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Chemistry Lesson 1
Atomic and Molecular Structure

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Instruction 1-1

Connection Among the Location in the Table, the Atomic Number, and Mass | How to Identify Metals, Semimetals, Nonmetals, and Halogens | How to Identify Alkali Metals, Alkaline Earth Metals, and Transition Metals | Lanthanide, Actinide, Transactinide, and Transuranium Elements | Ionization Energy, Electronegativity, Relative Sizes | How Many Electrons Can Bond? | Size and Mass | Location and Quantum Electron Configuration | Summary


Because some elements have similar properties as others, they were grouped together in an arrangement called the Periodic Table of the Elements. A modern Periodic Table of the Elements is shown in the first figure http://upload.wikimedia.org/wikipedia/commons/8/84/Periodic_table.svg.

periodic table

The Periodic Table of the Elements is a chart of all the elements from which all matter in the universe is made. We’ll explain it a bit more in a minute. But first we thought it might be a good idea to go back and review a few of the basic principles of chemistry.

Chemistry is the study of matter and how it behaves and changes. What’s matter? Matter is the name scientists give to anything you can see, feel, touch or smell. Everything in the universe is made up of matter. A good definition for matter is anything that has mass and occupies space.
Matter can exist in four states: solid, liquid, gas, and plasma. It can have physical properties like color or smell; it can have chemical properties like how well it reacts with oxygen. And it can undergo both physical and chemical changes. Matter is made up of elements. And elements are made up of atoms. An atom is the smallest part of an element that has all the properties of that element. http://science.howstuffworks.com/atom.htm

But even an atom is made up of smaller things. Those things are: electrons, protons and neutrons, http://education.jlab.org/atomtour/ which you should have learned about in your previous science classes. As you may know, scientists have discovered even tinier things inside atoms like quarks and nucleons – but it’s the basic electron-proton-neutron arrangement that chemists are interested in. And it’s also what makes every element in the universe different from every other element. Because it’s the specific number of electrons, protons and neutrons inside its atoms that makes each element unique.

At the moment, we know of 118 different elements and new ones are being discovered all the time. A Periodic Table of the Elements with all the discovered elements is provided here. But it’s the first 18 that really matter (small pun intended).


We need a little more detail about the arrangement of the electrons in order to better understand the chemistry of the elements. The electrons are not just moving around the nucleus of an atom. There are specific places where the electrons can be in the atom. These places occur in layers around the nucleus called shells, much like the chocolate and candy shell on a peanut M&M. There is even more restriction on the electrons, but we can postpone that information. Scientists have different names for some items because different people contributed different ideas and we put them together; these shells are also called “energy levels.”

There can be several different shells around the nucleus of an atom. Each shell (or energy level) has been given a name according to its distance from the nucleus. Actually, the names for these shells are letters of the alphabet. They go like this:
shell 1 (energy level 1),- the shell closest to the nucleus, is called “k”
shell 2 (energy level 2),- the next closest shell to the nucleus, is called “l”
shell 3 (energy level 3),- the next closest shell to the nucleus, is called “m”
The other shells – counting out from the nucleus – are “n,” “o,” “p,” and “q” – with many more, but we don’t need to worry about them. But it’s the first three shells that are the most important for our 18 most important elements.

Not all shells hold the same number of electrons; there are rules. The first shell (k) can only hold two electrons, the second shell (l) can hold eight electrons and the third shell (m) can hold eighteen electrons (shell n needs a couple electrons after m gets 8 electrons before shell m can be completely filled. There will be more information about this later).

It’s the electrons in the outermost shell of the atom that bond (combine) with the electrons in the outermost shell of other atoms; atoms combine with other atoms that are the same to form elements, or with different atoms to form compounds.
Elements are substances where all the atoms are alike. Compounds are substances made up of different kinds of atoms.
But since this instruction was supposed to be about the Periodic Table of the Elements, maybe now we’d better take a look at it.

Here is one representation of the Periodic Table of the Elements: http://www.webelements.com/

As you can see, the Periodic Table of the Elements has the elements arranged in a grid.

atomic number

Elements are arranged in cells according to their atomic numbers http://education.jlab.org/glossary/atomicnumber.html , beginning in the upper left-hand corner of the grid. The atomic number is the number you see at the top of each box – and it indicates the number of protons each atom of the element has. Hydrogen, for example, has the atomic number 1 because it has only 1 proton.
The next thing you see in each cell is the element’s symbol -- H, for example, which is the symbol for hydrogen. Some versions of The Periodic Table of the Elements also include the full name of the element and its atomic mass – which is something else we need to explain.

The atomic mass of each element is compared to the mass of Carbon-12. This mass is measured in units called Daltons (Da) (named for a chemist named John Dalton who lived in the1800’s), or in atomic mass units (u). Each u is approximately equal to 1/12th the mass of one carbon-12 atom, which is the mass of one hydrogen atom. You might notice that carbon is not exactly 12 u. That’s because there is a little carbon-13 and carbon-14 mixed in with carbon-12. We will explain what the difference is between the different carbons in a later lesson.

Start reading left to right and you’ll see that the atomic mass increases cell by cell, just as the atomic numbers do. Fluorine (F), for example, has the atomic number 9 and an atomic mass of 18.99840 u That means fluorine has less mass and a lower atomic number than calcium (Ca), which has the atomic number 20 and an atomic mass of 40.06 u.  Or, to put it another way, calcium has more mass and a higher atomic number than fluorine.

The Periodic Table of the Elements is arranged in rows going from left to right, which are called Periods and in columns going from top to bottom, which are called Groups.

 All the elements in each row use the same number of shells for their electrons. For example, all the elements in the top row need only one shell for their electrons. The other shells are present, but are not used. As you can see, there are only two elements in this row: hydrogen (H) and helium (He) -- the elements that were there at the beginning of the universe.

All of the elements in the second row need only two shells for their electrons. These elements are lithium (Li), beryllium (Be), boron (B), carbon (C), nitrogen (N), oxygen (O), fluorine (F), and neon (Ne).

All of the elements in the third row need only three shells for their electrons, and so on. The elements in the third row are sodium (Na), magnesium (Mg), aluminum (Al), silicon (Si), phosphorous (P), sulfur (S), chlorine (Cl) and argon (Ar).

The first three rows are the rows that contain the basic 18 elements – the elements that almost everything is made of.

So now you know about rows (Periods). We will postpone the information about the columns (Groups) until the second and third lessons.

There are a number of different ways to look at The Periodic Table of the Elements. There is also a lot of interesting information, click on the following link. See if you can determine why the elements are arranged in different patterns. Dr. Stowe’s table is probably the most difficult one to figure out, but it does show some important information (although there is an error on the d shell of n = 6).


For most of The Periodic Tables of the Elements on the web, you need to click on the element to get the name of each element plus its atomic number, atomic mass, electron configuration, number of neutrons, melting point, boiling point, date of discovery and crystal structure. For another site, click: http://www.chemicalelements.com/

All this is a lot to take in, and we’ll be talking more about the elements and the Periodic Table of the Elements in future Instructions. But we thought you might want to know who thought the whole thing up in the first place.

There are three important figures in the history of the Periodic Table of the Elements. The first is Russian chemist Demitri Mendeleyev, who made the first stab at organizing the elements in the 1870’s. His chart had a few inconsistencies, so in 1914 a British Physicist, Professor Mosley, rearranged the chart according to atomic number. This took care of the inconsistencies – and it is basically his chart we use today. Then in the 1950’s it was further refined and rearranged by Dr. Glenn Seaborg of the University of California at Berkeley, who predicted a few elements that have just been synthesized or discovered.

To learn more about these scientists, the elements or the Periodic Table of the Elements, click on any of the web sites we’ve mentioned within this instruction. 


Video Instruction
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for Students, Parents and Teachers

Now let's do Practice Exercise 1-1 (top). 


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