Name_________________________________ Period_____ Date__________
What is an Isotope?
You already know that all atoms of the same element have the same
number of protons in their nucleus. But protons are not the only
subatomic particles in the nucleus of an atom. There are also neutrons
in the nucleus.
Let’s look at the element Chlorine as it is represented on the Periodic
Table. That number above the element symbol is
17
the atomic number for chlorine. The atomic number
Cl
of an element tells you how many protons are in one
35.453
atom of that element. So, every single atom of
Chlorine
chlorine in the universe has 17 protons in its nucleus.
The number at the bottom of the Chlorine element square is the
atomic mass for chlorine. The atomic mass tells you how heavy atoms
are when compared to atoms of other elements. Atomic mass is
measured in atomic mass units, or amu. Chlorine’s atomic mass 35.453
amu.
25% of all chlorine atoms are Chlorine-37. Chlorine-37 is heavier than
Chlorine-35 because it has those two extra neutrons.
Questions:
1.Complete the sentence.
Answers:
All atoms of the same element
have the same number of
___________________.
2. What subatomic particles are
in the nucleus of an atom?
3. What does the atomic number
tell you?
4. What does the atomic mass
tell you about an atom?
5. Why is the atomic mass on the
periodic table a decimal and not a
whole number?
Why is the atomic mass on the periodic table a decimal and not a
whole number? Well, the atomic mass you see on the periodic table is
actually just an average mass. Not all atoms of a given element have
the same atomic mass. So, for example, every chlorine atom has 17
protons no matter what, but the number of neutrons can vary. Most
atoms of chlorine have 18 neutrons but some atoms of chlorine have
20 neutrons.
6. What is an isotope?
Isotopes are atoms of the same element with different numbers of
neutrons. These extra neutrons have no effect on the chemical
properties of the atoms but they do affect the atomic mass of the
atom. An isotope is called by its mass number – the number of
protons plus neutrons in the nucleus of the atom. For example,
Chlorine-35 has 17 protons and 18 neutrons in the nucleus of one
atom. Chlorine-37 also has 17 protons, but it has 20 neutrons in the
nucleus. About 75% of all chlorine atoms are Chlorine-35 and about
8. Which isotope is heavier?
Carbon-12 OR Carbon-14
9. Why is the isotope Oxygen-18
heavier than Oxygen-16?
7. Complete the sentence.
Extra neutrons have no effect on
the ____________ ____________
of atoms BUT they do affect the
___________ ___________ of the
atom.
The isotope Oxygen-18 is heavier
because ____________________
___________________________.