1. The Atom
Chapter 6

2. Atoms – The Inside Story
About 100 years ago, scientists came up with a term for the smallest piece of matter: the atom.
The word atom comes from the Greek word “atomus” which means “indivisible.”

3. Atoms – The Inside Story
Since scientists didn’t have very good laboratory equipment over one hundred years ago, they were slightly mistaken about atoms.
The atom is the smallest piece of one specific element.
Atoms, however, can be broken down into three smaller pieces.

4. TYPES OF SUBATOMIC PARTICLES
There are three particles inside the atom:
Protons
Neutrons
Electrons

5. Protons:
Protons are positive particles inside the nucleus (middle core) of the atom.
They have a charge of +1.
Protons are given a mass of 1 amu (atomic mass unit).

6. Neutrons: Neutrons are also located in the nucleus of the atom.
As their name suggests, they are neutral particles; they have a charge of zero.
Neutrons also have a weight of 1 amu.

7. Electrons:
Electrons are negatively charged particles. They have a charge of -1. As we’ll learn later in the course, they are responsible for electricity.
Electrons are not located in the nucleus of the atoms. Electrons spin outside of the nucleus in energy levels called “orbits” or “shells.”
Electrons are extremely light. They are 2000 times lighter than a proton or a neutron. Since they are so small and light, we say they have a mass of zero.

8. IMPORTANT NUMBERS AND ATOMS
The number of protons in an atom determines what element an atom is.
An atom with six protons will always be carbon.
An atom with two protons will always be helium.
The number of protons in an atom is called the atomic number.
The atomic number will always tell you how many protons AND how many electrons are in an atom.

9. Atoms ALWAYS have an overall charge of zero, therefore atoms are neutral.
For example, let’s look at nitrogen. The atomic number of nitrogen is seven. This means that nitrogen has seven protons and seven electrons.
Every proton is positive and every electron is negative, therefore:
(+7) + (-7) = 0
Therefore, every atom of nitrogen is neutral (no charge).

10. Another important number we need to consider is the “atomic mass number.” The atomic mass number tells us how heavy the atom is.
The atomic mass number of an atom only weighs the nucleus.
Do you remember which two particles are in the nucleus?

11. The protons and the neutrons
And each of these weigh 1 amu.
Therefore, the atomic mass number is the sum of the number of protons and the number of neutrons in an atom.

12. Example: Fluorine
Fluorine has an atomic number of 9, and an atomic mass number of 19.
Therefore, fluorine has 9 protons (the same number as the atomic number) and 9 electrons (also the same as the atomic number).

13. If there are 9 protons, how many neutrons are there?
The atomic mass number of 19 tells us there are a total of 19 protons AND neutrons in the atom (specifically in the nucleus of the atom).
If there are 9 protons, how many neutrons are there?
Number of Neutrons = Atomic Mass Number - Atomic Number

14. Fluorine
So for fluorine:
Number of Neutrons = = 10 neutrons

15. Cesium What is the atomic number of Cesium? 55.
What is the atomic mass number of cesium?
133.
Now let’s see how many of each subatomic particle cesium has.

16. Let’s use the formula to find out how many neutrons cesium has:
Cesium has 55 protons and 55 electrons (since the atomic number tells us how many protons and electrons are in an atom).
Let’s use the formula to find out how many neutrons cesium has:
Number of Neutrons = Atomic Mass Number – Atomic Number =
= 78 neutrons

17. If an atom has 10 protons and 19 neutrons, what is the atom’s atomic mass number?
Remember that the atomic mass number is the number of protons and neutrons added together.

18. Therefore the atomic mass number for this example is:
Number of Neutrons = Atomic Mass Number – Atomic Number
19 = Atomic Mass Number - 10
= Atomic Mass Number
29 = Atomic Mass Number

19. When the atomic mass number of an atom is not a whole number (for example: the atomic mass number of magnesium is 24.3), round the number to the nearest whole number (24.3 rounds to 24).

20. Classwork/Homework Element Worksheet (in class)
AIM 14 – Atomic weight (in class)
AIM 6 – How small is an atom?
AIM 7 – What is an element?