How Electronegativity Changes Across Periods and Groups
An atom can attract shared electrons in a covalent bond. It is seen as the higher the electronegativity value, the more strongly the elements attract the shared electron. Electronegativity predictably varies in the periodic table. It increases from bottom to top in groups and increases from left to right across periods. Hence, fluorine becomes the most electronegative element, and francium becomes one of the least electronegative element.
Talking about the electronegative trends, they are not so smooth among the transition and inner transition metals. But the electronegativity trend is fairly regular for main group elements.
Ionic Bond
It is defined as when atoms with an electronegativity difference of greater than 2 units are joined together, the bond between them is named as an ionic bond. Here, the less electronegative elements have a positive charge and the more electronegative element has a negative charge.
For example, We will include Sodium and Chlorine for further understandability of ionic bonds.
Sodium has an electronegativity of .93 whereas Chlorine has an electronegativity of 3.16. When Sodium and Chlorine form an ionic bond, where Chlorine takes away the electron from Sodium cation Na+ and Chloride anion Cl-. Chloride and Sodium ions attract each other very strongly due to the opposite charges, therefore form a crystal lattice.
Covalent Bond
When atoms with an electronegativity difference of fewer than two units are joined, the bond is said to be a covalent bond. Here, electrons are shared by both atoms. When the same atom shares electrons in a covalent bond, this results in no electronegativity difference between them. Hence, there is a symmetrical distribution of electrons between bonded atoms. These bonds are nonpolar covalent bonds. For this let us assume as an example of when two chlorine atoms are joined together, the electron spends as much time close to one chlorine as it spends with another. This results in a nonpolar molecule.
When the electronegativity difference is between 0 and 2, the more electronegative element attracts the shared more strongly. They are not strong enough to remove the electrons completely to form an ionic compound. The electrons are shared in an unsymmetrical distribution of electrons between the bonded atoms. These bonds are called polar covalent bonds. There are two things included further in a covalent bond. These are discussed further:
Partial Negative Charge, D- : The more electronegative atom has a partial negative charge. It is because the electrons spend time closer to the atom.
Partial Positive Charge, D+ : The less electronegative atom has a partial positive charge. It is because the electrons are not completely pulled away from the atom.
Let us take an example of a hydrogen chloride molecule to understand it even further. In the hydrogen chloride molecule, chlorine is electronegative than hydrogen by 0.96 units. Hence, the shared electron spends more time close to the chlorine atom making the chlorine molecule slightly negative, whereas the hydrogen end of the molecule is slightly positive. This results in a polar molecule.
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Patterns of Electronegativity In The Periodic Table
The distance of electrons from the nucleus remains constant in a periodic table row but talking about a periodic table column. The distance is not relatively constant. Coulomb’s law is the force decided between charges. It is given as below:
F = k ( Q1Q1) / x2
In this expression, Q represents a charge, k represents constant and r is the distance between the charges. The statement concluded from the equation is that the distance between the charges increases, the force decrease. This is also called quadratic change. This change results in an increase in electronegativity from bottom to top in a column in the periodic table. Elements at the topmost of the column have higher electronegativities than elements at the bottom of a given column.
The overall electromagnetic trend in the periodic table is diagonal from the lower-left corner to the upper right corner. Since there are exceptions in every concept, here as well we find this trend not applicable to all elements. We need to memorize some of the elements and their trend. The sequence is followed by F, O, Cl, N, Br, I, S, C, H, and lastly metals.
Electronegativity Across a Period
The positively charged protons attract the negatively charged electrons. As the number of protons increases, the electronegativity will increase. Looking at this explanation, the electronegativity increase from left to right in a row in the periodic table.
As we go down a group, electronegativity decreases. The attraction that a pair feels for a particular nucleus depends upon the following factors:
Number of protons in the nucleus
Amount of screening by inner electrons
Distance from the nucleus
FAQs on Electronegativity of Elements in the Modern Periodic Table
1. What exactly is electronegativity as a property of an element?
Electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself when it forms a chemical bond. It is not a property of an isolated atom but rather a measure of its electron-attracting power within a molecule. It is commonly measured on the Pauling scale, where higher values indicate a stronger attraction for electrons.
2. How does the electronegativity of elements change when moving from left to right across a period in the modern periodic table?
As you move from left to right across a period, the electronegativity of elements generally increases. This is because, within the same period, electrons are added to the same valence shell. However, the number of protons in the nucleus (nuclear charge) increases, which pulls the electron cloud closer and results in a smaller atomic radius. This stronger attraction from the nucleus for the shared pair of electrons leads to higher electronegativity.
3. Why does electronegativity decrease on moving down a group in the periodic table?
Electronegativity decreases as we move down a group for two main reasons:
- Increased Atomic Radius: Each step down a group adds a new electron shell, increasing the distance between the nucleus and the valence electrons.
- Electron Shielding: The inner electron shells shield the valence electrons from the full attractive force of the nucleus. This combined effect weakens the nucleus's pull on the shared electrons in a bond, thus lowering the electronegativity.
4. Which element has the highest electronegativity and why are noble gases not typically assigned a value?
Fluorine (F) is the most electronegative element, with a Pauling value of approximately 3.98. Its high electronegativity is due to its small atomic size and high effective nuclear charge, allowing its nucleus to exert a very strong pull on shared electrons. Noble gases like Helium and Neon are generally not assigned electronegativity values because they have a stable, completely filled valence shell and do not readily form chemical bonds to share electrons.
5. How does the difference in electronegativity between two atoms predict the type of chemical bond they will form?
The difference in electronegativity (ΔEN) between two bonding atoms is a crucial indicator of bond type:
- If the ΔEN is close to zero (e.g., in H₂ or Cl₂), the electrons are shared equally, forming a nonpolar covalent bond.
- If the ΔEN is small to intermediate (approx. 0.4 to 1.7), the electrons are shared unequally, creating a polar covalent bond with partial positive and negative charges (e.g., in H₂O).
- If the ΔEN is large (typically > 1.7), one atom pulls the electron pair so strongly that it is essentially transferred, forming an ionic bond (e.g., in NaCl).
6. What is the key difference between electronegativity and electron gain enthalpy?
While both relate to an atom's attraction for electrons, they are fundamentally different concepts.
- Electronegativity is a relative, qualitative measure of an atom's ability to attract a shared pair of electrons in a chemical bond. It has no units.
- Electron Gain Enthalpy is a quantitative, measurable energy change (in kJ/mol) that occurs when an electron is added to an isolated, gaseous atom to form a negative ion. It is a property of a single atom.
7. What is the 'diagonal relationship' in the context of electronegativity?
The diagonal relationship refers to the similarity in properties, including electronegativity, between certain elements in the second and third periods of the periodic table that are diagonally adjacent. For instance, Lithium (Li) is similar to Magnesium (Mg), and Beryllium (Be) is similar to Aluminium (Al). Their electronegativities are close because the effect of moving one step right (increasing EN) is roughly cancelled out by moving one step down (decreasing EN), resulting in comparable chemical behaviour.
8. Are there any significant exceptions to the standard electronegativity trends in the periodic table?
Yes, there are some notable exceptions, particularly in the p-block elements. For example, in Group 13, the electronegativity does not decrease smoothly. The trend is approximately B > Tl > In > Ga > Al. This irregularity arises from the poor shielding effect of the d- and f-orbitals in heavier elements like Gallium and Thallium, which increases the effective nuclear charge more than expected and thus raises their electronegativity.
