Page 333 - ABCTE Study Guide_Neat

As we know, a theory is modified when new information is obtained. As new data were obtained, the
model of the atom changed. The current model is a probabilistic model of electron location. Instead of
thinking of electrons as planets orbiting a sun, we currently believe the placement of electrons about the
nucleus of an atom is based on cloud-like probability structures, not orbits. It is best to envision different
types of electrons in clouds like the diagram below.

In the modern model of the atom, the electrons are no longer portrayed as moving in fixed orbits around
the nucleus. Instead, their position is indicated by a "cloud" that represents a probability of the electron's
position about the nucleus. The farther the electron is located from the nucleus, the higher its level of
energy. There is a limit to the number of electrons that can occupy a given energy level. In most atoms,
the highest, or outer, energy level can accommodate up to eight electrons. The lowest energy level can
only handle two electrons, so hydrogen and helium are exceptions to this octet rule.!

The number of protons in the atomic nucleus is called the atomic number of the element. The atomic
number of helium is 2. The mass of an atom is located almost entirely in its nucleus. The mass
number of an atom is the sum of the number of protons and the number of neutrons. Most carbon atoms
have six neutrons and all carbon atoms have six protons, so most carbon atoms have a mass number of
12.

Question

How many neutrons does an atom of zinc-64, , have?

A 30

B 34