ATOMIC STRUCTURE

Atoms Are Building Blocks

Atoms are the foundation of chemistry. They are the basis for everything in the Universe. As you know, matter is composed of atoms. Solids are made of densely packed atoms while gases have atoms that are spread out. We're going to cover basics like atomic structure and bonding between atoms. As you learn more, you can move to the reactions and biochemistry pages and see how atoms form compounds that help the biological world survive.

Are there pieces of matter that are smaller than atoms? Sure there are. Super-small particles can be found inside the pieces of atoms. These subatomic particles include nucleons and quarks. Nuclear chemists and physicists work together at particle accelerators to discover the presence of these tiny, tiny, tiny pieces of matter. However, science is based on the atom because it is the smallest distinct unit of matter.

Three Easy Pieces

Even though many super-tiny atomic particles exist, you only need to remember the three basic parts of an atom: electrons, protons, and neutrons. What are electrons, protons, and neutrons? Electrons are the smallest of the three particles that make up atoms. Electrons are found in shells or orbitals that surround the nucleus of an atom. Protons and neutrons are found in the nucleus. They group together in the center of the atom. That's all you have to remember. Three easy pieces!

There are almost 120 known elements in the periodic table. (117 as we write this) Chemists and physicists are trying to make new ones every day in their labs. The atoms of different elements have different numbers of electrons, protons, and neutrons. Every element is unique and has an atomic number. That number tells you the number of protons in every atom of the element. The atomic number is also called the proton number.

Charges of Atoms

You can see that each part of the atom is labeled with a "+", "-", or a "0." Those symbols refer to the charge of the particle. Have you ever heard about getting a shock from a socket, static electricity, or lightning? Those are all related to electric charges. Charges are also found in tiny particles of matter.

The electron always has a "-", or negative, charge. The proton always has a "+", or positive, charge. If the charge of an entire atom is "0", or neutral, there are equal numbers of positive and negative charges. Neutral atoms have equal numbers of electrons and protons. The third particle is the neutron. It has a neutral charge, also known as a charge of zero.

Since the number of protons in an atom does not change, fewer or extra electrons can create a special atom called an ion. Cations have fewer electrons and have a positive charge. Anions have extra electrons that create a negative charge.

Always in Motion

As you know, electrons are always moving. They spin very quickly around the nucleus of an atom. As the electrons zip around, they can move in any direction, as long as they stay in their shell. Any direction you can imagine — upwards, downwards, or sidewards — electrons can do it. Electrons are constantly spinning in those atomic shells and those shells, or orbitals, are specific distances from the nucleus. If you are an electron in the first shell, you are always closer to the nucleus than the electrons in the second shell.

Shell Basics

Let's cover some basics of atomic shells:
1. The center of the atom is called the nucleus.
2. Electrons are found in areas called shells. A shell is sometimes called an energy level.
3. Shells are areas that surround the center of an atom.
4. Each of those shells has a name (K, L, M...).

There are a couple of ways that atomic shells are described. The most general terms are the basic regions where you find electrons. Chemists use an "n" value, or the letters K, L, M, N, O, P, and Q. The "K" shell is the one closest to the nucleus, and "Q" is the farthest away. For simple atoms, those "n" values usually match the row number on the periodic table and are also known as energy levels. The second description looks at how electrons act inside of the shells. There are certain patterns of movement. Chemists have described those patterns with the "l" value. The "l" values tell you what suborbital an electron is found in. You will see the lowercase letters s, p, d, f, g, and h for the suborbitals.

For example, the electron in a hydrogen (H) atom would have the values n=1 and l=0. The single electron would be found in the "K" shell and the "s" suborbital. If you go on to learn about chemistry, you may see its description written as 1s1. Helium (He) is still in the K shell (top row), but it has two electrons. The first electron would be 1s1 and the second would be 1s2. What about lithium (Li) at atomic number three with three electrons? It would be described as 1s2 2s1. Why is that?

Not all shells and suborbitals hold the same number of electrons. For the first eighteen elements, there are some easy rules. The K shell only holds two electrons. The L shell only holds eight electrons. The M shell only holds eight electrons. The M shell can actually hold up to 18 electrons as you move to higher atomic numbers. The maximum number of electrons you will find in any shell is 32.

Suborbital Basics

We talked a little bit about s, p, d, f, g, and h suborbital descriptions. While the electrons are found in energy levels and regions around the nucleus, they can also be found in special areas within those energy levels. A guy named Schrödinger started realizing that all electrons weren't the same and they didn’t move in the same way. So, looking back at lithium we saw 1s2 2s1.

Those values describe where you can find the three electrons. Two are in energy level one in suborbital s. The third electron is in energy level two and suborbital s. Are they both in the same suborbital s? No. The letter of the suborbital references the shapes of regions you will find electrons. Suborbital "s" is in a spherical shape. Suborbital "p" is shaped kind of like barbells or a figure eight. Then you have "d" with two possible shapes, and it just gets crazy from there. Just remember that those letters refer to regions where you are likely to find the electrons within their energy level.

One last example: silicon (Si) at atomic number 14. You have fourteen electrons. Written out the long way, it looks like 1s2 2s2 2p6 3s2 3p2. Do you see how the numbers add up to fourteen? Row one has a shell that can hold two electrons. That’s covered by 1s2. Row two of the periodic table corresponds to shell two, which can hold eight electrons. You can see those eight in 2s2 and 2p6. Finally, we have shell/row three. Since suborbitals can only hold so many electrons, you see them divided into "s" and "p". Silicon only has four electrons in the third shell. Suborbital "s" can hold two, and the other two are found in "p". When you get past argon (Ar) at atomic number 18, you will start finding the "d" suborbitals in the transition elements.

Where Are the Electrons?

We've been telling you that electrons reside in specific shells or move in specific patterns in suborbitals. We can't really tell you exactly where an electron is at any moment in time. We can only approximate, or guess, where an electron is located. According to something called quantum theory, an electron can be found anywhere around the nucleus. Using advanced math, scientists are able to approximate the general location of electrons. These general areas are the shells and suborbitals.

Charge It!

Electrons are the negatively charged particles of atom. Together, all of the electrons of an atom create a negative charge that balances the positive charge of the protons in the atomic nucleus. Electrons are extremely small compared to all of the other parts of the atom. The mass of an electron is almost 1,000 times smaller than the mass of a proton.

Shells and Shapes

Electrons are found in clouds that surround the nucleus of an atom. Those clouds are specific distances away from the nucleus and are generally organized into shells. Because electrons move so quickly, it is impossible to see where they are at a specific moment in time. After years of experimentation, scientists discovered specific areas where electrons are likely to be found. The overall shape of the shells changes depending on how many electrons an element has. The higher the atomic number, the more shells and electrons an atom will have. The overall shell shape will also be more complex (because of the suborbitals) as you have more electrons.

Creating Bonds

Electrons play a major role in all chemical bonds. There is one type of bonding called electrovalent bonding (ionic), where an electron from one atom is transferred to another atom. You wind up creating two ions as one atom loses an electron and one gains one. The second type of bonding is called covalent bonding, where electrons are actually shared between two or more atoms in a cloud. Both types of bonds have specific advantages and weaknesses.

Power Up!

Electrons are very important in the world of electronics. The very small particles can stream through wires and circuits, creating currents of electricity. The electrons move from negatively charged parts to positively charged ones. The negatively charged pieces of any circuit have extra electrons, while the positively charged pieces want more electrons. The electrons then jump from one area to another. When the electrons move, the current can flow through the system.

Looking at Ions

We've talked about ions before. Now it's time to get down to basics. The atomic number of an element, also called a proton number, tells you the number of protons or positive particles in an atom. A normal atom has a neutral charge with equal numbers of positive and negative particles. That means an atom with a neutral charge is one where the number of electrons is equal to the atomic number. Ions are atoms with extra electrons or missing electrons. When you are missing an electron or two, you have a positive charge. When you have an extra electron or two, you have a negative charge.

What do you do if you are a sodium (Na) atom? You have eleven electrons — one too many to have an entire shell filled. You need to find another element that will take that electron away from you. When you lose that electron, you will you’ll have full shells. Whenever an atom has full shells, we say it is "happy." Let's look at chlorine (Cl). Chlorine has seventeen electrons and only needs one more to fill its third shell and be "happy." Chlorine will take your extra sodium electron and leave you with 10 electrons inside of two filled shells. You are now a happy atom too. You are also an ion and missing one electron. That missing electron gives you a positive charge. You are still the element sodium, but you are now a sodium ion (Na⁺). You have one less electron than your atomic number.

Ion Characteristics

So now you've become a sodium ion. You have ten electrons. That's the same number of electrons as neon (Ne). But you aren't neon. Since you're missing an electron, you aren't really a complete sodium atom either. As an ion you are now something completely new. Your whole goal as an atom was to become a "happy atom" with completely filled electron shells. Now you have those filled shells. You have a lower energy. You lost an electron and you are "happy." So what makes you interesting to other atoms? Now that you have given up the electron, you are quite electrically attractive. Other electrically charged atoms (ions) of the opposite charge (negative) are now looking at you and seeing a good partner to bond with. That's where the chlorine comes in. It's not only chlorine. Almost any ion with a negative charge will be interested in bonding with you.

Electrovalence

Don't get worried about the big word. Electrovalence is just another word for something that has given up or taken electrons and become an ion. If you look at the periodic table, you might notice that elements on the left side usually become positively charged ions (cations) and elements on the right side get a negative charge (anions). That trend means that the left side has a positive valence and the right side has a negative valence. Valence is a measure of how much an atom wants to bond with other atoms. It is also a measure of how many electrons are excited about bonding with other atoms.

There are two main types of bonding, covalent and electrovalent. You may have heard of the term "ionic bonds." Ionic bonds are electrovalent bonds. They are just groups of charged ions held together by electric forces. Scientists call these groups "ionic agglomerates." When in the presence of other ions, the electrovalent bonds are weaker because of outside electrical forces and attractions. Sodium and chlorine ions alone have a very strong bond, but as soon as you put those ions in a solution with H⁺, OH^-, F^- or Mg⁺⁺ ions, there are charged distractions that break the Na-Cl bond.

Look at sodium chloride (NaCl) one more time. Salt is a very strong bond when it is sitting on your table. It would be nearly impossible to break those ionic/electrovalent bonds. However, if you put that salt into some water (H₂O), the bonds break very quickly. It happens easily because of the electrical attraction of the water. Now you have sodium (Na⁺) and chlorine (Cl^-) ions floating around the solution. You should remember that ionic bonds are normally strong, but they are very weak in water.

Neither Here nor There

Neutrons are the particles in an atom that have a neutral charge. They aren't positive like protons. They aren't negative like electrons. But don't start thinking that they aren't important. Every piece of an atom has huge importance to the way the atom acts and behaves. Neutrons are no exception.

So, if an atom has equal numbers of electrons and protons, the charges cancel each other out and the atom has a neutral charge. You could add a thousand neutrons into the mix and the charge would not change. However, if you add a thousand neutrons, you will be creating one super-radioactive atom. Neutrons play a major role in the mass and radioactive properties of atoms. You may have read the page on isotopes. Isotopes are created when you change the normal number of neutrons in an atom.

Inside the Nucleus

You know that neutrons are found in the nucleus of an atom. Under normal conditions, protons and neutrons stick together in the nucleus. During radioactive decay, they may be knocked out of there. Neutron numbers are able to change the mass of atoms, because they weigh about as much as a proton and electron together. If there are many atoms of an element that are isotopes, the average atomic mass for that element will change. We have spoken about carbon (C) having an average mass of 12.01. It's not much different than you would expect from an atom with 6 protons and 6 neutrons. The number of carbon isotopes doesn't change the atomic mass very much. As you move higher in the periodic table, you will find elements with many more isotopes.

One Special Element

Did we say that all atoms have neutrons? Oops. All elements have atoms with neutrons except for one. A normal hydrogen (H) atom does not have any neutrons in its tiny nucleus. That tiny little atom (the tiniest of all) has only one electron and one proton. You can take away the electron and make an ion, but you can't take away any neutrons. Hydrogen's special structure becomes very important when you learn how hydrogen interacts with other elements in the periodic table. If you learn about nuclear fusion you will learn about deuterium and tritium. Deuterium is a hydrogen atom with an extra neutron and tritium has two extra. You won't find much deuterium in your backyard. It's mainly in oceans. Don't worry if you do find it, it's not radioactive. It's a stable isotope.

Neutron Madness

We have already learned that ions are atoms that are either missing or have extra electrons. Let's say an atom is missing a neutron or has an extra neutron. That type of atom is called an isotope. An atom is still the same element if it is missing an electron. The same goes for isotopes. They are still the same element. They are just a little different from every other atom of the same element.

For example, there are a lot of carbon (C) atoms in the Universe. The normal ones are carbon-12. Those atoms have 6 neutrons. There are a few straggler atoms that don't have 6. Those odd ones may have 7 or even 8 neutrons. As you learn more about chemistry, you will probably hear about carbon-14. Carbon-14 actually has 8 neutrons (2 extra). C-14 is considered an isotope of the element carbon.

Messing with the Mass

If you have looked at a periodic table, you may have noticed that the atomic mass of an element is rarely an even number. That happens because of the isotopes. If you are an atom with an extra electron, it's no big deal. Electrons don't have much of a mass when compared to a neutron or proton.

Atomic masses are calculated by figuring out the amounts of each type of atom and isotope there are in the Universe. For carbon, there are a lot of C-12, a couple of C-13, and a few C-14 atoms. When you average out all of the masses, you get a number that is a little bit higher than 12 (the weight of a C-12 atom). The average atomic mass for the element is actually 12.011. Since you never really know which carbon atom you are using in calculations, you should use the average mass of an atom.

Bromine (Br), at atomic number 35, has a greater variety of isotopes. The atomic mass of bromine (Br) is 79.90. There are two main isotopes at 79 and 81, which average out to the 79.90amu value. The 79 has 44 neutrons and the 81 has 46 neutrons. While it won't change the average atomic mass, scientists have made bromine isotopes with masses from 68 to 97. It's all about the number of neutrons. As you move to higher atomic numbers in the periodic table, you will probably find even more isotopes for each element.

Returning to Normal

Bonding Basics

You must first learn why atoms bond together. We use a concept called "Happy Atoms." We figure that most atoms want to be happy, just like you. The idea behind Happy Atoms is that atomic shells like to be full. That's it. If you are an atom and you have a shell, you want your shell to be full. Some atoms have too many electrons (one or two extra). These atoms like to give up their electrons. Some atoms are really close to having a full shell. Those atoms go around looking for other atoms who want to give up an electron.

Let's take a look at some examples.

We should start with the atoms that have atomic numbers between 1 and 18. There is a 2-8-8 rule for these elements. The first shell is filled with 2 electrons, the second is filled with 8 electrons, and the third is filled with 8. You can see that sodium (Na) and magnesium (Mg) have a couple of extra electrons. They, like all atoms, want to be happy. They have two possibilities: they can try to get to eight electrons to fill up their third shell, or they can give up a few electrons and have a filled second shell. It is always easier to give away one or two electrons than it is to go out and find six or seven to fill your shells.

What a coincidence! Many other atoms are interested in gaining a few extra electrons.

Oxygen (O) and fluorine (F) are two good examples. Each of those elements is looking for a couple of electrons to make a filled shell. They each have one filled shell with two electrons, but their second shells want to have eight. There are a couple of ways they can get the electrons. They can share electrons, making a covalent bond, or they can just borrow them, and make an ionic bond (also called electrovalent bond).

So, let’s say we've got a sodium atom that has an extra electron. We've also got a fluorine atom that is looking for one.

When they work together, they can both wind up happy! Sodium gives up its extra electron. The sodium then has a full second shell and the fluorine (F) also has a full second shell. Two happy atoms! When an atom gives up an electron, it becomes positive like the sodium ion (Na⁺). When an atom gets an extra electron, it becomes negatively charged like the fluorine ion (F^-). The positive and negative charges continue to attract each other like magnets. The attraction of opposite charges is the way they form and maintain the bond. Any atoms in an ionic/electrovalent bond can get or give up electrons.

Compound Basics

Let’s start with molecules. Molecule is the general term used to describe any atoms that are connected by chemical bonds. Every combination of atoms is a molecule. A compound is a molecule made of atoms from different elements. All compounds are molecules, but not all molecules are compounds. Hydrogen gas (H₂) is a molecule, but not a compound because it is made of only one element. Water (H₂O) can be called a molecule or a compound because it is made of hydrogen (H) and oxygen (O) atoms.

There are two main types of chemical bonds that hold atoms together: covalent and ionic/electrovalent bonds. Atoms that share electrons in a chemical bond have covalent bonds. An oxygen molecule (O₂) is a good example of a molecule with a covalent bond. Ionic bonds occur when electrons are donated from one atom to another. Table salt (NaCl) is a common example of a compound with an ionic bond.

You may also learn about a third type of bond. Metallic bonds occur between metal atoms. We’re going to focus on ionic and covalent bonds.

Physical and Chemical Traits of Compounds

When we discuss phase changes in matter, we are looking at physical changes. Physical forces alone (unless you're inside of the Sun or something extreme) rarely break down compounds completely. You can apply heat to melt an ice cube, but there will be no change in the water molecules. You can also place a cup of water in a container and decrease the pressure. The water will eventually boil, but the molecules will not change.

Chemical changes in compounds happen when chemical bonds are created or destroyed. Forces act on the bonds between atoms, changing the molecular structure of a substance. You can pour liquid acid on a solid and watch the solid dissolve. That process is a chemical change because molecular bonds are being created and destroyed. Geologists pour acids on rocks to test for certain compounds.

There are millions of different compounds around you. Probably everything you can see is one type of compound or another. When elements join and become compounds, they lose many of their individual traits. Sodium (Na) alone is very reactive. But when sodium and chlorine (Cl) combine, they form a non-reactive substance called sodium chloride (table salt, NaCl). New compounds have few or none of the physical or chemical traits of the original elements. They have a new life of their own.

Different Bonds Abound

If you look at sodium chloride, it is held together by one ionic/electrovalent bond. What about magnesium chloride (MgCl₂)? It contains one magnesium (Mg) and two chlorine (Cl) atoms. There are two ionic bonds. Methane (CH₄) is made up of one carbon (C) and four hydrogen (H) atoms. There are four bonds and they are all covalent.

Those examples have very simple chemical bonds. However, most compounds have combinations of ionic and covalent bonds. Let's look at sodium hydroxide (Na-OH)...

You can see the sodium (Na) part on the left and the hydroxide (-OH) part on the right. The bond that binds the hydrogen (H) to the oxygen (O) is covalent. The sodium is bonded to the hydroxide part of the compound with an ionic bond. This is a good example of how there can be different types of bonds within one compound.

Whole Lotta Rules Going On

The process of naming compounds is just a set of rules. We're going to show you some of the basics. There are some advanced ways of naming things that we're going to skip right now.

When you have two different elements, there are usually only two words in the compound name. The first word is the name of the first element. The second word tells you the second element and how many atoms there are in the compound. The second word usually ends in IDE. That's the suffix. When you are working with non-metals like oxygen (O) and chlorine (Cl), the prefix (section at the beginning of the word) of the second element changes based on how many atoms there are in the compound. It's like this...

Do you notice anything about the chalkboard? You can see that the prefixes are very similar to the prefixes of geometric shapes. You know what a triangle is. Right? Well the prefix tri- means three. So when you have three chlorine atoms, you would name it trichloride.

Look at the other names too. You may know about a pentagon, a hexagon, or an octagon. The naming system in chemistry works the same way!

Let's put these ideas together! Remember, we're only talking about simple compounds with no metal elements. Most simple compounds only have two words in their names. Let's start with carbon monoxide (CO). That name tells you that you have one carbon (C) atom and one oxygen (O) atom (you can also use the prefix MONO to say one atom). Remember that the second word ends in -ide. So...

(1) Carbon + (1) Oxygen = Carbon monoxide (CO)

Now we'll build on that example. What if you have one carbon (C) and two oxygen (O) atoms?

(1) Carbon + (2) Oxygen = Carbon dioxide (CO₂)

One last example and we'll call it quits. Now you have one carbon (C) and four chlorine (Cl) atoms.

(1) Carbon + (4) Chlorine = Carbon tetrachloride (CCl₄)

You should be getting the idea now. The compound name can tell you how many atoms are inside. Take a look at some of the examples and see if you understand what is happening in the name.

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Dangerous Particles

Radioactivity occurs when an atomic nucleus breaks down into smaller particles. There are three types of particles: alpha, beta, and gamma. Alpha particles are positively charged, beta particles are negatively charged, and gamma particles have no charge. The particles also have increasing levels of energy. Alpha has the lowest energy, beta has a bit more, and then gamma is the fastest and most energetic of all the emission particles.

The term half-life describes the time it takes for the amount of radioactivity to go down by one-half. Let's say you have some uranium (U) (don't try this at home!) and it's radioactive. When your measurements tell you that the level of radioactivity has gone down by one-half, the amount of time that has passed is the half-life. Every isotope has its own unique half-life. The half-life of uranium-235 is 713,000,000 years. The half-life of uranium-238 is 4,500,000,000 years. That's a long time to wait for the radioactivity to decrease.

Harnessing the Energy

Nuclear energy is the energy released when the nuclei (nuclei is the plural of nucleus) of atoms split or are fused. You know the nucleus is made up of protons and neutrons. Nuclear forces hold all of the pieces together. Fusion is when two nuclei come together. Fission is when one nucleus is split into two or more parts. Huge amounts of energy are released when either of these reactions occurs. Fusion reactions create much of the energy given off by the Sun. Fission creates the much smaller particles that make up the protons and neutrons that physicists are studying every day. In our nuclear reactors, fission is the main process. In the Sun, fusion is the big process.

Atoms from the Mirror Universe

Since we're talking a little bit about atomic and nuclear physics, we wanted to tell you about antimatter. It's not just found in television shows. Scientists have proved that it is real. While a regular atom

Atoms Around Us

Atoms are building blocks. If you want to create a language, you'll need an alphabet. If you want to build molecules, you will need atoms of different elements. Elements are the alphabet in the language of molecules. Each element is a little bit different from the rest.

Why are we talking about elements when this is the section on atoms? Atoms are the general term used to describe pieces of matter. You have billions of billions of atoms in your body. However, you may only find about 40 elements. You will find billions of hydrogen (H) atoms, billions of oxygen (O) atoms, and a bunch of others. All of the atoms are made of the same basic pieces, but they are organized in different ways to make unique elements.

Common Elements

Let's work with that idea for a bit. If you read a book, you will find words on each page. Letters make up those words. In English, we only have twenty-six letters, but we can make thousands of words. In chemistry, you are working with almost 120 elements. When you combine them, you can make millions of different molecules.

Molecules are groups of atoms bonded together in the same way that words are groups of letters. An "A" will always be an "A" no matter what word it is in. A sodium (Na) atom will always be a sodium atom no matter what compound it is in. While the atoms have different masses and organization for each element, they are all built with the same parts. Electrons, protons, and neutrons make the Universe the way it is.

From Simple to Complex

If you want to do a little more thinking, imagine the smallest particles of matter. Super-tiny subatomic particles are used to create the parts of atoms. Protons, neutrons, and electrons can then organize to form atoms. Atoms are then used to create the molecules around us. As we just learned, there are almost 120 elements that can be found in the molecules we know. Smaller molecules can work together and build macromolecules. It just goes on. Everything you see or imagine is built from something else.

You could start really small...
- Particles of matter
- Atoms
- Molecules
- Macromolecules
- Cell organelles
- Cells
- Tissues
- Organs
- Systems
- Organisms
- Populations
- Ecosystems
- Biomes
- Planets
- Planetary Systems with Stars
- Galaxies
- The Universe
...And finish really big.

Wow. All of that is possible because of atoms