Periodic Trends

What is the Periodic Table of Elements?

The Periodic Table is an arrangement of elements in the order of increasing proton number/atomic number, which is the number of protons in the nucleus of each atom of the elements. As the elements move from left to right in the periodic table, the number of protons in their nucleus increases by one.

Recall: Elements are substances that contain one type of atoms. Common misconceptions are that elements are made up of a single atom. This statement is not true. Elements are not made up of a single atom. Instead, they are made up of many atoms that are identical to one another.

The number of protons in an atom of an element determines the identity of the element since each element has a unique proton number. In other words, atoms of an element with the same number of protons are the same element.

How Are the Elements Arranged?

The elements are arranged in the periodic table such that the proton number of the atoms increases by one as the elements move to the across the period (horizontal section of the periodic table) and down the group (vertical section of the periodic table).

The periodic table consists of:

  1. 7 horizontal rows called periods.
  2. 8 vertical columns called groups.

*When you are looking at the periodic table, remember to look for the legend at the bottom of the periodic table, which will indicate the proton number/atomic number and the nucleon number.

Key terms

Group - This refers to the vertical section of the periodic table. There are a total of 18 periods, and they are read from left to right. E.g. group one, group two..

Groups are numbered from I to VIII/O in Roman numerals.
Group I elements are also known as Alkaline Metals.
Group II elements are also known as Alkaline Earth Metals.
Group VII elements are also known as Halogens.
Group VIII elements are also known as Noble gases.

Period - This refers to the horizontal sections of the periodic table. They are read from top to bottom, e.g., period one, period two...

How to use the periodic table to determine the identity of any element?

Let's say you are asked to identify an element with its atom having a proton number of eight and neutron number of ten.

In other words, the atom contains eight protons and ten neutrons in each nucleus of an atom.

Now, look at the periodic table and find the element with an atom that contains 8 protons. The element with eight protons in the periodic table is the element you are looking for.

Note: The nucleon number (sum of the proton number and neutron number) does not help us identify an element. In fact, the number of neutrons varies among in the atoms of the same element. These atoms of the same element with the same number of protons but a different number of neutrons are called isotopes.

(Extra) Why is there an exact nucleon number written at the top of the element in the periodic table when there are isotopes? This is because nucleon number is the average nucleon number of all the naturally existing atoms of the elements. In fact, most of the elements have nucleon number that is not a perfect number, but for the periodic table used in O level chemistry, most of the nucleon number is rounded to the nearest whole number, except for chlorine (Mr= 35.5).

What does the group number tell us about an atom.

The group number (vertical column) tells us the number of valence electron of an atom, which is essential to know the identity of the ions formed since ions are formed by gaining or losing electrons.

Elements in the same group have the same number of valence electrons. Hence, all elements in Group 1 have one valence electrons in their atoms.

Why is this the case?

Remember the electronic configuration of atoms. The first electron shell can only contain a maximum of two electrons, and the rest of the electron shells can fill a maximum of eight electrons.

There is a total of two groups in the first period and a total of eight groups from the second period onwards, excluding the transition metals. The reason why the group number tells us the number of valence electrons because the total number of groups in a period is the same as the maximum number of valence electron it can hold.

Note: although hydrogen has one valence electron, it does not belong to group one. This unique case only applies to hydrogen. Also, hydrogen is a non-metal, not a metal.

Similarities between elements of the same group

When an element undergoes a reaction, only the valence electrons are involved. This is because the valence electrons are located furthest away from the positively charged nucleus. Therefore, they are the most weakly attracted electrons in the atoms and can lose or gain electron with little energy relative to the inner electrons.

As mentioned above, elements in the same group has the same number of valence electrons (which are the only electrons involved in reactions). This means that they have same chemical properties (such as their reactivity to water, acid etc).

Metals, non-metals, and metalloid

Metals are generally located at the left-hand side of the periodic table. Non-metals, on the other hand, are found at the right-hand side of the periodic table.

Using this information, you can easily predict the properties of an unknown element if you know the position of the component of the periodic table. For example, you can predict calcium, which is located on the left-hand side of the periodic table, as a solid in room temperature.

This is because most metals (except mercury) exist in a solid state at room temperature and pressure due to its high melting and boiling points.

As we move across the period, the metallic character of changes to the non-metallic character. In between metals and non-metals, we have the metalloids, which have properties of metals and non-metals. Metalloids can be found near the zigzag line of the periodic table.

What does the period number tells us?

The period number tells us the number of electron shells in an atom.

This can be explained easily. In the first period, there are two atoms and the maximum number of valence electrons in period 1 is two. This is also the maximum number of valence electron that the first shell can hold. Therefore, at the end of period one,

In the same period, the element in Group I has one valence electron, and in Group II has two valence electrons, and in Group VII has seven valence electrons and in Group 0 has eight valence electrons.

The first electron shell can hold up to 2 electrons, and the subsequent electron shell can hold up to 8 electrons.

When the outer most/last electron shell contains the maximum number of electrons, it has a full valence electron and obtains a noble gas configuration.

(Recap) When atoms have a noble gas configuration, they have full valence electrons. Atoms will lose/gain electrons to obtain a noble gas configuration because it is more stable. When atoms gain/lose electrons, they form ions.

Properties of Group 1 elements

Group 1 elements have only one valence electron, that is, only one electron in the outermost electron shell.

Because it only has 1 valence electron, it requires just a small amount of energy to lose the valence electrons when it is taking part in the chemical reaction. The ease of losing electrons explains why group 1 metals are highly reactive.

This explanation is valid for all the elements in a group because elements in the group have the same number of valence electron shell and therefore similar chemical reactivity.

© 2020