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1.4  The Periodic Table

Dmitry Mendeleev

The periodic table is an incredible invention and a valuable tool in the world of chemistry. All elements known to man have been arranged together in patterns that can be used to make predictions about how they will react with each other.

The modern periodic table was first introduced in the 19th century by Russian chemist Dimitry Mendeleev. The picture below shows what it looks like. Its good to familiarize yourself earlier on with it because you will end up using it throughout your chemistry education. Memorizing the symbols and atomic numbers can help you do calculations faster.

The structure of the table is fairly straightforward. The rows are called periods. The columns are known as groups. Periods and groups help chemists categorize the elements. They will be explained in detail later in the section. The elements are arranged in order of increasing atomic number. The number on the top left of each element is the atomic number. The number on the bottom is the mass number.

Electron Shells

Before we discuss the periodic table any further, lets talk a bit more about the arrangement of electrons in atoms. As stated in the earlier chapter, electrons are extremely small sub-atomic particles that can be found at certain discrete energy levels away from the nucleus. This is pretty difficult to understand at first but gradually as you learn more about the atomic structures and reactivity, it will sink in. So don't worry.

Since the nucleus is positively charged, electrons are attracted to the center. The more energy the electrons have the more they can resist the attractive forces and move away from the nucleus.

The electrons surrounding the nucleus have fixed energy levels. Depending on that fixed amount of energy, they can be found at a certain range away from the nucleus. We call this range, a shell. The atom of an element can have multiple shells for the multiple electrons it has with different fixed energy levels.

Og has the highest number of electrons in the periodic table and therefore has the highest number of shells,7. In simpler terms it has a 7 rings where the energy level increases as you progress from the first shell to the seventh shell.

It is important to understand that electrons exist in pairs. It is difficult to explain why they exist in pairs without using wavefunctions. This is out of the scope of IGCSE; therefore, just understand that they are in pairs as this will help when studying how electrons are distributed in the shells.

Each of these shells actually have sub-shells. These sub-shells have different energy levels. The types of sub-shells are denoted using the following letters: s, p, f, d.

The 1st shell has 1 sub-shell called 1s.

The 2nd shell has 2 sub-shells called 2s and 2p.

The 3rd shell has 3 sub-shells called 3s,3p and 3d.

The 4th shell has 4 sub-shells called 4s,4p,4d and 4f. This will go on until the 7th shell. Lets stop here because going further will make things complicated.

The order in which the energy level of the sub-shells increases is as follows: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f. Notice that 4s has a lower energy than 3d. There is an explanation for this but learning it is not important for now.  The s type of sub-shells can only have 1 pair of electrons. The p type of sub shells can have 3 pairs of electrons. The d type of sub-shell can have 5 pairs electrons. f type of sub-shell can have 7 pairs of electrons.

Now we can calculate to see the number of maximum electrons each shell can have. Calculation is shown in the table below to make things easier to understand.

Let's try to apply this to an atom of calcium. If you look at the periodic table you can see that calcium has an atomic number of 20; therefore, it has 20 electrons. If you look at the ' Total No. of Electrons ' column in the table above Calcium should have 3 shells because 20 electrons will fit in that amount of shells. However, calcium actually has 4 shells. This is clear if look at the table below. As we noticed earlier 4s sub shell is at a lower energy than 3d. Therefore, electrons will fill up all sub-shells in shell 1 and 2 but in shell 3, only 3s and 3p will fill first. Then 4s subshell will be filled before going on to 3d sub-shell.

The electron arrangement for calcium will look as follows:

The first shell will have 2 electrons. The 2nd shell will have 8 electrons. The 3rd shell will have 8 electrons. The fourth shell will only have 2 electrons.

Even though the 3d subshell has more energy that 4s subshell, we can say that the 4th shell as a whole has more energy that 3rd shell when we draw the configuration. Always having to draw the configuration can be inconvinient. Therefore the configuration can also be wriiten as follows : 2882 or 2,8,8,2 or 2.8.8.2

Classifying Elements

Now that we know how electrons are arranged in atoms let us go back and study the periodic table.

As explained periods are the rows of the periodic table and groups are the columns.

The group number actually tells you the number of outer-shell electrons each element has. These outer shell electrons are also called valence electrons. Valence electrons are the electrons that the element uses when reacting with a substance. If an an element has valence electrons it means that it doesn't have a full shell; therefore it is unhappy. It will either get rid of some electrons or gain more electrons in order to get a full shell.

If you look at where calcium is in the periodic table, it's under group 2. This means it has 2 electrons in its outer shell. As you can see it matches the electron configuration we made of calcium. What about the other elements under group 2? They also have 2 electrons in their outer shell. Since all group 2 elements have the same number of valence electrons, they  show similar trends in chemical properties and reactions.

The period number of an element is the number of shells the element has or the number of the shell with the highest energy. Calcium's period number is 4, which once again matches the configuration we drew above.

If you look at the periodic table you can see that there is a group 0. That is it has no valence electrons. What does this mean? It means that the shells of the atoms of the elements are full and happy. They don't need to react with anything and are stable. Elements in group 0 are called the noble gases. Other elements that have valence electron always try to achieve a stable electron configuration that is similar to the noble gases by reacting to form bonds among each other. This will be clear in the next section.

Groups and periods help us easily identify elements in the periodic table. It can be used to predict chemical properties. If you look at the table above you will see that batches of elements are color coded. These represent types of elements that can be classified together. We will talk about each of these groups later on. We can generally categorize these into metals and non-metals. All elements to the left of basic metals are metals while the elements to the right are non-metals.

The properties listed below are not applicable to all metals and non-metals. Some non-metals can form structures that are stronger than metals. They can also have higher melting and boiling points. These physical and chemical properties will be discussed in detail later on.

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