Edexcel A Level Chemistry:复习笔记1.5.1 Electronegativity

Defining 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

1.3-Chemical-Bonding-Electronegativity-of-Elements

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

1.3-Chemical-Bonding-Electronegativity-of-Fluorine

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

3.1.5-Electronegativity-trends-down-a-group

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

1.3-Chemical-Bonding-Trends-Across-a-Group

Electronegativity increases going across the periods of the Periodic Table

 

Bond Polarity

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

1.3-Chemical-Bonding-Nonpolar-Diatomic-Molecule

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

1.3-Chemical-Bonding-Polar-DIatomic-Molecule

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

 

 

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