Which neutral atom is isoelectronic with each of the following ions: F−, Ca2+, P3+, I−, Pb2+?

F⁻ :
The atomic number of F is 9.
F atom has 9 electrons, and thus F⁻ ion has 9 + 1 = 10 electrons.
Ne (neon) is the element with atomic number 10, and Ne atom has 10 electrons.
Hence, Ne atom is isoelectronic with F⁻ ion.

Ca²⁺ :
The atomic number of Ca is 20.
Ca atom has 20 electrons, and thus Ca²⁺ ion has 20 - 2 = 18 electrons.
Ar (argon) is the element with atomic number 18, and Ar atom has 18 electrons.
Hence, Ar atom is isoelectronic with Ca²⁺ ion.

P³⁺ :
The atomic number of P is 15.
P atom has 15 electrons, and thus P³⁺ ion has 15 - 3 = 12 electrons.
Mg (magnesium) is the element with atomic number 12, and Mg atom has 12 electrons.
Hence, Mg atom is isoelectronic with P³⁺ ion.

I⁻ :
The atomic number of I is 53.
I atom has 53 electrons, and thus I⁻ ion has 53 + 1 = 54 electrons.
Xe (xenon) is the element with atomic number 50, and Xe atom has 54 electrons.
Hence, Xe atom is isoelectronic with I⁻ ion.

Pb²⁺ :
The atomic number of Pb is 82.
Pb atom has 82 electrons, and thus Pb²⁺ ion has 82 - 2 = 80 electrons.
Hg (mercury) is the element with atomic number 80, and Hg atom has 80 electrons.
Hence, Hg atom is isoelectronic with Pb²⁺ ion.

Are you sure that is P3+, not P3-? I don't think there's such thing as P3+. If it's P3-, then it's isoelectronic with Ca2+