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ALevel

Chemistry-9701

Electrons Energy Levels and Atomic Orbitals

Paper 1 & 2

Electrons Energy Levels and Atomic Orbitals
 

1: What Is Meant By The Following Terms?


>Shell: The electron shell is known as an energy level present on the outside part of an atom around the atomic nucleus. 

>Sub-Shells: Inside the shell we have subshells. These Sub-Shells of an atom are subdivisions of electron shells (energy levels) represented by s, p, d, f. These are then further separated by electron orbitals.

>Orbitals: Orbitals are those regions in space where there is the highest amount of probability to find an electron (negatively charged).

Note: Each orbital can hold 2 electrons in opposite directions. Why the opposite direction? Electrons (negatively charged) are placed in a set of two in an orbital which contains the same amount of energy. They (electrons) occupy the orbital at first singularly and then a pairing of two takes place. Naturally, both of these are negatively charged and if are placed in the same direction. They would repel distorting the orbital they are present in. Hence, they come together as a pair in opposite directions creating a “spin movement” this results in lesser repulsion.
 
                                                 
                                              chemrevise.org
The diagram above shows by the arrows moving in the opposite directions which represent the different spins of the electrons present inside the orbital
  
>Principal Quantum Number (n): As we know electrons are arranged in energy levels which we call shells, each shell is represented by a Principal Quantum number e.g. 1, 2, 3. 

Hence, the energy that an electron contains illustrates the most probable distance of the electron from the nucleus. Naturally, referring to the size of the orbital and the shell (energy level) an electron is placed in.

Note: The principal quantum shells are regions at various distances from the nucleus which may contain a certain number of electrons. The first (1) quantum shell contains up to 2 electrons, the second (2) up to 8 and the third (3) up to 18. Each number in the bracket is a Principal Quantum Number (n). They are categorically numbered according to how far (distant) they are from the nucleus (center of the atom). The lowest energy level, n=1, is closest to the nucleus, the energy level n=2 is further out, and the same phenomenon is followed with the shells which follow.  Also, the electrons in quantum shells further away from the nucleus have more energy and are held less tightly to the nucleus. 
       
                                                                       
                                       Cambridge International Examinations Lawrie Ryan, Roger Norris                                                     Cambridge International Examinations Lawrie Ryan, Roger Norris
 

The above diagram shows us the Lithium atom in the first Principal Quantum Shell (1) it has two electrons and in the second Principal Quantum shell it has one. With Neon and Chlorine we have the same number of electrons in the first (1) and second (2) principal quantum shell.  Chlorine is the only atom of the three atoms present to have the third principal quantum shell (3) which contains 7 electrons.

The table above shows us the first 11 elements in the periodic table and their electronic configuration. Illustrating the number of electrons present in each Principal Quantum shell according to their energy level. 

>Ground State (Limited to Electronic Configuration): The ground state configuration is the lowest energy (most stable) arrangement of an atom.

Note: The arrangement of electrons, present in an atom, in the atomic orbitals of an atom, is called the electron configuration.

Example 1: What is ground state electron configuration of Carbon which has the atomic number 6?  

Solution: Carbon has atomic number 6, indicating it has 6 protons and since a stable atom has the same number of protons and electrons. It would have six electrons as well. So we represent Carbon’s ground state through: 

 
                                                                   http://www.chem.ucalgary.ca/

2: What Are The Number Of Orbitals Making Up s, p and d Sub-shells, And The Number Of Electrons That Can Fill s, p and d Subshells?


>A s subshell contains only 1 orbital. As we remember, each orbital can hold only 2 electrons hence the subshell only contains 2 electrons.

                                                           
                    Cambridge International Examinations Lawrie Ryan, Roger Norris 
                            Illustration of s orbital and the 2 electrons it contains


> A p subshell contains 3 orbitals; hence it can hold 6 electrons (2 electrons in each orbital)
 
                                                          
                     Cambridge International Examinations Lawrie Ryan, Roger Norris 
                             Illustration of p orbital and the 6 electrons it contains

> A d subshell has 5 orbitals; hence it can hold 10 electrons (2 electrons in each orbital)
                                
                                                            
                                                            
  
                         Cambridge International Examinations Lawrie Ryan, Roger Norris 
                                 Illustration of d orbital and the 10 electrons it contains  


Short Recap:


The principle quantum number (n) of the shells is 1,2,3,4.  1 is closest to the nucleus.

Sub shells are s, p, d and f, present within the shells.

>s holds up to 2 electrons.

>p holds to 6 electrons.

 >d holds up to 10 electrons.

Orbitals are within the subshells which hold up to 2 electrons of opposite spin.
Principle quantum shell  
  1     
  2    
     3
4
Sub-shell
1s
  2s, 2p   
 3s, 3p, 3d
4s, 4p

3: What Is The Order Of Increasing Energy Of The Sub-Shells Within The First Three Shells And The 4s and 4p Sub-shells?


> An atom is filled up in the sub shells by the order of increasing energy (REMEMBER 3d is higher in energy than 4s and so gets filled after the 4s)
  • The Order is:
                 1s>2s>2p>3s>3p>4s>3d>4p



 
Cambridge International Examinations Lawrie Ryan, Roger Norris 
Note: n in the above diagram shows the Principal Quantum Number (n) of each shell. Also 


4: What Are The Electronic Configurations That Include The Number Of Electrons In Each Shell, Sub-shell And Orbital?


 
Cambridge International Examinations Lawrie Ryan, Roger Norris 

>The notation of the electron configuration of an atom is represented by the above format.

Examples: 
>Helium has 2 electrons hence both of the electrons can go into the orbital of 1s. This can hold a maximum of 2 electrons, so the electronic structure of helium is 1s2

>Lithium has 3 electrons. The 1s orbital can only hold a maximum 

5: Explain The Electronic Configurations In Terms Of Energy Of The Electrons And Inter-Electron Repulsion. 


> Electronic configuration is affected by:

The Energy Of The Electrons: Electrons are added one by one for successive elements, filling each subshell in order of increasing energy. The further the shell is from the nucleus the higher the energy of the shell.

Inter-Electron Repulsion: Electrons which are present in the same region of space repel each other because they have the same charge. Consequently, electrons will occupy different orbitals in the same subshell to minimize the repelling of the electrons. These electrons have their “spin” in the same direction. Electrons are only paired when there are no more empty orbitals available within a subshell. The spins are then opposite to minimize the repelling of the electrons.   

>An illustration of inter-electron repulsion is seen with:

                                           
               
                                           Cambridge International Examinations Lawrie Ryan, Roger Norris 
The diagram shows us that electrons are only added to a particular subshell, revealing electrons are only paired when no more empty orbitals are available.


6: How To Determine The Electronic Configuration Of Atoms And Ions Given The Atomic Or Proton Number And Charge, Using Either Of The Following Conventions:


a) Full Electron Configuration: The table below shows us that from the proton number of the atom. We can write the electronic configuration of the atom because a stable atom contains the same number of electrons and protons. Similarly, if we have the atomic number (total of protons and neutrons) knowing the number of protons from the total can help us write the electron configuration.

                            
                              Cambridge International Examinations Lawrie Ryan, Roger Norris 

b) Shorthand Electron Configuration: The electronic configurations of the elements after argon are shown in the table below. In this table part of the electronic configuration of each element is represented by [Ar]. This element illustrates the electronic configuration of argon: 1s2 2s2 2p6 3s2 3p6. This method is a shorthand way of writing electronic configurations of atoms with many electrons.

                          
                          Cambridge International Examinations Lawrie Ryan, Roger Norris 

Note: 
Electronic Configuration Of Potassium: Potassium has the electronic structure 1s2 2s2 2p6 3s2 3p6 4s1. The outer electron goes into the 4s subshell first rather than the 3d subshell because the 4s is below the 3d in terms of its energy of the overall shell.

Filling The 3d Subshell: Take note of how after calcium, the next electron goes into a 3d subshell rather than a 4p subshell. So scandium has the electronic configuration [Ar] 3d1 4s2. This is because electrons occupy the orbitals with the lowest energythe 3d subshell is just above the 4s subshell but below the 4p subshell.

Chromium And Copper: Electronic configurations of chromium and copper do not follow the expected pattern. Chromium has the electronic configuration [Ar] 3d5 4s1 (rather than the expected [Ar]3d4 4s2 ). Copper has the electronic configuration [Ar] 3d10 4s1 (rather than the expected [Ar]3d9 4s2 ). These two elements are an exception to the pattern.

Gallium To Krypton: The electrons are added to the 4p subshell because this is the next highest energy level after the 3d.

c) Electronic Configuration Of Ions: Positive ions are formed when electrons are removed from atoms. The sodium ion, Na+ (proton number = 11), has 10 electrons. So, its electronic configuration is 1s2 2s2 2p6. Note that this is the same as the electronic configuration of the Nobel gas neon, the element with 10 electrons in each atom. 

Negative Ions are formed when atoms gain electrons. The sulfide ion, S2– (proton number = 16), hence would have 18 electrons. The electronic configuration is 1s2 2s2 2p6 3s2 3p6, which is the same as argon, the element with 18 electrons in each atom.

 Note: The d-block elements behave slightly differently. Taking note of the elements across the Periodic Table from K to Zn, the 4s subshell fills before the 3d subshell. However, when atoms of a d-block element lose electrons to form ions, the 4s electrons are lost first.

For Example: 



Note: in both examples ions lose electrons from the 4s subshell first.

7: How To Use Electrons In Boxes Notation?


 
                         
                                         chemrevise.org

> In line with the above illustration, each box represents an atomic orbital. The orbitals (boxes) are arranged from bottom to top in order of increasing energy. The arrow and the direction of the arrow represent electrons and the spins of the electrons. An electron whilst filling an orbital of a subshell first occupies each orbital of a subshell singly before starting to pair up with other electrons in a different orbital. 

8: Describe And Sketch The Shapes Of s And p Orbitals:


s Orbital: 

                               
                  Cambridge International Examinations Lawrie Ryan, Roger Norris 

a)  Spherical in shape 
b)  Increases in size as the Principle Quantum number of the shell increases.

p Orbital: 

                          
               
                        Cambridge International Examinations Lawrie Ryan, Roger Norris 

a)  Dumbbell Shape: The shape is like an hourglass with two ‘lobes’. The three sets of ‘lobes’ are arranged at right angles to each other along the x, y and z axes. Hence the three 2p orbitals are named 2px, 2py and 2pz.

b)  There are three 2p orbitals in the second quantum shell. Each of these has the same shape. 

c)  The three 2p orbitals have the same energy as each other. There are also three 3p orbitals in the third quantum shell. Their shapes are similar to the shapes of the 2p orbitals.

9: Describe a free radical as a species with one or more unpaired electrons:


> A free radical is a very reactive atom or molecule that has one or more unpaired electron.

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