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Periodic Table of Elements

  • Last Updated : 20 Oct, 2021

There are now 118 elements known, and studying the characteristics of all of these elements independently is quite challenging. As a result, all of the elements have been grouped into a few groups such that elements in the same group have comparable qualities. Dobereiner’s triads, Newlands’ law of octaves, and Mendeleev’s periodic table arranged some elements, but there were many demerits with all these arrangements. The modern periodic table, which we now use, is an updated and improved version of the previous tables.

Modern Periodic Table

Bohr is credited for developing the current periodic table. It is also known as the periodic table in its extended version. In the contemporary table, the elements are grouped in horizontal rows called periods in increasing atomic number order. All other elements with the same number of valence electrons are arranged beneath each element, such that all elements with the same number of valence electrons are in the same vertical column termed group. Because they all contain the same amount of valence electrons, all elements in a specific group of the periodic table have comparable characteristics. The electronic configurations of elements are based on their arrangement in the modern (long form) periodic table.

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What are Periods?

In a periodic table, periods are the horizontal rows of elements. There are seven periods in the long form of the periodic table. The elements in the periodic table have consecutive atomic numbers. Each period has a different number of elements, as shown below.

  • The first period contains 2 elements. It is called a very short period.
  • The second period contains 8 elements. It is called a short period.
  • The third period contains 8 elements. It is also called a short period.
  • The fourth period contains 18 elements. It is called a long period.
  • The fifth period contains 18 elements. It is called a long period.
  • The sixth period contains 32 elements. It is also called a long period.
  • The seventh period contains the rest of the elements. It is incomplete.

The maximum number of electrons that may be accommodated in the various shells of an atom determines the number of elements in a period. For example, the first period includes two elements because an atom’s first electron shell (K shell) can only contain 2 electrons. Similarly, the second period of the periodic table includes eight elements because an atom’s second shell (L shell) can contain a maximum of 8 electrons. The digits 2, 8, 18, 32, and so on, which represent the number of elements in certain periods, actually represent the maximum number of electrons that may be placed in each of the atom’s shells. 

To decide whether an element comes first or last in a period?

The first period begins with hydrogen and finishes with a noble gas, helium. All other periods begin with alkali metals such as lithium, sodium, and potassium and end with noble gases such as neon, argon, and krypton. Every period’s first element has one valence electron, whereas the last element, commonly known as noble gas, has eight valence electrons (except the first period in which the last element helium has only 2 electrons). It can be concluded that the number of valence electrons in the atoms of elements determines which element is the first and which is the last in a period. 

Since each element in a given period has a different number of valence electrons in its atoms, their electronic configurations also differ. Since the electrical configurations of elements in a period differ, their characteristics differ. As a result, each element in a period has its own set of properties.

What are Groups?

In a periodic table, groups are the vertical columns. In the long form of the periodic table, there are 18 groups. These groups are numbered from one to eighteen. In a particular group, the elements do not have consecutive atomic numbers. On the left side of the periodic table, is group 1 whereas on the right side of the periodic is group 18. 

  • Groups 1 and 2, and 13 to 17 contain the normal elements. All the inner shells are completely filled with electrons, only the outermost shells are incomplete in these normal elements. In their atoms, all elements of a given group of normal elements have the same number of valence electrons. It can be said that all the elements in a group have similar electronic configurations and show similar properties.
    • Group 1 contains elements containing only 1 valence electron in their atoms, these are called alkali metals such as lithium, sodium, potassium, etc. Since all the alkali metals have the same number of valence electrons, so they show similar chemical properties.
    • Group 2 contains alkaline earth metals like beryllium, magnesium, calcium, and so on have 2 valence electrons. Since all the alkaline earth metals have the same number of valence electrons,  so they show similar chemical properties.
    • Group 17 contains halogens like fluorine, chlorine, bromine, etc. all having 7 valence electrons. Since they contain the same number of valence electrons, so all the halogens show similar properties.
  • Group 18 elements are known as noble gases or inert gases. These elements are helium, neon, argon, krypton, and so on. All these elements except helium have 8 electrons in their valence shell. Thus the valence shell of the noble gases is completely filled with electrons.
  • Transition elements are the elements from group 3 to group 12. In these elements, the outermost shell, as well as the next to the outermost shell, are incomplete which are in the process of being filled with electrons.

From the atomic number 57 to 71, the elements are called lanthanide series (as the first element is lanthanum) and from atomic number 89 to 103, the elements are called actinide series ( as the first element is actinium). The elements of these two series have similar properties. These two series have been placed in two rows at the bottom of the periodic table so that the periodic table can fit on a single page.

Metals and Non-metals in the Modern Periodic Table.

The elements have been roughly divided into metals and non-metals, in the modern periodic table. The elements on the left side of the table are metals whereas the elements on the right side of the table are non-metals. The metals and the non-metals of the periodic table are separated by some elements placed diagonally in the periodic table which are called metalloids. Some elements like boron, silicon, germanium, arsenic, antimony, tellurium, and polonium are metalloids. The properties of these elements are intermediate between those of metals and non-metals. The metals lie on the left side of the metalloids and the non-metals are placed on the right side of the metalloids.

Position of Hydrogen in the Modern Periodic Table.

Since hydrogen has an electronic configuration that is similar to that of alkali metals, it has been placed at the top of group 1 above them. Both hydrogen and alkali metals have a single valence electron. Since the hydrogen atom is so small, many of its properties differ from those of alkali metals. As a result, hydrogen is never included while discussing group 1 alkali metals. Hydrogen is treated as a unique element and is placed at the top of the periodic table on its own.



Solved Questions

Question 1: Give some examples of elements with only one electron in their outermost shells.

Answer:

The elements of group 1 has only single electron in their outermost shells. Some elements belonging to group 1 are lithium, sodium, potassium. So, the elements that have a single electron in their outermost shells are lithium, sodium, potassium.

Question 2: Can an element having atomic number 2.5 be placed between helium and lithium?

Answer:

Atomic number is a whole number. It can either be 2 or 3. There can not exist an element with atomic number 2.5. So, an element having atomic number 2.5 cannot be placed between helium and lithium.

Question 3: The atom of an element has electronic configuration 2,8,7. What are the atomic number and the name of the element?

Answer:

The atomic number of an element can be obtained by adding all the electrons present in the electronic configuration. So, the atomic number is, 2+8+7=17. The element with atomic number 17 is chlorine. 

Question 4: Name an element that has only two shells, and both these shells are completely filled with electrons.



Answer:

All the shells of the noble gases (group 18) are completely filled with electrons. Since the element has only two shells, and the maximum occupancy of first shell is 2 and second shell is 8, so its electronic configuration is 2, 8. So its atomic number will be 2+8=10 and the element with atomic number 10 is Neon. Thus an element which has only two shells, having both these shells completely filled with electrons is Neon.

Question 5: Determine whether Calcium with atomic number 20 is a metal or non-metal.

Answer:

Calcium has atomic number 20. So, its electronic configuration 2, 8, 8, 2. It has 2 electrons in its outermost shell, so it belongs to group 2. In the modern periodic table group 2 is on the left side of the table and all the elements on the left side of the table are metals. So calcium is a metal.

Question 6: Name an element that contains three times the number of electrons in its second shell as it does in its first.

Answer:

Since the maximum occupancy of first shell is 2. So the number of electrons in first shell is 2. The second shell has three times the number of electrons as its first shell, this means, 3×2=6. So the number of electrons in the second shell is 6. There fore, the electronic configuration is 2, 6 and the atomic number of the element is 8 (2+6=8). The element with atomic number 8 is oxygen.




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