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/.

Polar Covalent Bond: Water

Elements share electrons to form covalent bonds. We assume that they both have equal possession of shared pair of electrons. But it isn’t the case every time. You know each element is different to others and every element has its unique qualities, strengths and weaknesses.

Water Molecule 3D view

As you know electrons continuously move around, they look like a cloud hanging around the nucleus. When you look on the picture of water molecule, you will see the cloud is a little denser near the Oxygen atom. It means the shared electron pairs are pulled by Oxygen atom to get a greater share of it. How does Oxygen manage to do it? What makes Oxygen more powerful than Hydrogen?

The subatomic particles build an element’s strength/ weakness. You remember protons are positively charged and stay inside the nucleus. Because of these protons nucleus behaves like a magnet for electrons. Nucleus attracts electrons to bind them closer to it. Magnetic power of nucleus doesn’t work effectively on the electrons of distant orbits.

Let’s have a close look on Oxygen atom (⁸O). Its electronic configuration is 1s², 2s², 2p⁴. It has 2 orbits and 8 protons in its nucleus. So it wouldn’t be difficult for nucleus to bind electrons and even to attract electrons of bonding pair.

Oxygen Atom

When you see Hydrogen atom, it has only one proton. Its nucleus is not strong enough to withstand in the competition with Oxygen.

The capability of element to pull bonding electrons is called “electro negativity”. In water molecule Oxygen is more electro negative than Hydrogen. You don’t have to work out every time that which element is more electro negative than other. You can find this type of information about any element in the periodic table. You will be amazed to know that elements are arranged in the order of strengths and weaknesses in the periodic table. I will elaborate it in my next post.

When one element in a covalent bond is stronger than the other one, the shared electron cloud shifts towards stronger element (more electro negative) and that element develops partial negative charge. The weaker element (less electro negative) on the other hand, loses some of the cloud and develops partial positive charge. That’s how two different poles (“+” and “-“) are developed in the same molecule. Thus, covalent bond develops a bit of ionic characteristic, and such molecules are called polar molecules.

This type of polar covalent bond is the reason behind the wonderful properties of water. That’s why water is a liquid and remains liquid up to large range of temperature. It boils at quite higher temperature (100 ͦ C). Its solid form Ice is lighter than its liquid form is also due to its polar nature. It can dissolve a number of chemical substances, ions and gases. These are a few qualities of water I have mentioned. Water is the unique creation of the nature that supports life in the earth.​

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/.

Sharing in Elements: Covalent Bond

Do Elements love sharing? Yes, they step forward to share their electrons to achieve octet. If you write the electronic configuration of oxygen (⁸O): 1s², 2s², 2p⁴ you see it has 6 electrons in its outer most orbit (2s², 2p⁴). To complete its octet it has to find 2 more electrons anyhow. And if you write electronic configuration of Hydrogen (¹H): 1s¹ it has single electron. For hydrogen to achieve octet is next to impossible because orbit 1 allows maximum 2 electrons. So there is a relaxation in octet rule for hydrogen. Hydrogen has to complete its outer most orbit that is orbit 1. To do so it has to find 1 electron from anywhere.

Bonding inWater Molecule

When we look at the situation of Oxygen and Hydrogen, both has lesser electrons and they in no condition to borrow or to donate to each other. So they decide to share electrons and help each other to achieve octet. But the problem is that Oxygen wants 2 electrons and Hydrogen has 1. If Hydrogen manages to bring another Hydrogen atom with it, the problem will be solved. Now 1 Oxygen atom and 2 Hydrogen atoms come forward to share their electrons. Let’s see how they share electrons.

Type of Covalent Bonds

These shared pair of electrons form the bonds and called as bonding electrons. When there is single shared pair between 2 atoms, it forms single bond. As above, Oxygen forms single bond with each of the Hydrogen atoms. When there is 2 shared pairs of electrons, they form double bond and 3 shared pair of electrons form triple bond.

Now Oxygen and Hydrogen are not individual elements they become H₂O molecule. Oxygen and Hydrogen are gases but as they combine together they form the most abundant substance of the earth and also of our body. Yes, you guessed right, H₂O is the water.

When elements come together they form “Molecule”. Molecule has its own identity and its characteristics are totally different from its constituent elements. As you see, water is liquid while Oxygen and Hydrogen are gases.

How do we write that how many elements bond together and how many pair of electrons they share? There are few scientific manners of presenting all these information in a precise way.

·         Formula: It shows the number of elements and their ratio. As formula of water (H₂O), tells us that a molecule of water is formed by 2 atoms of Hydrogen and 1 atom of Oxygen.

·         Lewis dot structure:  It is named after American chemist Gilbert Newton Lewis. In this method electrons of outer orbit are shown in the form of dots around the symbol of element.

Lewis dot structure

·         Kekulé structures: in this method element’s symbol and bonding electrons are drawn as lines.

Kekulé structure

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/.

Bonding in Elements

Do elements make relations? Yes they do; just as we make relations. Their relations are called “Bond”. Every element has a particular nature and it likes to make relation with a specific kind of element. Like us some elements are very friendly and make a number of friends, some are a little bit choosy and make a few selective friends and some like to be alone, these are royal type of elements.

Which factor guides their behaviour? Similar to our life where money factor decides most of the decisions, electron plays a major role in an element’s behaviour. To understand this you have to know how these electrons are distributed in an atom. In other words, you have to study financial condition of an element.

In the previous post you have learnt that every track allows a particular number of electrons. These tracks are called as “Orbits” in which electrons move around the nucleus. Orbit 1 allows only 2electrons, Orbit 2 allows 8, Orbit 3 also allows 18 and Orbit 4 allows 32 electrons.

Elements are not greedy like us. They just want to have 8 electrons in their outer most Orbit. If they have less than that, they like to borrow and if they have extra they love to give it. In their community 8 is the holy figure that every element tries harder to achieve. And those who already have this Octet (In Greek it means 8) are called Nobel elements. They like to live alone and rarely make any relation.

Let’s have a look on an element Sodium ¹¹Na, its symbol is ‘Na’ which is short form of its Latin name Natrium. It has 11 electrons. When you distribute them in orbits, you find 2 electrons in 1^storbit, 8 electrons in 2^nd orbit, and remaining 1 electron in 3^rdorbit. If it loses 1 electron it will have 8 electrons in 2^nd orbit.

When you study electronic distribution of element chlorine ¹⁷Cl, you find 2 electrons in 1^st orbit, 8 electrons in 2^nd orbit and 7 electrons in 3^rd orbit. There is one less than eight electrons in outer most orbit. Cl is looking for someone who can give him one electron.

Cl wants 1electron and Na wants to lose 1 electron. So they come together and make a bond. Thus they both achieve octet and stay together as NaCl (sodium chloride). You can find it in your kitchen. Without it nothing can taste better. It’s your table salt.

You know atoms are neutral because there electrons and protons are present in equal number. When atom loses or borrows electrons in order to achieve octet state, it disturbs the balance of charge. It means either negatively charged electrons or positively charged protons become in excess and the majority decides the charge of the atom. Charged atom is called “ion”. Charge of an ion is written in the right superscript.

When sodium donates an electron, it becomes positively charged because now protons are in majority. It is now called sodium ion Na⁺. Similarly Chlorine becomes negatively charged as it borrowed an electron. It is called as chloride ion Cl^- .

Now these oppositely charged ions attract each other and make a bond. This bond is called “ionic bond”.​

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/.

How to Write Electronic Configuration

Electrons are much disciplined runners. As you know electrons move around the nucleus in certain orbits. Like any racing game, electrons also have to follow some rules. These rules decide the orbit of an electron, space ship it will travel on, and also the particular seat for it. When you see an atom you will find it much organised, even you can guess the arrangement of electrons in it. But you must know the rules.

Rule 1 is about orbit.

It is named as “The Aufbau Principle” (aufbau is German for “building up”). Orbit nearer to the nucleus, which has lower energy, is to be filled first. It means orbit 1will be filled first then 2 then 3 and so on.

If you take an example of element Lithium (³Li), it has 3 electrons. First they have to fill orbit 1 and then move on to the next one. Orbit 1 allows 2 electrons so 2 of 3 electrons will there and remaining 1 electron will go to the orbit 2.

Rule 2 is about space ship.

Space ship is called “Sub-Shell”. You know every orbit has different sub-shells. Orbit 1 has “s” sub-shell, orbit 2 has “s” and “p” sub-shells, orbit 3 has “s”, “p” and “d” sub-shells and orbit 4 has “s”, “p”, “d” and “f” sub-shells.

Every sub-shell has different energy and its energy also depends on the energy of the orbit it belongs to. Orbital “s” is the lowest energy sub-shell and “f” is the highest energy sub-shell.

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

Let’s take an example of Boron (⁵B), it has 5 electrons. First 2 electrons will go in 1s, next 2 electrons will go in 2s and remaining 1 will go in 2p. Now you are able to distribute electrons in orbits and sub-shell. How do you write your finding in a precise manner? 

·               ​First write orbit number before sub-shell,

·               Number of electrons is to be written in the right superscript.

This way of presentation of electrons distribution is called “Electronic Configuration”. Now try to write electronic configuration of Boron​.⁵ B = 1s², 2s², 2p¹

Rule 3 is about assigning the seats in a spaceship.

You know every spaceship or sub-shell has different number of compartments. These compartments are named “Orbitals”. Orbitals of the same sub-shell have equal energy and these are called “Degenerate Orbitals”. Every orbital can accommodate maximum 2electrons.

This rule is called “Hund’s Rule” (given by the scientist Friedrich Hermann Hund). It states that when there are degenerate orbitals electron will occupy an empty orbital before it will pair up with another electron.

Let’s take an example of Oxygen (⁸O), it has 8 electrons. And its electronic configuration will be ⁸O = 1s², 2s², 2p⁴

​​

If you remember I have told you that electrons are like spin top. Every electron revolves clockwise or anticlockwise in its axis while it travels in the orbit. When they share single orbital one of the electron revolves clockwise and the other one anticlockwise. Clockwise rotation is represented by half up side arrow and anticlockwise rotation by half down side arrow. Let’s place electrons of oxygen in their corresponding orbitals.

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/.

Elements in human body

Out of 118 elements 29 are found in our body. 99% mass of the human body is made up of just six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. But there are a number of elements found in our body which play vital role.

No.	Name of Element	Where it is Found in our body
1.	Oxygen (65%)	Predominantly found in the form of water in our body
2.	Carbon (18%)	Every cell is made up of it
3.	Hydrogen (10%)	Predominantly found in the form of water in our body
4.	Nitrogen (3%)	Found in Muscles and DNA
5.	Calcium (1.5%)	Found in Bones
6.	Phosphorus (1.0%)	Found in Bones and in energy generator cells
7.	Potassium (0.35%)	Vital for electrical signalling in nerves
8.	Sulphur (0.25%)	Found in two amino acids that are important for giving proteins their shape
9.	Sodium (0.15%)	Vital for electrical signalling in nerves. It also regulates the amount of water in the body.
10.	Magnesium (0.05%)	Found in Skeleton and muscles. It is also necessary in more than 300 essential metabolic reactions.
11.	Iron (0.70%)	Found in Blood and gives red colour to it.
12.	Chlorine (0.15%)	It is important for maintaining normal balance of fluids
13.	Copper (0.0001%)	Without enough copper, iron won't work properly in the body
14.	Iodine (0.000016%)	It is required for making of thyroid hormones which is responsible for brain and body growth
15.	Fluorine (0.0037%)	Found in teeth and bones
16.	Selenium (0.000019%)	It is essential for certain enzymes, including several anti-oxidants.
17.	Zinc (0.0032%)	Several proteins contain structures called "zinc fingers" which help to regulate genes. Zinc deficiency has been known to lead to dwarfism.
18.	Chromium  (0.0000024%)	It helps regulate sugar levels by interacting with insulin
19.	Manganese  (0.000017%),	It is essential for certain enzymes in particular the ones that protect mitochondria (the power house of cell) from dangerous oxidants.
20.	Molybdenum  (0.000013%)	It is important for transforming sulphur into a usable form.
21.	Cobalt, Lithium, Strontium, Aluminium, Silicon, Lead, Vanadium, Arsenic, Bromine (trace amounts)	Roles not known

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/.