Answer
(a) $^3H$ has 1 proton
$^3H$ has 2 neutrons
(b) $^{40}Ar$ has 18 protons
$^{40}Ar$ has 22 neutrons
(c) $^{40}Ca$ has 20 protons
$^{40}Ca$ has 20 neutrons
(d) $^{239}Pu$ has 94 protons
$^{239}Pu$ has 145 neutrons
Work Step by Step
(a) $^3H$ has atomic number 1
$^3H$ has 1 proton
Note that $3-1 = 2$
$^3H$ has 2 neutrons
(b) $^{40}Ar$ has atomic number 18
$^{40}Ar$ has 18 protons
Note that $40-18 = 22$
$^{40}Ar$ has 22 neutrons
(c) $^{40}Ca$ has atomic number 20
$^{40}Ca$ has 20 protons
Note that $40-20 = 20$
$^{40}Ca$ has 20 neutrons
(d) $^{239}Pu$ has atomic number 94
$^{239}Pu$ has 94 protons
Note that $239-94 = 145$
$^{239}Pu$ has 145 neutrons
