Download 37.19. Model: For a neutral atom, the number of electrons is the

37.19. Model: For a neutral atom, the number of electrons is the same as the number of protons, which is the
atomic number Z. An atom’s mass number is A = Z + N, where N is the number of neutrons.
Solve: (a) For a 207Pb atom, Z = 82. So, N = 207 – 82 = 125. The neutral 207Pb atom contains 82 protons, 82
electrons and 125 neutrons.
(b) The electric potential at the surface of a lead nucleus is
V=
9
2
2
−19
1 Q (9.0 × 10 N m / C )(82)(1.60 × 10 C)
=
= 1.66 × 10 7 V
7.1 × 10 −15 m
4πε 0 r
The electric field strength at the surface of a lead nucleus is
E=
2
2
−19
9
1 Q (9.0 × 10 N m / C )(82)(1.60 × 10 C)
=
= 2.34 × 10 21 V / m
2
−15
4πε 0 r 2
7.1 × 10 m
(
)