nucleus. However, it was proved by Weaver Heisenberg’s Uncertainity Principle:–It is impossible at any
movement to predict the exact position and velocity of an electron in an atom.

MASS SPECTROMETER

Is an instrument which is used to determine the relative atomic mass of an element.

Mass spectrometers have an ability to determine the relative molecular mass of a compound and formula of a compound. In a spectrographic plate.

Mass spectrograph is a plate of mass spectrometers which detect and record the relative atomic mass of an element. The spectrographic plate is a detector or recorder. The following is a diagram of mass spectrometer which has different part.

 

 

 

Vaporization. The instrument is evacuated in order to prevent the interference of air. The solid sample must be heated until it forms gaseous atoms and vaporized before it’s introduced into the mass spectrometer.

 

Ionization. The vaporized sample of element is introduced in the ionization chamber. This region has electric filament which emits fast moving electrons. These electrons collide with atom of an element as a result the atom split which leave only nucleus of positive charge. The nucleus left is where mass of atom is concentrated.

 

Acceleration. The ion particles formed in the ionization chamber are accelerated by two plate of negative charge toward the magnetic field. These two plates are connected to negative potential which accelerate ions as a beam of light toward the magnetic field.

 

Deflection. The beam of ions deflected in the magnetic field. The extent of deflection depends on the mass charge ratio m/e. The light particle deflected more than the heavy particles. The deflected ions strike on the detector. The ions at the same mass charge ratio strike on the same mass spot.

 

Detection. Once the ions strike the detector in the collector that converts the intensity of the ions into electric signals which is amplified by the amplifier into large electric current.

 

Recording. The current is used to operate a pen that moves on a paper tracing the peak of the isotope. The ion is recorded in terms of atomic mass and relative abundance. When they
fall on the photographic plate they produce a mass spectrum consisting of a series of line at different point. The mass of the ions detected on the photographic plate. Relative
abundance is the percentage of an isotopic atom in the element. The ions particles when strike the detector produce mass spectrum. The mass spectrum recorded as a peak.

The following include example of mass spectrum.

 

CALCULATION OF RELATIVE ATOMIC MASS

Relative atomic mass of an element is the average value of all the known isotope atomic weight relative to the proportional abundances. Relative atomic mass is more useful in chemistry than its simple atomic mass of element because simple atomic weight does not consider existence of isotopic forms of elements. The relative abundance of isotopic element is applied to determine relative atomic mass. Example isotopic element X contained Y% of zaX and W% of zbX, the relative atomic mass of element X is determined as given below.

 

Since y + w = 100

The length of peaks of each isotopic atom applied to find the relative atomic mass. Each atom have peak due to the different mass charge ratio m/e.

 

 

 

       DALTON’S ATOMIC THEORY

Dalton’s atomic theory includes the following main points:-

i.    Matter is made up of small indivisible particles called atoms.

ii.   Atom is neither created nor destroyed.

iii.  Atoms of the same elements are similar especially in mass.

iv.  Atoms of different elements are different especially in mass.

v.   Atoms of different elements when combine they do so in small ratio whole numbers.

 

RECENT MODIFICATION OF DALTON’S ATOMIC THEORY

Dalton’s atomic theory was modified because all points were not valid. This resulted into discover of modern atomic theory. The following include point of modern atomic theory:-

i.  Matter is made up of small indivisible particles called atoms was not valid due to the existence of three particles in atom. Matter is made up of a small divisible particle called atoms.

 

Particle Nature of change symbol mass Position
Proton +1 11P 1.00 Nucleus
Neutron   0 01n 1.00 Nucleus
Electron  -1 -10e 1/1840 Shell

 

ii.   Atom is neither created nor destroyed was not valid due to the existence of radioactivity therefore Atom can be created or destroyed by either nuclear fission or fusion.

iii.  Atoms of the same elements are similar especially in mass was not valid due to the existence of isotopes. Atoms of the same elements have either same or different mass.

 

iv.  Atoms of elements are different especially in mass was not valid. Atoms of different elements have either same or different mass.

 

v.  Atoms of different elements when combine they do so in small ratio of whole number was not valid because different elements combine by using variable ratio of whole number.

THOMPSON’s EXPERIMENT (DISCOVERY OF ELECTRONS)

 

Thompson’s conducted an experiment to investigate if air conducts electricity. The following circuit was used during the experiment.

• The emission tube have electrode at each end which is connected to the external circuit. The emission tube is connected to the vacuum pump in order to maintain the low pressure in         the emission tube.

The circuit is switched  on which results into the following observation:-

• The bulb emitted light which indicate that the gas conduct electricity.

• There is glowing of emission tube or emission of light.

• There is fluorescence of emission tube.

• The stream of rays running from cathode to the anode. Through investigation of properties of cathode rays by using magnetic field, electric field and gold electroscope results into
discover of electron. The cathode rays were the electrons.

EXPLANATION OF THOMPSON’S EXPERIMENT IN TERMS OF ATOMIC STRUCTURE

Ground state: Is lowest energetic state of an atom. Is a state when an electron filled in the lowest energy level before filling the highest energy level available. The electron filled in atom in order of increasing energy level. This state make atom to be stable.

 

Excited state: Is a state of an atom when electron filled in the highest energy level before filling lowest energy level available. If the electron excited jumps to the extent that the nuclear attractive force act upon it result pulling back of electrons. When return back to the ground state release all amount of energy which was absorbed inform of radiation.

 

RADIATION: This energy causes glowing of emission tube. When the radiation strikes the emission tube causes florescence of emission tube.

 

Convergent limit: Is a state of atom when an electron is removed completely from ground state to the infinite. The convergent limit occurs if atom gain high energy which result electrons to jump to the highest energy level where the nuclear attractive force cannot act up on it. This electron cannot return back to the ground state it result the atom left to positively charged. The convergent limit is a factor which causes some electrons to move from cathode to anode. These were stream of rays called cathode rays which later was electrons.

 

SIGNIFICANCE OF CONVERGENT LIMIT

These include the following:-

• It resulted into discover of the ionization energy. This ionization energy used in the inorganic section.

• It resulted into formation of ion particles. The ionic particle is more reactive when take part during chemical reaction.

• It resulted into production of rays. These rays are known as atomic spectrum.

THOMSON MODEL OF THE ATOM

After the discovery of electrons and protons, the next question was to know how these particles are arranged in an atom. The first simple model of the atom was proposed by J.J.Thomson in 1898.The Thomson atomic model is popularly known as the Thomson’s “plum-pudding” model of the atom.

Thomson considered an atom to be a sphere (radius = 10-10 m or 10-8 cm) of uniform positive charge into which the negatively charged electrons were embedded. This model is like plum-pudding dotted with raisins

 

This model of an atom could not explain many experimental facts. So, it was abandoned.

 

RUTHERFORD’S SCATTERING EXPERIMENT

In 1911 Rutherford performed an experiment which is now known as Rutherford’s scattering experiment. In this experiment, he bombarded a thin sheet (0.00006 cm thickness) of gold with alpha α particles. The α-particles were obtained from a radioactive substance. The -particles are doubly ionized helium atoms (He2*).

The scattered α -particles produced tiny flashes on striking with the zinc sulphide screen. These tiny flashes were observed with a movable microscope. The experimental set up used in the famous α-scattering experiment is shown in figure below. The following observations were made from the scattering experiment.

Structure of atom

 

The α -scattering from metal foils. The -particles are produced by a radioactive source.Since lead absorbs α -particles a lead plate with a hole is used to obtain a beam of a particles.The α -particles scattered from the metal foil strike the fluorescent (zinc sulphide) screen and produce tiny flashes. A movable microscope is used to view the flashes.

i.        Most of the a-particles passed through the metal foil without any change in their path i.e. they remained undeflected.

ii.        Some of the α -particles   deflected through small angles.

iii.        Only a few of them (1 in 10,000) were actually deflected by as much as 90°, or even larger angles. One in 20,000 particles returned back suffering a deflection of 180°.

Explanation. The results of the scattering experiment could not be explained by the Thomson’s atomic. Calculations showed that a charge spread over a sphere of radius 10-8 cm could deflect α-particles only through    small angles. The deflections of onlyα -particles through larger angels as observed would be possible If the positive charge in the atom is spread over a sphere of radius of about 10-13 cm thus, the α-particles scattering result could not be explained by Thomson’s atomic model.

RUTHERFORD’S NUCLEAR MODEL OF THE ATOM

On the basis of α-particle scattering experiment, Rutherford put forwarded in 1912, his nuclei model of the atom. According to this assumptions.

i.    An atom consists of a positively charged nucleus surrounded by a system of electronics. Electrons are moving around the nucleus. The positive charge of the nucleus is due to the
protons present in it.

ii.   Electrons and the nucleus are held together by coulombic force of attraction,

iii.  The effective volume of the nucleus is extremely small as compared to the effective volume of the atom. From the experiments, it was found that, Approximate radius of the
nucleus of an atom  10-14– 10-15 m (10-12 – 10-13 cm), Approximate radius of sphere of electrons (or the radius of an atom)  10-10 m (10-8 cm).Since, the volume varies as r3,
hence the volume occupied by the nucleus is about 1012 times the volume of the atom.

iv.   Almost the entire mass of the atom is concentrated in the nucleus.

v.     The positive charge on the nuclei of different elements are always integral multiples of the electron charge, but opposite in sign. Since, each atom is electrically neutral, hence in
an atom, the number of positive charges on the nucleus of an atom is equal to the number of electrons in it. The Rutherford’s model of an atom is shown in the figure below

 

Rutherford, in 1912, put forwarded an atomic model. According to this model an atom consists of positively charged nucleus which is surrounded by a system of electrons. The electrons and I nucleus are held together by electrostatic forces. Because of the electrostatic attraction between the nucleus and the electrons, the electrons should ultimately fall into the nucleus. But, it does not happen. In order to explain why electrons not fall into the nucleus, Rutherford postulated that the electrons are not stationary, but are revolving about the nucleus in orbits. But, this explanation did not solve the problem completely. The physicists had observed that a revolving electric charge must emit radiation and thus lose energy. Thus, an electron revolving around a nucleus in an orbit should emit radiation and lose energy

If so, the continuous loss of energy should slow down electrons. As a result, the electron will not be able to stand the attraction of the nucleus and gradually move towards the nucleus. The electron should, therefore, follow a spiral path and ultimately fall into the nucleus within 10-8s. The atom should thus, collapse. But, it does not happen, as the atoms are stable. Thus, there must be something wrong in the Rutherford’s nuclear model of the atom.


RUTHERFORD ATOMIC MODEL

(i) The atom consists of extreme denser region at center called nucleus which have positive charged particles.
(ii) The negative particles (electron(s) of atoms revolve around the nucleus in a path called orbit.
(iii) The large area of atom is empty space.

SHORT COMING OF RUTHERFORD ATOMIC MODEL.

Rutherford atomic model accounted that electron revolve around the nucleus in the path called orbit. This is an imagination because no any scientist proved occurrence but discovered that when charged particle revolve around the opposite charge reach a point when they collapse due to the attraction force occurs between the nuclear and electron(s) hence electron capture. But this proved the truth of Rutherford because no any atom discovered to collapse.

BOHR’S ATOMIC MODEL The electrons revolve around the nucleus in a path called orbits which have certain energy.
The electron when revolve in stationery state does not radiate energy.
The electrons emit or absorb energy when shift from one energy level to another.
The electronic motions are those which its angular momentum is integral multiple of  where n = 1, 2, 3…

SHORT COMING OF BOHR’S ATOMIC MODEL

Bohr’s accounted that electrons revolve in a single plane. But discovered that movement of electrons is not restricted in only on plane.
The Bohr’s model did not explain spectral of multi-electrons atom but accounted only spectral line of uni-electron hydrogen atom.
The Bohr’s model did not account for electrons which were found in a chemical bond.
Bohr viewed an electron as being placed at a certain distance from the nucleus. However, it was proved by Weaver Heisenberg’s Uncertainity Principle:–It is impossible at any
movement to predict the exact position and velocity of an electron in an atom.

MASS SPECTROMETER

Is an instrument which is used to determine the relative atomic mass of an element.

Mass spectrometers have an ability to determine the relative molecular mass of a compound and formula of a compound. In a spectrographic plate.

Mass spectrograph is a plate of mass spectrometers which detect and record the relative atomic mass of an element. The spectrographic plate is a detector or recorder. The following is a diagram of mass spectrometer which has different part.

 

 

 

Vaporization. The instrument is evacuated in order to prevent the interference of air. The solid sample must be heated until it forms gaseous atoms and vaporized before it’s introduced into the mass spectrometer.

 

Ionization. The vaporized sample of element is introduced in the ionization chamber. This region has electric filament which emits fast moving electrons. These electrons collide with atom of an element as a result the atom split which leave only nucleus of positive charge. The nucleus left is where mass of atom is concentrated.

 

Acceleration. The ion particles formed in the ionization chamber are accelerated by two plate of negative charge toward the magnetic field. These two plates are connected to negative potential which accelerate ions as a beam of light toward the magnetic field.

 

Deflection. The beam of ions deflected in the magnetic field. The extent of deflection depends on the mass charge ratio m/e. The light particle deflected more than the heavy particles. The deflected ions strike on the detector. The ions at the same mass charge ratio strike on the same mass spot.

 

Detection. Once the ions strike the detector in the collector that converts the intensity of the ions into electric signals which is amplified by the amplifier into large electric current.

 

Recording. The current is used to operate a pen that moves on a paper tracing the peak of the isotope. The ion is recorded in terms of atomic mass and relative abundance. When they
fall on the photographic plate they produce a mass spectrum consisting of a series of line at different point. The mass of the ions detected on the photographic plate. Relative
abundance is the percentage of an isotopic atom in the element. The ions particles when strike the detector produce mass spectrum. The mass spectrum recorded as a peak.

The following include example of mass spectrum.

 

CALCULATION OF RELATIVE ATOMIC MASS

Relative atomic mass of an element is the average value of all the known isotope atomic weight relative to the proportional abundances. Relative atomic mass is more useful in chemistry than its simple atomic mass of element because simple atomic weight does not consider existence of isotopic forms of elements. The relative abundance of isotopic element is applied to determine relative atomic mass. Example isotopic element X contained Y% of zaX and W% of zbX, the relative atomic mass of element X is determined as given below.

 

Since y + w = 100

The length of peaks of each isotopic atom applied to find the relative atomic mass. Each atom have peak due to the different mass charge ratio m/e.

 

 

 

ready also:

FORM FOUR PHYSICS STUDY NOTES TOPIC 4: THERMIONIC EMISSION

CHEMISTRY FORM ONE TOPIC 5: MATTER,

GEOGRAPHY FORM THREE TOPIC 2: FORCES THAT AFFECTS THE EARTH

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