Counting Particles
What is an Isotope?Take a minute to look at the picture below. It shows three diferent types of Hydrogen atoms. Note which particles are the same in every atom, and which are different. Pay particular attention to the center of the atom, the nucleus.
You should have noticed that the number of protons is the same for all three atoms of hydrogen, and that the neutrons can change. For right now, we will always assume that the number of electrons is the same as the number of protons. It can change, but that is a topic for a different lesson. What is most important is that all atoms of hydrogen have only one proton, but can have a different number of neutrons. The protons define the element. Now we can get a definition for an isotope.
ISOTOPES - Atoms with same number of protons, but a different number of neutrons.
If we compare atoms to people, the protons would be like our skeleton: it never changes (except in drastic circumstances). However, we can gain and lose weight, just as an atom can gain and lose neutrons.
How Are Isotopes Written?
Isotopes are written in three different ways:
- 3H
- Hydrogen-3
- 31H
In all cases, the "3" represents the Isotopic Mass - the
total number of protons and neutrons. In the third case, the "1" represents
the Atomic Number - the number of protons. We only will use the first
two cases in this lesson. The third case is used mostly when studying nuclear
reactions.
How Can I Use The Periodic Table To Help?
Every element on the periodic table has two numbers in its box. The smaller number is always the Atomic Number, the number of protons. Let's look at Carbon as an example:
We see that every Carbon atom in the universe has six protons. That can never change. The larger number is the Atomic Mass. This is not the mass of any particular isotope, but rather a weighted average of all of the naturally occuring isotopes of an element. We will learn how to calculate Average Atomic Mass in Lesson Two. For right now, the only piece of information we will use from the periodic table is the number of protons.
Example: Calculate the number of protons, neutrons, and electrons in an atom of 8Li
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How Can I Use The Periodic Table To Help?
Every element on the periodic table has two numbers in its box. The smaller number is always the Atomic Number, the number of protons. Let's look at Carbon as an example:
We see that every Carbon atom in the universe has six protons. That can never change. The larger number is the Atomic Mass. This is not the mass of any particular isotope, but rather a weighted average of all of the naturally occuring isotopes of an element. We will learn how to calculate Average Atomic Mass in Lesson Two. For right now, the only piece of information we will use from the periodic table is the number of protons.
Example: Calculate the number of protons, neutrons, and electrons in an atom of 8Li
- To determine the number of protons, we look at the periodic table. Since
Lithium has an atomic number of "3", we know it has three protons.
# Protons# Neutrons# Electrons3
- Recall that the isotopic mass is the number of protons + the number of neutrons. Since this isotope has a mass of 8, it must have 5 neutrons (8 - 3 protons
= 5 neutrons).
# Protons# Neutrons# Electrons35
- Since we have 3 positively charged protons, we need 3 negatively charged
electrons to balance them. (Sometimes the number of electrons can change,
but we won't deal with that in this mini-course.)
# Protons# Neutrons# Electrons353
Site built and maintained by Mr. Fredericks
jfredericks@dallassd.com