Edexcel A Level Chemistry:复习笔记1.5.1 Electronegativity

• Electronegativity is the power of an atom to attract the pair of electrons in a covalent bond towards itself
• The electron distribution in a covalent bond between elements with different electronegativities will be unsymmetrical
• This phenomenon arises from the positive nucleus’s ability to attract the negatively charged electrons, in the outer shells, towards itself
• The Pauling scale is used to assign a value of electronegativity for each atom

First three rows of the periodic table showing electronegativity values

• Fluorine is the most electronegative atom on the Periodic Table, with a value of 4.0 on the Pauling Scale
• It is best at attracting electron density towards itself when covalently bonded to another atom

Electron distribution in the C-F bond of fluoromethane

Nuclear charge

• Attraction exists between the positively charged protons in the nucleus and negatively charged electrons found in the energy levels of an atom
• An increase in the number of protons leads to an increase in nuclear attraction for the electrons in the outer shells
• Therefore, an increased nuclear charge results in an increased electronegativity

Atomic radius

• The atomic radius is the distance between the nucleus and electrons in the outermost shell
• Electrons closer to the nucleus are more strongly attracted towards its positive nucleus
• Those electrons further away from the nucleus are less strongly attracted towards the nucleus
• Therefore, an increased atomic radius results in a decreased electronegativity

Shielding

• Filled energy levels can shield (mask) the effect of the nuclear charge causing the outer electrons to be less attracted to the nucleus
• Therefore, the addition of extra shells and subshells in an atom will cause the outer electrons to experience less of the attractive force of the nucleus
  • Sodium (period 3, group 1) has higher electronegativity than caesium (period 6, group 1) as it has fewer shells and therefore the outer electrons experience less shielding than in caesium

   

• Thus, an increased number of inner shells and subshells will result in a decreased electronegativity

Trends in electronegativity

• Electronegativity varies across periods and down the groups of the periodic table

Down a group

• There is a decrease in electronegativity going down the group
• The nuclear charge increases as more protons are being added to the nucleus
• However, each element has an extra filled electron shell, which increases shielding
• The addition of the extra shells increases the distance between the nucleus and the outer electrons resulting in larger atomic radii
• Overall, there is decrease in attraction between the nucleus and outer bonding electrons

Electronegativity decreases going down the groups of the periodic table

Across a period

• Electronegativity increases across a period
• The nuclear charge increases with the addition of protons to the nucleus
• Shielding remains relatively constant across the period as no new shells are being added to the atoms
• The nucleus has an increasingly strong attraction for the bonding pair of electrons of atoms across the period of the periodic table
• This results in smaller atomic radii

Electronegativity increases going across the periods of the Periodic Table

• When two atoms in a covalent bond have the same electronegativity the covalent bond is nonpolar

The two chlorine atoms have the same electronegativities so the bonding electrons are shared equally between the two atoms

• The difference in electronegativities will dictate the type of bond that is formed
• When the electronegativities are very different (difference of more than 1.7) then ions will be formed and the bond will be ionic
• When two atoms in a covalent bond have a difference in electronegativities of 0.3 to 1.7 a covalent bond is formed and the bond will be polar
  • The electrons will be drawn towards the more electronegative atom
• As a result of this:
  • The negative charge centre and positive charge centre do not coincide with each other
  • This means that the electron distribution is asymmetric
  • The less electronegative atom gets a partial charge of δ+ (delta positive)
  • The more electronegative atom gets a partial charge of δ- (delta negative)

   

• The greater the difference in electronegativity the more polar the bond becomes

Cl has a greater electronegativity than H causing the electrons to be more attracted towards the Cl atom which becomes delta negative and the H delta positive