How to determine or calculate the number of Valence electrons?
Do you know what are valence electrons and how to find these electrons? Well, this is probably the main reason why you are here. So, in this article, we will discuss all there is to know about How to find valence electrons from different types of methods.
Valence electrons are the building blocks of an atom’s electronic configuration. The electronic configuration of an element holds a prime significance in determining its nature, electronic behavior, and chemical reactivity.
That being said, a thorough understanding of how to find valence electrons by different methods is one of the most valuable skills for a chemist skillset.
In this article, we will learn:
- What are valence electrons?
- How to find valence electrons for main group elements and compounds?
- How to determine valence electrons from the Periodic Table?
- How to find valence electrons from the Bohr model of the atom?
- How to determine valence electrons from the electronic configuration?
- How to calculate valence electrons of d-block, transition metals
- How to find valence electrons from an orbital diagram?
- How to determine the valence electrons from a Lewis structure?
- How to Calculate valence electrons of ions and polyatomic ions?
What are valence electrons?
Valence electrons are negatively charged subatomic particles (electrons) present in the outermost shell of an atom. By this convention, valence electrons are ideally the electrons that get involved in chemical bond formation.
Valence electrons lie farthest away from the atomic nucleus therefore these electrons experience the least force of attraction from an atom’s positively charged nuclear center. The valence electrons thus possess maximum energy and can easily break free from the nuclear hold in order to participate in different kinds of chemical reactions.
How to find Valence electrons?
The Periodic Table is a tabular display of chemical elements, arranged in ascending order of atomic numbers. It consists of vertically arranged groups and horizontal rows called periods.
There are a total of 18 groups and 7 periods in the Periodic Table. The elements situated in groups (1, 2, and 13-18) are called main-group elements. Groups 1 and 2 of the Periodic Table together make an s-block. On the other hand, the block comprised of groups (13-18) is called a p-block.
For main-group elements, valence electrons can be determined from their position in the Periodic Table i.e., their group number and/or the electronic configuration of elements.
However, valence electrons are not always the electrons present in the outermost shell. All the elements lying in groups (3-12) constitute another block in the Periodic Table called a d-block. These d-block elements are transition metals that exhibit a variable valency.
Thus, the valence electrons of d-block elements, unlike the s and p blocks, do not exist in the outermost shells only. Rather, their valence electrons can be present in the inner/ core shells as well.
Hence, we need to consider a lot of different ways when finding the valence electrons of a specific chemical element from the Periodic Table.
For instance, Let’s check the number of valence electrons in the most common atoms.
|Atomic number||Name of the Elements||Number of valence electrons|
|1||Hydrogen Valence electrons||1|
|2||Helium Valence electrons||2|
|3||Lithium Valence electrons||1|
|4||Beryllium Valence electrons||2|
|5||Boron Valence electrons||3|
|6||Carbon Valence electrons||4|
|7||Nitrogen Valence electrons||5|
|8||Oxygen Valence electrons||6|
|9||Fluorine Valence electrons||7|
|10||Neon Valence electrons||8|
|11||Sodium Valence electrons||1|
|12||Magnesium Valence electrons||2|
|13||Aluminum Valence electrons||3|
|14||Silicon Valence electrons||4|
|15||Phosphorus Valence electrons||5|
|16||Sulfur Valence electrons||6|
|17||Chlorine Valence electrons||7|
|18||Argon Valence electrons||8|
|19||Potassium Valence electrons||1|
|20||Calcium Valence electrons||2|
Check – Valence electron calculator to count the number of valence electron for any atom
How to find the Valence electrons from the periodic table?
The position of a chemical element in the Periodic Table gives a major hint in determining that element’s valence electrons. For the s-block for instance, the number of valence electrons is equal to the group number in which the element is situated.
For neutral atoms, the number of valence electrons is equal to the atom’s main group number.
For examples –
- Lithium (Li) is present in group 1, and it has 1 valence electron. Similarly,
all the elements present in group 1 have a single valence electron only.
- Magnesium (Mg) and calcium (Ca) are both present in group 2 and have 2 valence electrons each. Similarly, all the elements present in group 2 have two valence electrons only.
Now, P-block contains the group 13, 14, 15, 16, 17, and 18. These groups are also written as – Group 13 or 3A, Group 14 or 4A, Group 15 or 5A, Group 16 or 6A, Group 17 or 7A, and Group 18 or 8A.
The valence electrons for P-block is also found by the same method as S-block. Just look at the Group number of element = it is equal to the number of valence electrons.
- The boron atom belongs to Group 3A, hence, the number of valence electrons in the boron atom is 3.
- Same, the oxygen atom belongs to Group 6A, hence, the number of valence electrons in the oxygen atom is 6.
That’s all, just remember the Group number of S and P-block elements, and calculate their valence electrons.
|Block||Periodic table Group||Number of valence electrons|
|S-Block||Alkali metals – Group 1 (I)||1|
|S-Block||Alkaline earth metals – Group 2 (II)||2|
|P-Block||Boron group – Group 13 or 3A||3|
|P-Block||Carbon group – Group 14 or 4A||4|
|P-Block||Nitrogen group – Group 15 or 5A||5|
|P-Block||Oxygen group – Group 16 or 6A||6|
|P-Block||Halogens – Group 17 or 7A||7|
|P-Block||Noble gases – Group 18 or 8A||8|
How to find valence electrons from the electronic configuration?
Electronic configuration describes the electronic structure of an atom i.e., the arrangement of electrons in its atomic orbitals. The total electrons present in a neutral atom are always equal to the total number of protons present in its nucleus. The total number of protons present in an atom is represented as its atomic number (Z). Thus, Z also signifies the total electrons present in an atom.
While writing the electronic configuration, we must remember that the integers 1,2,3… represent shell numbers, on the other hand, s, p, and d are sub-shells. Electrons are situated in the orbitals present in these sub-shells, each orbital having a maximum capacity of holding 2 electrons.
For examples –
Let’s find the valence electrons of the nitrogen atom through its electron configuration. (7N)
- Step 1 – Determine the atomic number from the elemental symbol in the Periodic Table. The atomic number of Nitrogen, Z=7
- Step 2 – Write the electronic configuration. Electronic configuration of N: 1s22s22p3.
- Step 3 – Identify the outermost shell of Nitrogen. Outermost shell = Shell number 2
- Step 4 – Find the number of valence electrons by counting the total number of electrons present in the outermost shell. The outermost shell of nitrogen has (2s22p3), therefore, the total valence electrons in nitrogen are 5.
All these steps to find valence electrons are applicable to the s-block elements as well.
Let’s take one more example to understand How to calculate the valence electron with the help of electron configuration.
- Calcium that has atomic number Z=20
- Electronic configuration of Ca: 1s22s22p63s23p64s2
- Outermost shell: Shell number 4
- Valence electrons of calcium = 2
How to find valence electrons from the orbital diagram?
An orbital diagram is a diagrammatic representation of electrons occupying atomic orbitals. It can prove useful for determining the valence electrons present in an atom. Atoms use their valence electrons to form new bonds and achieve a stable electronic configuration. Let’s see how it is done.
The process of finding the valence electrons from the orbital diagram is the same as the electron configuration.
The main difference between the orbital diagram and electron configuration is that an orbital diagram shows electrons in form of arrows whereas an electron configuration shows electrons in form of numbers.
Two nitrogen atoms, each using three of their five valence electrons available can combine to form a nitrogen (N2) molecule. A triple covalent bond will form with three bond pairs. While 2 unbonded electrons on each nitrogen will stay as a lone pair. In this way, each nitrogen atom will achieve an octet configuration.
You can easily find the valence electrons for various atoms using an orbital diagram and electron configuration.
How to find valence electrons in compounds?
In chemistry, a compound is a substance that results from a combination of two or more different chemical elements.
Let’s say we have to find a number of valence electrons in a CO2 compound.
CO2 is a compound that is formed between two non-metal atoms (carbon and oxygen)
Valence electrons in CO2 –
To find the total valence electron in CO2, look at the periodic group of carbon and oxygen atoms.
By looking at the periodic table, we come to know carbon belongs to the 4A group and oxygen belongs to the 6A group in the periodic table. Hence, carbon has 4 valence electrons and oxygen has 6 valence electrons.
⇒ Valence electron of Oxygen = 6 [∴ Periodic group of oxygen = 16 or 6A]
⇒ Valence electron of Carbon = 4 [∴ Periodic group of carbon = 14 or 4A]
∴ Total valence electron available in CO2 compound = 4 + 2*6 = 16 valence electrons [∴ CO2 molecule has one carbon and two oxygen atoms]
Let’s take one more example to understand, how to determine the valence electrons in the compound.
For example – Find valence electrons in the PBr3 molecule.
An easy way to find the valence electron of atoms in the PBr3 molecule is, just to look at the periodic group of phosphorous and bromine atoms.
As the phosphorous atom belongs to the 5A group in the periodic table and bromine is situated in the 7A group, hence, the valence electron for the phosphorous is 5, and for the bromine atom, it is 7.
⇒ Total number of the valence electron in Phosphorous = 5
⇒ Total number of the valence electrons in bromine = 7
∴ Total number of valence electrons available in PBr3 = 5 + 7×3 = 26 valence electrons [∴ PBr3 molecule has one phosphorous and three bromine atoms]
How to determine valence electrons through the Bohr model?
Bohr model describes the visual representation of orbiting electrons around the small nucleus. It used different electron shells such as K, L, M, N…so on.
“Valence electrons are found in the outermost shell of an atom”
Let’s say you have the Bohr model of a Nitrogen atom, By looking at this, we can easily find the valence electrons for the nitrogen atom.
Nitrogen Bohr model
From the Bohr diagram of an atom, we can easily find the number of valence electrons in an atom by looking at its outermost shell.
So, we have to find a valence electron in the Nitrogen atom, for this, look at its Bohr diagram.
Bohr’s diagram of Nitrogen has only two electron shells (K and L), the inner shell is the K-shell and the outermost shell is L-shell.
Hence, the electrons found in the L-shell of the Nitrogen atom are its valence electrons because it is the outermost shell also called the valence shell.
The L-shell or outer shell of the Nitrogen Bohr model contains 5 electrons, therefore, the number of valence electrons in the Nitrogen atom is 5.
Let’s take one more example to clarify the concept of determination of valence electrons through the Bohr diagram.
⇒ Find the Valence electron of Oxygen through its Bohr diagram
Oxygen Bohr model
So, we have to find a valence electron in the Oxygen atom, for this, look at its Bohr diagram.
Bohr’s diagram of Oxygen has only two electron shells (K and L), the inner shell is the K-shell and the outermost shell is L-shell.
Hence, the electrons found in the L-shell of the Oxygen atom are its valence electrons because it is the outermost shell also called the valence shell.
The L-shell or outer shell of the Oxygen Bohr model contains 6 electrons, therefore, the number of valence electrons in the Oxygen atom is 6.
Therefore, by using the above explanation, you can easily determine the valence of electrons by looking at the Bohr model of the atom.
|Name of element||Number of electrons||Electronic configuration based on shell||Outermost electrons (Valence shell electrons)|
|Lithium (Li)||3||[2, 1]||1|
|Beryllium (Be)||4||[2, 2]||2|
|Boron (B)||5||[2, 3]||3|
|Carbon (C)||6||[2, 4]||4|
|Nitrogen (N)||7||[2, 5]||5|
|Oxygen (O)||8||[2, 6]||6|
|Fluorine (F)||9||[2, 7]||7|
|Neon (Ne)||10||[2, 8]||8|
|Sodium (Na)||11||[2, 8, 1]||1|
|Magnesium (Mg)||12||[2, 8, 2]||2|
|Aluminum (Al)||13||[2, 8, 3]||3|
|Silicon (Si)||14||[2, 8, 4]||4|
|Phosphorus (P)||15||[2, 8, 5]||5|
|Sulfur (S)||16||[2, 8, 6]||6|
|Chlorine (Cl)||17||[2, 8, 7]||7|
|Argon (Ar)||18||[2, 8, 8]||8|
|Potassium (K)||19||[2, 8, 8, 1]||1|
|Calcium (Ca)||20||[2, 8, 8, 2]||2|
How to determine valence electrons from Lewis structure?
If you have the structural representation of an atom in front of you, then you can tell the number of valence electrons present in it, without using the Periodic Table. Lewis structures are the simplest form of representing valence electrons using a line and dot model.
Let’s say you have given a lewis structure of CH4 –
Let’s take more examples to understand, How to count valence electrons using the Lewis structure of any compound or atoms.
Example – Lewis structure of CHCl3
So, we have given a lewis structure of CHCl3, let’s find valence electrons in it.
In the above picture, (there are 18 dots of electrons + 4 single bonds means 8 electrons = 26 valence electrons in CHCl3).
Example – Lewis structure of NCl3
So, we have given a lewis structure of NCl3, let’s find valence electrons in it.
In the above picture, (there are 20 dots of electrons + 3 single bonds means 6 electrons = 26 valence electrons in NCl3).
That’s all, by looking at the lewis structure of any molecule, you can easily count the valence electrons in molecules or atoms.
Also Read –
How to determine the valence electrons of ions?
Let’s see how to determine the valence electrons of positive ions.
- Positively charged ions are formed by the loss of valence electrons.
- To find valence electrons of positively charged ions, consider the magnitude of charge present.
- From the electronic configuration of the corresponding neutral atom, remove the electrons from the outer shell equal to the charge.
- Consider the remaining electrons as the valence electrons of the ion.
Let’s see how to find the valence electrons of negative ions.
- Negatively charged ions are formed by a gain of electrons in the valence shell.
- Consider the electronic configuration of neutral atoms for adding electrons in the outermost shell equal to the charge present.
- The total electron count after this addition is the valence electrons of the negative ion.
Let’s check the valence electrons for some common ions.
|Ions||Gain or loss of electrons||Valence electrons|
|Li+||Lithium loses one electron||2|
|Be2+||Beryllium loses two electrons||2|
|B3+||Boron loses three electrons||2|
|C4-||Carbon has gained four extra electrons||8|
|N3-||Nitrogen has gained three extra electrons||8|
|O2-||oxygen has gained two extra electrons||8|
|F--||Fluorine has gained one extra electron||8|
|Na+||Sodium loses one electron||8|
|Mg2+||Magnesium loses two electrons||8|
|Al3+||Aluminum loses three electrons||8|
|Si4+||Silicon loses four electrons||8|
|P3-||Phosphorus gains three electrons||8|
|S2-||Sulfur gains two electrons||8|
|Cl--||Chlorine gains one electron||8|
|K+||Potassium loses one electron||8|
|Ca2+||Calcium loses two electrons||8|
How to calculate the number of valence electrons in polyatomic ions?
Poly means many and ions are charged particles. Thus, polyatomic ions are species based on more than one different type of ions.
For example –
Sulfate (SO42-) is a polyatomic ion, composed of sulfur and oxygen atoms. Let’s find total valence electrons are present in its structure.
Step I: Count the valence electrons of each atom present in this polyatomic ion.
- Valence electrons of sulfur =6
- Valence electrons of oxygen = 6
- 4 oxygen atoms mean 4×(6) =24 valence electrons
Step II: Consider the overall charge present on the ion
A minus two charge means two electrons are gained
Step III: Count all the electrons present
The total number of valence electrons in SO42- = valence electrons of sulfur + total valence electrons of oxygen + electrons gained = 6 + 24 + 2= 32 valence electrons are present in a sulfate ion, as can be spotted from its Lewis structure below.
Let’s take one more example, BrO2-- molecule
⇒ Total number of the valence electrons in oxygen = 6
⇒ Total number of the valence electrons in bromine = 7
∴ Total number of valence electron in BrO2– = 7 + 6(2) + 1 = 20 valence electrons [∴ one bromine, two oxygen and one negative ion in BrO2– that also count as a one valence electron]
So, this is how you can easily find the valence electrons for any polyatomic ions. You just have to count the valence electrons for individual atoms then add all of them and if a positive charge is given, then subtract the electrons, and if a negative charge is given then add the electrons.
How to determine valence electrons of d-block
As we already noted, the elements lying at the center of the Periodic Table in a rectangular block called d-block behave differently from all the other elements. These are called transition metals. These also include the f-block elements lying in a small block, positioned below the rest of the table.
In transition metals, electrons occupy outer shells without first completely filling inner core shells. Similarly, transition metals can use their core-shell electrons in addition to the electrons from the outermost shell while bonding. That is why transition metals possess multiple valencies simultaneously.
The electronic configuration of a transition metal generally ends as ns2(n-1)d. We need to identify the shell with the highest quantum number (n) to find valence electrons in transition metals. Let’s understand this concept better with the help of the examples given below.
⇒ Transition metal: Iron (26Fe)
⇒ Electronic configuration of 26Fe : 1s22s22p63s23p64s23d6, it can also written as [Ar] 4s23d6
⇒ 4s is filled before 3d because s subshell is symmetrical in shape and closer to the nucleus thus having lower energy. But once all the electrons are placed in their respective orbitals, the electronic configuration can be rearranged as [Ar] 3d64s2. The total number of electrons that can be involved in bond formation = is 6+2 = 8. But Fe rarely uses all eight electrons available.
⇒ Fe loses only 2 of its valence electrons during chemical bonding which results in a ferrous (Fe2+) ion. Therefore, it is often said that Fe has 2 valence electrons only although it has a total of 8. We must note that 8 is also the Group number of iron.
⇒ Ferric (Fe3+) ions are also formed from Fe by loss of 3 electrons i.e., 2 electrons from 4s and 1 electron from 3d. In that case, the valency of iron is 3. One electron from the core 3d subshell also acts as a valence electron. Thus, a proof of the statement: transition metals exhibit a variable valency.
The valence electrons present in a transition metal can be determined by writing its electronic configuration with the help of its atomic number and the charge present if any.
Provide another example below for you to make sure you understand this concept well.
- Transition metal: Copper (29Cu)
- 29Cu is present in Group 11 of the Periodic Table.
- The total number of valence electrons = 11.
- Electronic configuration of 29Cu : [Ar] 3d104s1
- Only 1 electron is placed in 4s because 3d104s1 is more stable than 3d94s2 due to a completely filled d subshell.
- Valence electrons usually used for bond formation: 1 or 2
- Copper ions possible: Cu+ and Cu2+
- Preferrable valencies: 1 (from 4s) or 2 (1 electron from 4s and 1 electron from 3d can be removed to facilitate chemical bonding)
The first ten transition metal elements of the d-block, located in Period 4 hold a significant position among d-block elements. So, in the table given below, we are providing you with a list of the total number of valence electrons present in these elements and their preferrable valencies.
How to determine the valence electrons for P-block elements?
The latest updated version of the Periodic Table uses numbers 13-18 for p block elements. Previously, the Periodic Table was believed to consist of 8 groups only. In that case, the p block was represented by Roman numerals (III-VIII).
According to that convention, elements present in group III have three valence electrons. Those present in groups IV and V have four and five valence electrons, and so on until group VIII with eight valence electrons except helium which consists of two valence electrons.
Eight valence electrons in the outermost shell represent a complete octet configuration thus extremely stable, inert chemical entities such as Noble gases.
How many valence electrons are present in the polyatomic ions: i) PO43- and ii) NH4+
i) Valence electrons of phosphorus + 4 (valence electrons of oxygen ) + 3 electrons gained
∴ 5+4(6)+3= 32 Valence electrons in PO43-
ii) Valence electrons of nitrogen + 4 (valence electrons of hydrogen) – 1 electron lost
∴ 5 + 4(1) -1 = 8 valence electrons in NH4+
The atomic number of chlorine (Cl) is 17. Its electronic configuration can be written as: 1s22s22p63s23p5. Can you determine the valence electrons of Cl?
|Outer shell of calcium = shell number 3, therefore, the number of valence electrons in calcium outermost shell (3s23p5) is = 2+5= 7|
How can oxygen use its valence electrons to form a chemical bond for achieving a stable octet configuration?
- Valence electrons are very important in controlling the structure, properties, and, chemical reactivities of all the elements present in the Periodic Table.
- Elements use their valence electrons to form new bonds with the ultimate goal to achieve a stable electronic configuration.
- Therefore, it is essential for a chemistry student to know how to find valence electrons.