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Electronegativity is the ability of an atom to attract the bonding pair of electrons in a covalent bond to itself. This is why its values can be used by chemists in order to predict whether bonds between different types of atoms are polar, non-polar, or ionic. Many factors affect electronegativity within atoms; there are also trends relating the elements in the periodic table to electronegativity.
Electronegativity is the power and ability of an atom to attract and pull a pair of electrons in a covalent bond towards itself.
Let's look at a few factors which impact electronegativity.
The distance between the outer electrons in the outermost shell and the nucleus is known as the atomic radius.
The shorter the distance between the negative outer electrons and the positive nucleus, the stronger the attraction between them. This means that if the electrons are further away from the nucleus, the attraction will be weaker. Therefore, if you decrease the atomic radius, electronegativity increases.
The nuclear charge is also known as the number of protons in the nucleus.
The negatively charged electrons and positively charged protons are attracted to each other. Due to this, if the number of protons is increased, then the nuclear attraction towards the outer electrons also increases.
Beware! Do not confuse nuclear charge with an element or compound having a charge.
If you increase the number of subshells and inner shells of an atom, the electronegativity decreases. This is because the electrons of the inner shells 'shield' the outer electrons from the attraction of the nucleus, causing the attraction between the outer electrons and the nucleus to decrease. Therefore, the addition of extra subshells and inner shells causes outer electrons to be less attracted to the nucleus.
Let's look at some basic trends in electronegativity, which generally hold true in the periodic table.
Electronegativity decreases going down a group in the periodic table. The nuclear charge increases as protons are added to the nucleus. However, the effect of shielding is also increased as there is an extra filled electron shell in each element going down a group. The atomic radius of the atom increases as you go down the group since you are adding more shells of electrons, which makes the atom larger. This leads to an increase in the distance between the nucleus and the outermost electrons, meaning that there is a weaker force of attraction between them.
As you go across a period in the periodic table, electronegativity increases. The nuclear charge increases because the number of protons in the nucleus increases. However, shielding remains constant since no new shells are being added to the atoms, and electrons are being added to the same shell each time. As a result of this, the atomic radius decreases because the outermost shell is pulled closer to the nucleus, so the distance between the nucleus and the outermost electrons decreases. This results in a stronger attraction for the bonding pair of electrons.
Diagram showing the electronegativity trends in the periodic table. Sahraan Khowaja, StudySmarter Originals
The Pauling scale is a numeric scale of electronegativities that can be used to predict the percentage ionic or covalent character of a chemical bond. The Pauling scale ranges from 0 to 4.
Halogens are the most electronegative elements in the Periodic Table, with fluorine being the most electronegative element of all, with a value of 4.0. The elements that are least electronegative have a value of approximately 0.7; these are caesium and francium.
Single covalent bonds can be formed by the sharing of a pair of electrons between two atoms.
Examples of molecules made up of a single element are diatomic gases, and molecules such as H2, Cl2, and O2. Molecules made up of a single element contain bonds that are purely covalent. In these molecules, the difference in electronegativity is zero since both atoms have the same electronegativity value and, therefore, the sharing of electron density is equal between the two atoms. This means that the attraction towards the bonding pair of electrons is equal, resulting in a non-polar covalent bond.
However, when atoms with different electronegativities form a molecule, the sharing of electron density is not equally distributed between the atoms. This results in the formation of a polar covalent bond. In this case, the more electronegative atom (the atom with the higher value in the Pauling scale) attracts the bonding pair of electrons towards itself. Due to this, partial charges appear on the molecule, since the more electronegative atom gains a partial negative charge, while the less electronegative atom gains a partial positive charge.
An ionic bond is formed when one atom completely transfers its electrons to another atom which gains the electrons. This occurs when there is a large enough difference between electronegativity values of the two atoms in a molecule; the least electronegative atom transfers its electron(s) to the more electronegative atom. The atom which loses its electron(s) becomes a cation which is a positively charged species, whilst the atom which gains the electron(s) becomes an anion, which is a negatively charged species. Compounds such as magnesium oxide (MgO), sodium chloride (NaCl), and calcium fluoride (CaF2) are examples of this.
Usually, if the difference in electronegativity is greater than 2.0, the bond is likely to be ionic. If the difference is less than 0.5 then the bond will be a non-polar covalent bond. If there is an electronegativity difference between 0.5 and 1.9, then the bond will be a polar covalent bond.
Let's have a look at some examples. Take LiF:
The electronegativity difference for this is 4.0 - 1.0 = 3.0, therefore this represents an ionic bond.
HF:
The electronegativity difference for this is 4.0 - 2.1 = 1.9, therefore this represents a polar covalent bond.
CBr:
The electronegativity difference for this is 2.8 - 2.5 = 0.3, therefore this represents a non-polar covalent bond.
As shown above, one can see all the Pauling electronegativity values of the elements from a dedicated Periodic Table. To calculate the bond polarity of a molecule, you have to subtract the smaller electronegativity value from the larger one.
Carbon has an electronegativity value of 2.5, and chlorine has a value of 3.0. So, if we were to find the electronegativity of the C-Cl bond, we would know the difference between the two.
Therefore, 3.0 - 2.5 = 0.5.
If the two atoms have similar electronegativities, then the electrons sit in the middle of the two nuclei; the bond will be non-polar. For instance, all diatomic gases such as H2 and Cl2 have covalent bonds which are non-polar as the electronegativities are equal in the atoms. Therefore, the attraction of electrons to both nuclei is also equal.
If two atoms have different electronegativities, however, the bonding electrons are attracted towards the atom which is more electronegative. Because of the uneven spread of electrons, a partial charge is assigned to each atom as mentioned under the previous heading. As a result, the bond is polar.
A dipole is a difference in charge distribution between two bonded atoms that is caused by a shift in electron density in the bond. The electron density distribution depends on the electronegativity of each atom.
A bond is said to be more polar if the difference in electronegativity is larger. Therefore, there is a larger shift in electron density.
You can read about this in more detail in Polarity.
Diagram showing the bond dipole. Sahraan Khowaja, StudySmarter Originals
Electronegativity is the power and ability of an atom to attract and pull a pair of electrons in a covalent bond towards itself.
The nuclear charge increases because the number of protons in the nucleus increases. The atomic radius decreases as the distance between the nucleus and the outermost electron decreases. Shielding remains constant.
The larger the difference between the electronegativity of the elements forming the bond, the higher the chance of the bond being ionic.
To calculate the polarity of a bond in a molecule, you have to subtract the smaller electronegativity from the larger one.
In a molecule such as hydrogen chloride, the chlorine atom drags the electrons towards itself slightly because it is the more electronegative atom and gains a partial negative charge, whereas hydrogen gains a partial positive charge.
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