Name_________________________________________________Date________________________
CScope Unit 3
Modern Chemistry • CHAPTER 3-4
HOMEWORK 3-3
Isotopes and Atomic Number
The atomic number of an element is represented by the symbol Z. The atomic
number is the number of protons in the nucleus of an atom. Because an atom is electrically
neutral, the number of electrons must equal the number of protons. The number of
protons determines the identity of the element.
Isotopes are atoms of an element that are exactly alike chemically but slightly
different in mass. Isotopes have the same number of protons but a different number of
neutrons. The different number of neutrons is what accounts for the differences in mass
of isotopes of an element. The number of neutrons determines the particular isotope of
the element. The particles that make up an atomic nucleus are called nucleons. The total
number of nucleons (protons plus neutrons) in an atom is called the mass number, A. Thus,
the number of neutrons = A – Z. Isotopes are often symbolized by placing the atomic
number and mass number to the left of the chemical symbol. The mass number is a
superscript; the atomic number is a subscript. Both occur before the symbol. The isotope
of radon, an indoor air pollutant, is shown as follows.
222
Mass number (protons + neutrons) = 222
Atomic number (protons) = 86
The name of this isotope is radon-222.
Rn
Fill in Table 1 below.
Table 1.
Name
Nuclide
Symbol
Protons Electrons Neutrons Atomic
Number
Mass
Number
Magnesium-25
29
Si
11
12
6
14
3
7
Name_________________________________________________Date________________________
Problems using frequency, wavelength, the speed of light, and energy. Show formula used,
numbers substituted in, and answer with correct units and correct significant figures.
1. A microwave oven produces electromagnetic radiation of wavelength 12.2 cm. What
is the wavelength expressed in meters and in nanometers?
2. An FM radio station broadcasts on a frequency 91.5 MHz. What is the wavelength
of these radio waves in meters?
3. Most of the light emitted by a sodium vapor lamp has wavelength of 589 nm. What
is the frequency of this radiation?
4. The lowest frequency light that can produce a photoelectric effect on potassium
metal is 4.2 x 1014/s. What is the energy of one photo of this light?
5. What is the energy, in kJ/mol, associated with monochromatic radiation of
wavelength 225 nm? Remember that 1 mole = 6.022 x 1023 of anything.
6. What is the wavelength associated with electrons traveling at one-hundredth the
speed of light?