The number of lone pair on central atom in BrF₅ and SF₄ are:

CONCEPT:

Lone Pairs on Central Atom

• The number of lone pairs on the central atom in a molecule can be determined using the valence shell electron pair repulsion (VSEPR) theory.
• Lone pairs are non-bonding pairs of electrons that are found on the central atom.
• To determine the number of lone pairs, we need to know the total number of valence electrons around the central atom and how many of these are used for bonding with other atoms.

EXPLANATION:

• For BrF₅:
  • Bromine (Br) has 7 valence electrons.
  • Fluorine (F) has 7 valence electrons and there are 5 fluorine atoms.
  • In BrF₅, bromine forms 5 single bonds with 5 fluorine atoms.
  • Each single bond uses 1 pair of electrons, so 5 bonds use 5 pairs (10 electrons).
  • Therefore, bromine has 7 - 5 = 2 electrons (or 1 pair) left as lone pairs.
    
• For SF₄:
  • Sulfur (S) has 6 valence electrons.
  • Fluorine (F) has 7 valence electrons and there are 4 fluorine atoms.
  • In SF₄, sulfur forms 4 single bonds with 4 fluorine atoms.
  • Each single bond uses 1 pair of electrons, so 4 bonds use 4 pairs (8 electrons).
  • Therefore, sulfur has 6 - 4 = 2 electrons (or 1 pair) left as lone pairs.
    

Therefore, the number of lone pairs on the central atom in BrF₅ and SF₄ are 1 and 1, respectively.