Ionic Bond

Ionic bond is a type of chemical bond that exist between metal and nonmetal forming  an ionic compound which involves transfer of electrons.  Metals having low in ionization energy and electronegativity, tends to lose their valence electrons to nonmetals.  Nonmetals on the other hand accept valence electrons from metal due to their higher ionization energy and electronegativity.

Below is an example of ionic compound LiF:

Complete transfer of electrons occur between metal and nonmetal when the electronegativity difference is 2.0 above.  Just like the example above Li has electronegativiy of 1.0 and F has electronegativity of 4.0.  Their difference is 3.0 therefore there is a complete transfer of valence electrons from  Li to F.  

Let us look at this:

Li has 3 electrons and F has 9 electrons  with the electron configuration of:

Li =  1s² 2s¹      and     F =  1s² 2s² 2p⁵

Li has 1 valence electron while F has 7 valence electrons.  To become stable the atoms should complete 8 valance electrons, but there are some exceptions that we have to consider.

The valence electrons of Li upon giving its 1 valence electron to F,  is 2 from 1s² configuration and the F will have the configuration of 1s² 2s² 2p⁶      , which has the valence electrons of 8.

As you can see the valence electrons now of Li upon giving its valence electron will be 2, same with the valence electrons of H which is exempted to the Octet Rule, therefore stable, and the F will have now 8 valence electrons achieving the octet rule.  Therefore we can say that only one atom of Li is needed to make the F stable.  

The formula now of Lithium flouride is LiF, having 1:1 ratio.

Let us have more examples:

The reaction between Calcium with Chlorine is also an example of ionic bonding, with electronegativity difference of 2.0 (3.0 -1.0) looking at their valence electrons and lewis symbols;

Calcium has 2 valence electrons being in group IIA and chlorine has 7 valence electrons being in group VIIA.  The lewis symbol will be:

Since Ca has 2 valence electrons and Cl only need 1 valence electron, Ca will not be stable yet.  And so we need more another Cl atom to make Ca stable as shown below:

Therefore to become stable, calcium will react with 2 atoms of chlorine, and will have a formula of CaCl₂.

How about the reaction of Sr and N?  Could there be a transfer of electrons?  What is the electronegativity difference?  Electronegativity difference of 2.0 ( 3.0 - 1.0) therefore transfer of electrons will occur.   Sr is in group IIA having 2 valence electrons and N is in group VA having 5 valence electrons.

If we will only allow 1 Sr atom to react with 1 atom N, then both atoms will not be stable yet, so how many Sr is needed to react with N and vice versa.  Lets find out:

Therefore to become stable Sr needs 3 atoms to 2 atoms of N.  Chemical Formula is Sr₃N₂.

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Chemical Bond

Chemical bond is an attraction that hold atoms together forming molecule and formula unit.  There are three general types of bonds: the ionic, covalent and metallic.

Ionic bond refers to electrostatic attraction between ions of opposite charge, the cation and anion forming ionic compound.  The bond is formed by transfer of electrons from metal to nonmetal.  Further discussion about this on my next post.

Covalent bond is another type of bond which involve sharing of valence electrons between two nonmetals.  The best example is the sharing of electrons of diatomic molecules like H₂, O₂, etc. Detailed discussion on my next post.

Metallic bond on the other hand is a bond that exist in metals such as aluminum, copper, and iron. In metals each atom is bonded to several neighboring atoms.  The valence electrons in metallic bonding forms sea of valence electrons that are free to move throughout the three-dimensional structure of the metal.

Lewis Symbol

In chemical bonding, the one responsible are the valence electrons, the electrons occupying the highest energy level of an atom.  We also learn that the valence electrons of representative elements are the same with the group number of the elements.  

Now to easily understand covalent bonding, we need to learn how to write Lewis Symbol.  Lewis symbol for an element consist of the chemical symbol for the element plus dots representing the valence electrons surrounding the chemical symbol.  This Lewis symbol was formulated by an American Chemist G.N. Lewis (1875 - 1946) and now known as the Lewis electron-dot symbols or merely Lewis symbol. 

Below are the Lewis symbol of Representative elements:

The Octet Rule

Why atoms undergo reaction?  Do they want something?  Yes, atoms want to become stable.  Just like us we do something to our life to become stable financially, emotionally and more.  Atoms became stable either by giving up, accepting or sharing their valence electrons.  But atoms follow a certain rule to achieve stability.  And what is that rule,  and that's the Octet Rule.  Octet Rule states that atoms gain stability by achieving 8 valence electrons same with the noble gases.  

 Exception of the Octet Rule

There are many exceptions to the Octet Rule, one of this are the elements consisting of less than 4 valence electrons located in the first and second period, will not complete the 8 valence electrons such as H, Li and B.  There are also some elements that can have more that 8 valence electrons, those elements containing 5 or more valence electrons.

Trends in Properties of the Elements in the Periodic Table

To know more about the different elements in the periodic table, you need to know the different properties of the elements and their trends.   We need to recall that Periodic Table of Elements, the horizontal rows called periods and the vertical columns called groups or families.

Atomic Size

     Atomic size refers to the how big or small an atom is.  To determine the atomic size of an atom, atomic radius is used, which is a one-half the distance between the two nuclei in two adjacent metal atoms.

     The first figure above shows the atomic radius of iodine diatomic molecule, the atomic radius is measured from the nucleus of the first atom of the iodine to the next atom of the molecule.  The second figure shows how the atomic radius of two adjacent atoms is measured.  

     Usually the atomic size increases from top to bottom and decreases from left to right in a given period.  What could be the possible reason of this trend?  As we move down a group, let's say from H to Fr the atomic size increases that is because of the addition of the energy level.  What about in a given period where elements have the same energy level?  The atomic number increases as we move from left to right in a given period, this means that the number of protons and electrons increases and therefore there is an increase in the effective nuclear charge, this is why the atomic size of the atoms decreases in moving from left to right in a given period.

Below are the radii (in picometers) of representative elements according to their position in the periodic table.

Plotting atomic size versus atomic number will give some repeating patterns.  The elements in group IA are the elements on the highest point of the graph while elements in Group VIIA are the elements in the lowest point.  This only indicates that atomic size decreases in a given a period from left to right.

Ionization Energy

Ionization energy is another property of the elements, which is defined as the minimum energy required to removed an electron from a gaseous atom in its ground state.  It is usually expressed in kJ/mol).  The magnitude of the ionization energy is a measure of how tightly the electron is held in an atom.  The higher the ionization energy, the more difficult it is to removed the valence electrons from an atom.  For many atoms, the amount of energy required to remove the first electron from an atom is called first ionization energy.

The Ionization Energies of First 2 Elements in kJ/mol

What is the trend in ionization energies in going down a group and in moving from left to right in a given period?  Let us see the graph below: 

As we observed based from the graph above, the elements lie on the highest points are the elements in group VIIIA or group O/Noble Gases.  And the elements lie on the lowest point of the graph are the elements in group IA/Alkali metals.  Which means that in moving from left to right in a given period the ionization energy increases.  therefore metals have lower ionization energies than nonmetals.  This is the reason why metals tend to lose electrons forming cations or positive ions and nonmetals tend to gain electrons forming anions or negative ions.  One reason also of the increase of the ionization energy from left to right is the increase of the nuclear attraction between the protons and valence electrons due to the decrease in the atomic size of the atoms.

In going down a group let say in Group IA from Li to Cs, there is a decrease in ionization energy.  This means bigger atoms tend to lose electrons because of the lower nuclear attraction between the protons and the valence electrons.  

Example :  

1.  Which has lower ionization energy Li or Rb?  Na or Ar?

Answer:   Rb has lower ionization energy than Li since Rb is bigger in size than Li.   Na has lower ionization energy than Ar for the same reason.

Electron Affinity

Electron affinity is another property of elements which vary also in a group and within a given period.  It is defined as the energy released when an atom accepts electrons from another atom forming anion.  

Electrons Affinities of Some Representative Elements and Noble Gases in kJ/mol

From the above table, generally the electron affinity increases in moving from left to right in a given period.  This means that  nonmetals have higher electron affinity than metals.  In going down a group the electron affinity is decreased.

Two sign conventions are used for electron affinity, it can be positive or negative.  In most introductory text, the thermodynamic sign is used.  A negative sign indicates that the addition of electrons is an exothermic process.  Historically, however, electron affinity is defined as the energy released when an electron is added to a gaseous atom or ion, the electron by this convention is positive.  

Electronegativity

Electronegativity is another property of an element, it is defined as the ability of an atom to attract an electron to itself in relation to chemical bonding.  The higher the value of electronegativity , the greater is the ability to attract electrons to itself.  The electronegativity of an atom is related to ionization energy and electron affinity, which are also other properties of the elements.  An atom with very negative electron affinity and high ionization energy will attract electrons more easily and resist having its electrons attracted away, and this is called  highly electronegative atom.

Generally, electronegativity increases in moving from left to right in a given period, and decreases in moving down a group, as shown in Table below:

Since nonmetals have higher electronegativity values than metals, they have the ability to attract electrons than metals. That's the reason why nonmetals carries negative charge forming anion and metals form cation.

For general trends in the different properties of elements in the Periodic Table, refer on the table below:

Valence Electrons

What is valence electrons?

Valence means last, therefore valence electrons are the electrons located at the highest energy level.  Valence electrons are the one responsible for chemical bonding and also responsible for the emission of light or energy.

How do we determine the valence electrons of the elements?

There are two ways we can determine the valence electrons of an element, one of these is with the use of electron configuration.   If you are using electron configuration, you need to determine the highest energy level and count the number of valence electrons found in the highest energy level.  But these is only true for those Representative elements or the group A elements.

Let us have an example

1.  H, atomic number is 1

     Electron Configuration:   1s¹

     Since hydrogen has only one electron, therefore the valence electron of hydrogen is only 1.

2.  He, atomic number is 2

     Electronic Configuration:  1s²
     Valence Electron :  2

    Since helium has only one sublevel and orbital, valence electrons is 2.

3.  F, atomic number = 9
     Electron Configuration:   1s² 2s² 2p⁵
     Valence electrons :   7

     The valence electrons of Flourine is 7 , the highest energy level is 2 having two sublevels the s and p.  Two electrons in 2s sublevel and 5 electrons in 2p sublevel, which equals 7 valence electrons.  We need to add since both are in the same energy level.

4.  Ca, atomic number = 20
     Electronic Configuration:  1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
     Valence electrons:  2

5.  Al, atomic number = 13
     Electronic Configuration:  1s² 2s² 2p⁶ 3s² 3p¹
      Valence electrons:    3

Relationship Between the Valence Electrons and the Group Number

Representative elements (Elements from group IA to group VIIIA now group 0) valence electrons are related to their group number.  Based from the examples above, Hydrogen being in group IA has 1 valence electron, Flourine in group VIIA has 7 valance electrons, Calcium in group IIA has 2 valence electrons and aluminum group IIIA has 3 electrons.  Except helium which is an exception. In other words, valence electron is just equal to the group number of elements in Representative group of elements but not related to transition elements.

Below are some examples of elements:

Electron Configuration

Electron configuration is the arrangement of electrons in successive sublevels and orbitals.   To determine the electron configuration of the elements electron distribution mnemonics is used.

In the electron distribution mnemonics the sublevels are already arranged according to increasing energy as shown below: 

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f  6d 7p

Now in writing electronic configuration we have to follow the Pauli's Exclusion Principles which states that, no two electrons can have four quantum numbers, and that orbitals can only accommodate 2 electrons with opposite spins.    Since, we only have 1 s orbital in s sublevel, only two electrons can occupy in s sublevel, and we have 3 p orbitals in p sublevel, and therefore total of 6 electrons can be accommodated in p sublevel.  In d sublevel there are 5 d orbitals, accommodating 2 electrons in each orbital there are 10 total electrons that can be accommodated in d sublevel, and in f sublevel there are 7 f orbitals and having  2 electrons each orbital, total of 14 electrons are in sublevel f.  An so we have to take note in writing electrons configuration the following:  s can have only 1 or two electrons, p can have only 1 to 6 electrons, d can have 1 to 10 electrons, and f sublevels can have 1 to 14 electrons.  

We use atomic number in writing the electron configuration, since atomic number is the number of protons at the same time the number of electrons.  Let us have an example:

What is the electron configuration of first 10 elements? 

₁H has only 1 electron and so the the electron is only located at the lowest sublevel 1s.  The electron configuration is :  1s¹

₂He has 2 electrons. The electron configuration is 1s²

₃Li :  1s² 2s¹

₄Be:  1s² 2s²

₅B : 1s² 2s² 2p¹

₆C : 1s² 2s² 2p²

₇N : 1s² 2s² 2p³

₈O : 1s² 2s² 2p⁴

₉F :  1s² 2s² 2p⁵

₁₀Ne : 1s² 2s² 2p⁶

We have to take note that in writing electron configuration, electron distribution mnemonics must be followed.