How to solve a question related to avagadro constant and no. of particle...

How to solve a question related to avagadro constant and no. of particle...

How to solve a question related to precipation reaction?

Edexcel IAL AS Unit 1 Chemistry Question Paper May 2015 (Question 4)

Edexcel AS Chemistry Unit 1 Question Paper May 2015 Solution

Edexcel AS Chemistry Unit 1 Question Paper May 2015 Solution

Edexcel AS Chemistry Unit 1 Question Paper May 2015 Solution

Edexcel AS Chemistry Unit 1 Question Paper May 2015 Solution

Edexcel AS Chemistry Unit 1 May 2015 question no. 2 solution.

Edexcel question paper May 2015 Unit -1 ( question 1) Solution.

How do mass spectrometry works?

How do mass spectrometry works?

A mass spectrometer can split atoms and molecules based on the mass. It can also give us a series of data about the compounds and elements present in the sample. With that series of data about the atoms or molecules, that would be represent on a graph in a computer from which we can identify the elements and compounds present in the sample.

The mass spectrometry is an ideal device for measuring relative mass of  an element/ compound  in a given sample since it can measure very accurately.

In order to move through the mass spectrometer, sample must be

a) first vaporised,

b) secondly ionised.

The air is first pumped out of the mass spectrometer  to avoid ionisation of air.

Diagram:

Vaporisation :

The sample which is needed to analyze must be in gaseous state in order to move  easily through the mass spectroscopy. There is a high vacuum area in the 1^st section of the instrument where the given sample is vaporised. 

The air particles are first pumped out of the vacuum chamber from the mass spectroscopy in order to prevent air particles get ionised . This is because we want only the sample to be ionised. If there is any air molecules present, then that will also get ionised. So, it would be pretty difficult for us to analyse the actual ions present in the sample.

Then,  the desired sample is injected into the mass spectroscopy and is first vaporized. Here, the given sample is vaporised at a given temperature , if the sample is not in the gaseous state.   

                                                                                   

Ionisation :

Then in the next section , the vaporised sample is bombarded with high energy electrons. These high energy electrons knock one or more electrons in the valence making ions , molecular ions. It doesn’t make any significant differences in mass since the mass of electrons are negligible. Now the cations are formed which can move to the electric field.

X (g) + e ^- → X⁺ + 2e^-

Two types of ions and free radicals are formed in the ionisation:

1) Molecular ions    2) Fragmented ions

Diagram:

** Note : Later we will study the fragmentation pattern of molecular ions .

Acceleration :

Then the cat ions passes through the electric field to get accelerated. The positive ions pass through the slits and comes out like stream of  beams. The cat ions get accelerated but negative ions don’t get accelerated in the electric field.

Velocity selector :

Then the positive ions pass through the velocity selector where a fixed velocity is set for all the ions . The velocity selector makes sure that all the positive ions are travelling at constant speed .

This means that affect of the magnetic field in the next section would be due to the differing mass and charge / mass/ charge ratio (m/z) but not for the speed since the speed is constant.     

Uniform Magnetic field :

Then the ions passes through the uniform magnetic field where deflection of ions occurs. Deflection depends on both mass and charge. The ions with large mass and small charge would deflect least . On the other hand, the ions with small mass and large charge would deflect most .

The strength of magnetic field is gradually increased,  only ions with specific mass/charge ratio can pass through the passage at a selected settings of the magnetic field. Others would strike the wall by deflecting high or low and failed to move through the pathway to the detector.

Detector :

Then the detector detects the no. of positive ions pass through and transform them as a tiny currents and transmit as electric signal to the computer.

Display:

Then the Mass spectrum is obtained as a result. The computer would produce a graph of abundance against mass/ charge ratio (m/z) where you would have relative peaks and different m/z ratio values.  The relative height shows relative abundance cations . The m/z ratio gives us the information about relative mass of the particles present in the compound. Most of the charge of the ion is +1. So mass of the ions = m/z ratio of the ions.

***Note: We will later study the mass spectroscopy graph in later chapter.

What is Mass spectrometry ?

Mass spectrometry:

The mass spectrometer happens to be an important device to measure the relative mass associated with atom, a molecule or a particular ion accurately.

Typically the mass spectrometer separates atoms and molecules as reported by their mass and also shows the relative variety of the different atoms and molecules present.

Then the data is generated from ion detector of mass spectrometer which can be use to make a graph in the computer where we can identify different elements or compounds present in the sample.

Typically the masses of atoms, molecules and fragments of molecules are generally measured using a mass spectrometer.

Atoms are very tiny. It is almost impossible to measure the mass of an atom in the traditional way.  A mass spectrometry separates the atoms and molecules

Definition of Isotopes :

Isotopes are the atoms of the same element with same atomic no. but differing mass no.

The isotopes are the atoms of the same elements which have:

same atomic numbers

same no. of protons

same no. of electrons

similar chemical properties

same symbol

but

differing  mass numbers

differing  number of neutrons

differing physical properties.

We will find the following  things by using mass spectrometer:-

a) Relative atomic mass :

It is the average mass of an atom of an element compared to  1/12 th of the mass of 1 atom of Carbon-12 isotope.

b) Relative isotopic mass :

It is the mass of 1 atom of an isotope of an element compared to 1/12 th of the mass of 1 atom of Carbon – 12 isotope.

c) Relative molecular mass :

It is the mass of 1 molecule of a substance compared to 1/12^th of the mass of 1 atom of Carbon – 12 isotope.

The interpreting electronic structure in box notation:

Electron configurations using box notation:

We can represent orbitals as box notations and arrows to represent electrons pair.

Points to know :

•       The electrons pair in any orbitals spin with equal amount of  energy.
•          Electrons are in opposite sides to minimize repulsion to be in high stable conditions.
•          Each orbitals of sub-shell get filled with 1 electron at a time.

represents an orbital

                                                                                                  

                       

 } represents an electron

The electrons configuration of some elements are given below in box notation : 

H ( z = 1)    = 1s¹

He  ( z = 2) = 1s²

Li ( z = 3) = 1s² 2s¹

Be ( z = 4) = 1s²  2s²

B  ( z = 5) = 1s² 2s² 2p¹

C ( z =  6) = 1s² 2s² 2p²

N  ( z = 7) = 1s² 2s² 2p³

O ( z =  8) = 1s² 2s² 2p⁴

F ( z =  9) = 1s² 2s² 2p⁵

Se ( z =  34)   =     [Ar] 4s² 3d¹⁰  4p⁴  

                

Br ( z =  35)   =   [Ar] 4s² 3d¹⁰  4p⁵  

                

Kr ( z =  36)   =   [Ar] 4s²  3d¹⁰ 4p⁶  

               

Stability of sub-shells:-

                                    s >  p > d > f

                                                                                                                   The stability of  sub-shells increases

.i.e. “ s ” sub - shells are the most stable and “ f ” sub – shell are the least stable.

Half – filled “ s ” sub-shell

 

Full – filled “ s ” sub - shell 

Half – filled “ p ” sub-shell

Partially – filled “ p ” sub-shell

Partially – filled “ p ” sub-shell

 

Full – filled “ p ” sub - shell   

Full – filled sub - shell > Half – filled sub-shell  > Partially – filled sub-shell

                                                                                                                            

                                       Stability increases

The reasons behind the electron configurations of Copper and Chromium:

The electronic structure of Copper was supposed to be like this:

**Cu ( z =  29)  = 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²3d⁹         Or        [Ar] 4s² 3d⁹

But Copper’s electronic structure doesn’t exist like that. If we look carefully at the last 3d sub-shell, we can see that it is partially-filled which makes the structure less stable. Since, there is small energy difference between 4s and 3d sub-shell, electrons can move easily from 1 sub – shell to another sub – shell. So, one electron from 4s sub – shell gets promoted to 3d sub – shell.                                                                         

Now, 4s sub – shell is half - filled and 3d sub – shell is full - filled which makes the structure more stable than before.

So, the structure would be :

Cu ( z =  29)  = 1s²2s²2p⁶ 3s² 3p⁶ 4s¹3d¹⁰              Or        [Ar] 4s¹ 3d¹⁰

Diagram:

The same thing happens in the case of Chromium:

**Cr ( z =  24) = 1s²2s²2p⁶ 3s² 3p⁶ 4s² 3d⁴        Or        [Ar] 4s² 3d⁴ 

The chromium has 3d sub – shell which is partially – filled. So, one electron from 4s sub – shell gets promoted to 3d sub – shell. Now, this makes both 4s sub-shell and 3d sub-shell half-filled. This is the most stable electronic configuration of chromium.

So, the structure would be :

Cr ( z =  24) = 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵        Or        [Ar] 4s¹ 3d⁵ 

Diagram:

Classification of elements in the periodic table according to the sub-shell filled by the last electron.

Classification of elements in the periodic table according to last sub-shell:

Elements in the periodic table can be classified by the sub-shell that is filled by the last electrons.

There are four blocks of elements in the periodic table:

1)  s – block elements

2)  d – block elements

3)  p – block elements

4) f – block elements

1)  s – block elements :

The elements which have their last electron is being filled on “ s ”  sub – shell.

The Group 1 and Group 2 elements in the periodic table are s – block elements.

For example :

Li ( z = 3) = 1s² 2s¹

Be ( z = 4) = 1s²  2s²

Na ( z =  11) = 1s² 2s² 2p⁶ 3s¹                                     Or            [Ne] 3s¹

Mg ( z =  12) = 1s² 2s² 2p⁶ 3s²                                    Or             [Ne] 3s²

2)  d – block elements :

The elements which have their last electron is being filled on “ d ”  sub – shell.

The elements between Group 2 and Group 3 that is transition metals in the periodic table are d – block elements.

For example :

Sc  ( z =  21) = 1s²2s²2p⁶ 3s² 3p⁶ 4s² 3d¹            Or         [Ar] 4s² 3d¹

Ti ( z =  22) = 1s²2s²2p⁶ 3s² 3p⁶ 4s² 3d²               Or        [Ar] 4s² 3d²

V ( z =  23) = 1s²2s²2p⁶ 3s² 3p⁶ 4s² 3d³                Or        [Ar] 4s² 3d³

Ni ( z =  28) = 1s²2s²2p⁶ 3s² 3p⁶ 4s² 3d⁸                 Or        [Ar] 4s² 3d⁸  

3)  p – block elements :

The elements which have their last electron is being filled on “ p ”  sub – shell.

The Group 3 to Group 8 elements in the periodic table are s – block elements.

For example :

Al ( z =  13) = 1s² 2s²  2p⁶  3s² 3p¹                           Or             [Ne] 3s² 3p1

Si ( z =  14) = 1s²  2s² 2p⁶ 3s² 3p²                             Or             [Ne] 3s² 3p3

P ( z =  15) = 1s² 2s² 2p⁶ 3s² 3p³                                Or             [Ne] 3s² 3p3

S ( z =  16) = 1s² 2s² 2p⁶ 3s² 3p⁴                                Or             [Ne] 3s² 3p4

4) f – block elements :

The elements which have their last electron is being filled on “ f ”  sub – shell.

We don’t have f – block elements in our AS level Chemistry specification.

Diagram:

Classification of elements in the periodic table according to last sub-shell.

The electron configuration of ions:

Li ( z = 3) = 1s² 2s¹

Li⁺ ( z = 3) = 1s² 2s⁰

Na ( z =  11) = 1s² 2s² 2p⁶ 3s¹                                     Or            [Ne] 3s¹   

Na⁺ ( z =  11) = 1s² 2s² 2p⁶ 3s⁰                                     Or            [Ne] 3s⁰

                                                                                                                                                             

Mg ( z =  12) = 1s² 2s² 2p⁶ 3s²                                    Or             [Ne] 3s²

Mg²⁺ ( z =  12) = 1s² 2s² 2p⁶ 3s⁰                                   Or             [Ne] 3s⁰

Al ( z =  13) = 1s² 2s²  2p⁶  3s² 3p¹                           Or             [Ne] 3s² 3p¹

Al³⁺ ( z =  13) = 1s² 2s²  2p⁶  3s² 3p¹                           Or             [Ne] 3s⁰ 3p⁰

N  ( z = 7) = 1s² 2s² 2p³

N³⁺  ( z = 7) = 1s² 2s² 2p⁰

O ( z =  8) = 1s² 2s² 2p⁴

O^2- ( z =  8) = 1s² 2s² 2p⁶

F ( z =  9) = 1s² 2s² 2p⁵

F^- ( z =  9) = 1s² 2s² 2p⁶