ATOMS STUDY GUIDE

 

What are atoms? Atoms are the basic building blocks of matter that make up everyday objects. A desk, the air, even you are made up of atoms! Atoms are created inside of stars, so every atom in your body has been through at least one star.  We are all made of star stuff!

Atoms are very small.  There are about 20 million hydrogen atoms in one millimeter!  Even so, atoms are made up of smaller parts called subatomic particles.  The three most important subatomic particles are protons, neutrons and electrons.  Below is an example of an atom.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The four circles in the center make up the nucleus of the atom.  The nucleus is made up of protons and neutrons.  Protons and neutrons are about the same size and have about the same mass ( 1 AMU or atomic mass unit).  Although they look the same, they have different charges.  A proton has a + or positive charge and a neutron has no charge at all.

 

All of the atoms in the periodic table have no net (total) charge.  They like to be neutral, or have a total charge of zero.  The atom above has two plusses (protons) in the nucleus. In order to have a total charge of zero, it needs to have two negatives as well (2+ + 2- = 0).  These negative charges are known as electrons.  Electrons are extremely small and move very fast around the nucleus in orbits.  The orbits are sometimes called shells.  The atom above has only one shell.  Because the first shell is so close to the nucleus, only two electrons can fit.  If the atom has more than two electrons, it must have more shells.  The second shell can hold as many as eight.  The shells further out can hold many more electrons, but for right now we will just say that they can only hold eight.

                Let’s look at a real atom from the periodical table of elements.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


The number at the top of the box is the atomic number.  This is important because it tells you how many protons are in the atom.  If you have an atom with 3 protons in the nucleus, it must be a Lithium atom. 

 

The bottom number is the mass number.  This number is important because it tells you the mass of the atom.  Since the mass of the electron is almost zero, the mass number is really the total of protons and neutrons in the nucleus.  The box tells us that we have 3 protons (atomic number).  The mass number tells us that the number of protons plus the number of neutrons is equal to 7.  So if we have 3 protons we must need 4 neutrons to get a total of 7.  Pretty easy huh?

 

Sometimes we have Lithium atoms that are a little heavier than normal.  In such cases we have mass numbers that are different.  The only way to have different mass numbers is to add or subtract protons or neutrons. But if you add or subtract a proton, you also change the atomic number which means that you have a different atom entirely.  So Lithium atoms with different  mass numbers occur because they have different numbers of neutrons.  Such atoms are called isotopes.

 

Atomic Forces

 

There are two important forces at work inside the atom.  The first is the electromagnetic force.

 

The electromagnetic force involves the force of attraction. Did you ever hear the saying that opposites attract? Well, it's true. Two things with opposite, or different charges (a positive and a negative) will attract, or pull towards each other. Things with the same charge (two positives or two negatives) will repel, or push away from each other.

 

Even though electrons are attracted to protons they never reach the protons.  That’s because the electrons are moving!  They are moving just fast enough so that they don’t get pulled into the proton, but also slow enough that they don’t fly away.

 

 

 

You may be thinking-“Wait a minute, there are a bunch of protons in the nucleus.  Why don’t they repel each other?”  Well, they do.  But the neutrons in the nucleus help create something called the strong force.  The strong force is stronger than the electromagnetic force, so even though the protons are repelling, the strong force keeps them together.  It’s like a spring.  You can push a spring down and it wants to spring back up.  If you push the spring harder than the spring is pushing up, the spring stays down.  If the spring is pushing up harder than you are pushing down on it, it will come up.  The strong force is pushing hard enough to keep the repelling protons together.

 

 

 

Hope this helps.  Good Luck on the test!