91影视

${}_{82}{}^{207}Pb$

Given,

Atomic number = 82

Atomic mass = 207

Number of protons = Atomic number = 82

Number of electrons = 82

We know,

Atomic mass = number of protons + number of neutrons

So,

Number of neutrons = atomic mass 鈥� number of protons

Number of neutrons = 207 鈥� 82 = 125

${}_5{}^{9}Be$

Given,

Atomic number = 5

Atomic mass = 9

Number of protons = Atomic number = 5

Number of electrons = 5

We know,

Atomic mass = number of protons + number of neutrons

So,

Number of neutrons = atomic mass 鈥� number of protons

Number of neutrons = 9 鈥� 5 = 4

${}_{33}{}^{75}As$

Given,

Atomic number = 33

Atomic mass = 75

Number of protons = Atomic number = 33

Number of electrons = 33

We know,

Atomic mass = number of protons + number of neutrons

So,

Number of neutrons = atomic mass 鈥� number of protons

Number of neutrons = 75 鈥� 33 = 42