The atoms and elements that make up all matter matter a whole lot on the GED Science exam. Learn the basics about these scientific building blocks with this guide to GED chemistry: atoms and elements.
Atoms and Elements
Everything physical in the universe is made up of matter. Trees, air, rocks, your computer, YOU… everything is made of matter.
Some key things to know about matter:
- Matter takes up space.
- Matter has mass.
- Matter cannot be created or destroyed— only changed. (If you want to get technical, this is called the Law of Conservation of Matter.)
An atom is the basic unit of matter. There are different types of atoms, based on how many protons, neutrons, and electrons they contain (more on that in a minute). Each unique type of atom is called an element. These elements are listed on the periodic table.
Parts of an Atom
Atoms contain three main parts: protons, neutrons, and electrons.
Protons have a positive (+) charge. To remember this, think “pro = positive.” On a pro-con list, the pros are the positives. PROtons have a positive charge.
Neutrons have a neutral charge (no charge). They are not positive or negative. That should be pretty easy to remember. NEUTRon = NEUTRal.
Electrons have a negative (-) charge. (No handy trick for remembering that one, but if you know that positive and neutral are already taken, negative is the only logical thing left!)
An atom has an overall neutral charge. The strength of its positive charge must equal the strength of its negative change. This means that atoms have the same number of protons (+) as electrons (-). That way, they balance each other out. Since neutrons have a neutral charge, their number doesn’t affect the charge of the atom. The number of neutrons per atom can be different than the number of protons and electrons.
The center of an atom is called the nucleus. This is where the protons and neutrons hang out. Electrons float around outside the nucleus. Depending on the atomic model, this may be in “rings” or “clouds.” For the purposes of the GED, you’ll see electrons on rings around the nucleus, like in the model below.
Image by XSapien
This is a beryllium atom. You’ll see that the protons (red in this diagram) and neutrons (white) are clustered together in the nucleus. The electrons (black) are further out in rings. Notice that there are 4 protons and 4 electrons, keeping the atom’s charge balanced (neutral). Beryllium happens to have 5 neutrons, but again, this does not affect the charge.
One other thing you might notice about the beryllium diagram we just saw is the number 9 next to the name: “beryllium-9.” That means that this beryllium atom has a mass of 9 atomic mass units (AMU). To estimate the mass of an atom, you can add up the number of protons and neutrons it has. Each proton and each neutron weighs about 1 AMU. So, looking at this beryllium atom, there are 4 protons and 5 neutrons, for a total of 9. This gives the atom its mass of 9 AMU. You might be thinking, “Hey, what about electrons?” Electrons are so teeny tiny that their mass is considered negligible. You can get a close estimate of atomic mass by just adding up the protons and neutrons.
You can also work backwards from mass to find the number of neutrons in an atom given its mass. If you look at the periodic table, each element has a number next to it (in the top-left on the periodic table above). This is its atomic number, and it is the same as the number of protons in an atom of the element. The number of protons in an atom is unique to each type of element. The periodic table also gives you the mass. So… you can subtract to find the number of neutrons.
Let’s take fluorine (F) for example. Looking at the periodic table, its atomic number is 9, so it has 9 protons. Its mass is about 19 AMU. That means it has 19 – 9 = 10 neutrons.
Need more practice? Check out this post on the periodic table.
When an Atom Isn’t an Atom
As we said before, an atom has a neutral charge and an equal number of protons and electrons.
Electrons arrange themselves in layers or rings called “shells.” One shell holds up to 8 electrons. An atom can have many shells, depending on how many electrons it has. Shells always fill up all the way to 8 before adding a new shell. Only the outermost shell may have fewer than 8 electrons. The electrons in the outermost shell are known as valence electrons.
Sometimes (such as during bonding), these valence electrons jump ship and attach to another atom. If an atom gains or loses electrons, its charge changes and it isn’t neutral anymore. The atom is then known as an ion.
- If an atom LOSES electrons, its charge becomes POSITIVE and it’s called a cation.
- If an atom GAINS electrons, its charge becomes NEGATIVE and it’s called an anion.
A molecule is two or more atoms that have bonded together. These atoms can be the same type (the same element) or different. You can write out the name of a molecule using letters and numbers to represent the different types of atoms in the molecule. This is called a chemical formula.
Let’s look at an example. Here’s a water molecule. Notice that the chemical formula (H2O) comes from the number of each type of atom in the molecule.
The letters tell you the name of the elements and the numbers tell you how many atoms of each element are in one molecule. If there’s no number, assume it’s a 1.
So H2O is 2 hydrogens + 1 oxygen.
Similarly, CO2 (carbon dioxide) is 1 carbon + 2 oxygen.
When writing chemical equations (see below), if you want to indicate more than one molecule of the same type, you do it with a coefficient (a regular-sized number in front of the entire chemical formula).
- 1 molecule of water is written as H2O.
- 2 molecules of water are written as 2H2O.
Chemical Reactions and Equations
Chemical reactions are what happen when two different kinds of molecules react to each other and change into new types of molecules. A chemical equation is just a way to write down what happens in a chemical reaction.
Let’s take a simple example.
When hydrogen and oxygen combine to form hydrogen peroxide, it looks like this:
H2 + O2 = H2O2
The two substances combined to form a new, different substance. Notice that there are still 2 hydrogen atoms and 2 oxygen atoms on each side of the equation, but now they’re just combined in a new way. (Remember: Law of Conservation of Matter!)
Sometimes, more than one new substance is formed. In this case, the elements might get rearranged a little more, but they’re all still there.
NaOH +HCl → NaCl + H2O
This is sodium hydroxide and hydrochloric acid reacting to form sodium chloride (salt) and water.
Wrapping It Up
If you study chemistry, all of this gets a lot more complex when looking at exactly how elements bond, how chemical reactions happen, and more. For the purposes of the GED, though, just make sure you have down the big picture ideas like:
- The definitions of matter, atoms, elements, and molecules
- The parts of an atom (proton, neutron, electron) and their charges
- How to pull information from a periodic table
- How to read a chemical formula and equation