Periodic Property: Ionization energy

Now you are quite familiar with the atom and you know very well that the atom is neutral because it has equal number of electrons and protons. If we remove one of the electrons or add an extra electron to it, this disturbs the balance of charge and the atom no longer remains neutral. It becomes a charged species and is now called as “ion”. It either develops an excess of positive charge or negative charge.

You also know that electrons are arranged in orbits and sub-shells and every electron has an address. What makes an electron different from other? Energy! Yes, it creates all the difference. Energy of an electron of 1st orbit is lower than the electron of 2^nd orbit. If certain amount of energy is supplied to 1st orbit electron it can jump to 2^nd orbit. Similarly, on supplying large amount of energy (sufficient energy to overcome the nuclear attraction) to the outer most electron, it can be removed from the atom. This process is called ionization and the energy required to remove an outer most electron is called the ionization energy.

It is possible to remove more than one electrons from an atom but you have to proceed step by step.

            Energy required to remove an electron from neutral atom is called “First Ionization Energy”.

            Energy required to remove an electron from A⁺ is called “Second Ionization Energy”.

            Energy required to remove an electron from A⁺²  is called “Third Ionization Energy”.

Removal of successive electrons from an ion becomes more difficult because of increased nuclear charge. That’s why 2^nd IE is always larger than the 1^st IE, because in A⁺ ion electrons are bound more tightly due to increased nuclear charge. And for the similar reason 3^rd IE is larger than the 2^nd IE.

When you go downwards in a group (column), ionization energy decreases. Though the nuclear charge increases when you go downwards in a group, but it is easier to remove an electron from a larger atom (Remember! number of orbits increases, it means the size of atom or the atomic radius increases.)

When you go towards right in a row (period), ionization energy increases, because atomic size decreses in that direction. And it would be difficult to remove an electron from a smaller atom than from a larger atom, because electrons in a smaller atom are bound tightly as compared to the larger one.

In general IE decreases down the group and increases in a period. You will find that group 1 elements have lowest IE in their respective period. Noble gases have highest IE in their respective period.  There are few deviations in the trend though, like

ü  ⁴Be has higher IE than ⁵ B. 

ü  ¹² Mg has higher IE than ¹³ Al

ü  Group 13 shows irregular trend of IE

ü  Elements of group 15 have higher IE than expected.

In the next post we will find out the reasons behind their remarkable behaviour. 

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.

Role of Quantum Numbers in Periodic Table

Periodic table is the most customised address book of elements. Elements are placed here in increasing order of their atomic number. Every element has got a specific place. As you know that every element has unique characteristics. So how is it possible to arrange them in rows and columns? You must have noticed the lengths of columns are different, and there are also a few incomplete rows and two rows are separately placed at the bottom of the table. Why there are 18 columns and 7 rows? In this post we will try to find some answers to these questions.

Rows in periodic table

Write the electronic configuration of first element of each row. Now find the principal quantum number for the outer most electron of each element. You will find “n” of the element is similar to the number of the row it belongs to.  

“s Block” 

When you look at the Periodic table, you will see there are two columns on the left side. Let’s write the electronic configuration of first two elements of the first column.

¹H = 1s¹

³Li = 1s², 2s¹

Notice the outer orbital of these elements; its “s” in both elements.

Now write the electronic configuration of first two elements of the second column.

⁴Be = 1s², 2s²

¹²Mg = 1s², 2s², 2p⁶, 3s²

Notice the outer orbital of these elements; its again “s” in both elements.

As you have noticed that, all the elements of these two columns have their last electron filled in “s” orbital. That’s why they are called “s Block” elements. Since “s” orbital can accommodate maximum 2 electrons that’s why “s block” comprises of two columns.

“p Block”

When you look on the right side of the table you will find 6 columns there. Now write the electronic configuration of at least first elements of each column.

In all these elements outer most electron is filled in the “p” orbital. That’s why these 6 columns are placed under “p Block”. You can guess why there are 6 columns in “p block”. Yes of course because “p” orbital can accommodate maximum 6 electrons. If you write the configuration of other elements of the same column you will find all of them have similar number of electrons in the “p” orbital. In other words, elements of the same column have same “l”, “m_l” and “m_s” quantum numbers.

“d Block”

You must have noticed the bridge of 10 columns in the periodic table. I know you guessed the right reason of the name of this block and reason behind the number of columns. Orbital d can accommodate 10 electrons that’s why it has 10 columns and each element has filled their outer most electron in “d” orbital.

Before you go to write the configuration, I want to tell you the (n+l) Rule. In the previous post about electronic configuration I have told you about energy order of the sub-shells.

Energy order of the sub-shells is as follows: s < p < d < f

When you compare the energy of sub-shells belonging to different orbits you will find

1s < 2s < 2p < 3s < 3p < 3d

The (n+l) rule, which I am going to explain, is about the energy of sub-shells. The sub-shell with higher (n+l) value has higher energy. Let’s check this rule in the energy order given above.

1s has n = 1, l = 0 so, (n+l) = 1

2s has n = 2, l = 0 so, (n+l) = 2

2p has n = 2, l = 1 so, (n+l) = 3

Yes this rule is working fine. Now workout yourself, the order of higher sub-shells 3d, 4s, 4p, 4d, and 4f.

3d has n = 3, l = 2 so, (n+l) = 5

4s has n = 4, l = 0 so, (n+l) = 4

So, the energy of 4s < energy of 3d.

4p has n = 4, l = 1 so, (n+l) = 5

There is tie between 3d and 4p. When there is a tie, the “Aufbau Principle” will decide which one has higher energy. According to this principle the orbit nearer to the nucleus has lower energy or in other words the orbit with higher “n” value has higher energy.

So, the energy of 4p > energy of 3d.

Now the complete energy order will be

1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p

Like p block elements, elements of d block also has similar set of “l”, “m_l” and “m_s” quantum numbers.

Count the number of rows after which “d Block” starts. You will find that it starts from 4^th row. You can justify it from the energy order of sub-shells. Yes, because “d” sub-shell is not present in 1^st and 2^nd  orbit and electron cannot be filled in “3d” sub-shell before filling the “4s” sub-shell. That’s why row 1, 2 and 3 have a gap between “s block” and “p block”. And “d block” starts in 4^th row after “4s”.

“f Block”

It is shown as the extended part of the periodic table. In fact it is present between 6s and 5d but to maintain the symmetry of the periodic table it is placed separately. It has 14 columns because “f” sub-shell can accommodate maximum 14 electrons and the outer most electron of each element is filled in “f” orbital.

Do you remember, that 4^th orbit has 4sub-shells s, p, d and f? However in the energy order of sub-shells discussed above, we didn’t mention 4f sub-shell. Let’s find out its place in the energy order.

4f has n = 4, l = 3 so, (n+l) = 7

5p has n = 5, l = 1 so, (n+l) = 6

6s has n = 6, l = 0 so, (n+l) = 6

5d has n = 5, l = 2 so, (n+l) = 7

There is tie between 4f and 5d and 5p and 6s. In this situation, “Aufbau Principle” will be the tie breaker and 5d wins over 4f and 6s wins over 5p because greater “n” has greater energy.

Now the complete energy order will be

1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s < 5f < 6d < 7p.

Position of ²He

After understanding the reasons behind the arrangement of blocks and the elements belonging to them, you will be able to guess the position of any element from its electronic configuration. When you write the configuration of ²He : 1s², you may place it in the “s Block” but, it is placed in “p Block”.

The arrangement of elements in perodic table also sets particular trends of their properties along the rows and columns such as size of atom, electonegativity, ionization enthalpy and electron gain enthalpy. These are known as periodic properties. In the coming posts I will discuss these properties and there trends one by one. 

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.

Quantum Numbers: Postal Address of an Electron

As you know, in an atom every electron has a particular place. Just as you have your postal address, every electron also has a particular address. Like a pin code, electrons have a set of Quantum Numbers. These quantum numbers give valuable information about an element. On the basis of quantum numbers of the outer most electron an element gets its particular place in the periodic table. In this post, I am going to explain the quantum numbers and in my next post I will correlate them to the periodic table.

Electronic Configuration of ⁵B

Write the configuration of ⁵B: 1s², 2s², 2p¹. Now arrange its electrons in the orbitals and spot the outer most electron. List down all the specifications you need to write the address of this electron.

1.      The orbit number
2.      The Sub-shell
3.   The orbital
4.      The orientation

You are correct; these 4 specifications are listed as quantum numbers and make the complete address of every element. I will explain these specifications one by one.

Orbit Number: Principal Quantum Number (n)

The principal quantum number tells about the orbit number. It is shown by “n”. Its value can be 1, 2, 3, 4, 5 …..

In the above example for the outer most electron, the value of “n” will be 2, as it is present in orbit number 2.

The Sub-shell: Azimuthalquantum number (l)

Azimuthal quantum number tells about the sub-shell. The symbol for it is “l”. Its value can be 0, 1, 2, 3, 4…(n-1). For any electron the value of “l” is always less than the value of “n”.

The value of “l” is assigned for every sub-shell. 

For “s” the assigned value of “l” is = 0

For sub-shell “p” the assigned value of “l” is = 1

For sub-shell “d” the assigned value of “l” is = 2

For sub-shell “f” the assigned value of “l” is = 3

In the above example for the outer most electron, the value of “l” will be 1, as it is placed in “p” sub-shell.

The Orbital: Magnetic quantum number (m_l)

Magnetic quantum number (m_l) tells about the particular orbital where electron is placed. As you know every sub-shell has different number of orbitals. These sub-shells have used the advance level of architecture, so that every orbital has a definite orientation in the space. As in “p” sub-shell, one orbital is oriented in “x” axis, other one is in “y” axis and the third one is in the “z” axis. They named as (p_x ) _, (p_y) and (p_z )   according to their orientation in the space.

Magnetic quantum number is the notation for that particular orbital. Like in car we refer different seats as driver seat, window seat, front seat, middle seat and back seat.

The value of Magnetic quantum number depends on the value of Azimuthal quantum number. Value of m_l = +(l) to –(l).

For the “l”= 1that is “p” sub-shell, the value of “m_l” will be= +1, 0, -1.

For the “l”= 0that is “s” sub-shell, the value of “m_l” will be= 0.

For the “l”= 2that is “d” sub-shell, the value of “m_l” will be= +2, +1, 0, -1, -2.

For the “l”= 3that is “f” sub-shell, the value of “m_l” will be= +3, +2, +1, 0, -1, -2, -3.

In the above example for the outer most electron, the value of “m_l” will be +1, as it is placed in the (p_x ) orbital of the “p” sub-shell.

The orientation: Spinquantum number (m_s)

Spin quantum number

Spin quantum number tells about the orientation of the electron. Either it revolves clockwise or anticlockwise in its axis. Its value can be +1/2 or -1/2.

In the above example for the outer most electron, the value of “m_s” will be +1/2.

If you try yourself a few examples, you will find that none of the two electrons of the same atom have the similar address or in other words have the same value of all the four quantum numbers. It was first discovered by “Wolfgang Pauli”. His finding is known today as “Pauli exclusion principle”  

Correlation Table of Quantum Numbers

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.

Periodic Table: Address Book for Atoms

Click on to view Periodic Table

Atoms are like us. Even though we all are made up of flesh and bone, everyone is different and unique. Similarly, all atoms are made up of subatomic particles like proton, electron and neutron. These subatomic particles give identity to an atom and make is different to another. Every atom has different set of subatomic particles.

Like us atoms also have names and address. The atom which has 1electron and 1 proton is named as Hydrogen. And an atom which has 2electrons and 2 protons is named as Helium. As one kind of animals make a species, one type of atoms combine to make an element. There are total 118 different elements. These elements are present everywhere in the universe. You can find their details in their address book. This address book is called “Periodic Table”. Elements are arranged there according to their Identity number.

Elements have Identity number. As we have now in the form of “Adhar card” in India, and Social Security number in United States. Element is defined by their atoms, that’s why this identity number is given on the credentials of the atom. For an element, identity number is a set of atomic number and atomic mass. Atomic number is the number of electrons present in an atom and atomic mass is the mass of an atom.

In the periodic table elements are arranged in an increasing order of their atomic number. It is divided into columns and rows. It has 18 columns and 7 rows. Elements are arranged in such a way that similar type of elements gets the same column. Like in cities cantonment area is mostly populated by defence persons.

As there is a particular way of writing our full name like we write our name first than surname. An element’s name is written with its identity number. It is much like a signature because it contains symbol instead of full name. Let’s see. Symbol for Oxygen is ‘O’ its atomic number is 8 and atomic mass is 16.

Atomic number is written in left super script and atomic mass in left subscript to the symbol of the element. 

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.