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NCERT Solutions for Exercise 8.4 Class 10 Maths Trigonometry
March 28, 2023
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Valence electrons: Valence electrons (VE) are generally negatively charged particles. These electrons are arranged in different orbits or shells. They are important for the formation of chemical bonds and for the interaction between atoms. Valence electrons are also called outer-shell electrons. Valence Electrons form a vital part of the Chemistry syllabus. Candidates must have a clear understanding of this topic. This topic forms the base for understanding chemical reactions.
By the end of the article, students will know how to calculate the valence electrons of oxygen and other elements. They will be able to answer questions such as ” the number of these electrons in cl- ion are?”, or calculate the number of iodine outer-shell electrons of fluorine outer-shell electrons by referring to the periodic table. Know what are ionic bonds and covalent bonds to understand the role of valence electrons in bond formation. Read the article to learn about these electrons.
Electrons are tiny subatomic particles that revolve around the nucleus in energy shells called orbits. These subatomic particles carry a negative charge of (1.602 times {10^{ – 19}}) Coulombs. The mass of an electron is (9 times {10^{ – 31}},{rm{kg}}{rm{.}}) The position of electrons around the nucleus is not certain. They continuously revolve around the nucleus in shells with increasing energy, i.e. the shell closer to the nucleus has less energy than the outermost shell. The electrons present in the outermost shell of the atom are called valence electrons or outer-shell electrons. In the periodic table, the vertical group represents the number of electrons present in the atom’s valence shell. But how do these tiny particles contribute to the chemical character of an atom?
Atoms are made up of neutrons, protons (left( + right)) and electrons (left( – right).) The nucleus of an atom is made up of positive protons and neutral neutrons. Electrons are found moving around the nucleus in energy shells. The shells can be numbered numerically as (1, 2, 3…) so on or by letters like ({rm{K,}},{rm{L,}},{rm{M}}…,) The energy shell ({rm{K,}}) denoted as ({rm{1,}}) has the lowest energy. The orbits are arranged in increasing energy levels. The lowest energy shell is closer to the nucleus, and the highest energy shell is farthest from the nucleus.
The vertical column or groups of the periodic table represents the number of these valence electrons present in all the elements of that group, i.e., the elements corresponding to a group have the same number of electrons in their valence shell.
For example- All alkali metals like Sodium (left( {{rm{Na}}} right),) Potassium (left( {rm{K}} right),) Lithium (left( {{rm{Li}}} right){rm{,}}) Caesium (left( {{rm{Cs}}} right){rm{,}}) Rubidium (left( {{rm{Rb}}} right)) and Francium (left( {{rm{Fr}}} right)) belong to group ({rm{1}}left( {rm{A}} right)) of the periodic table. All these elements have only one electron present in their valence shell. Hence, the group represents the number of electrons present in the valence shell.
Therefore, the easiest way to determine such electrons is by checking out the elements’ place in the periodic table.
Another way to find or determine these electrons is through the atomic number.
The atomic number of an element ( = ) number of electrons ( = ) number of protons
Electrons are distributed in different energy shells denoted by ({rm{K,}},{rm{L,}},{rm{M,}},{rm{N}}…) known as the electronic configuration of the element. By knowing the electronic configuration of an element, we can determine the number of valence electrons present in the atom’s valence shell.
However, if we consider the transition metals (groups (3 – 12)), finding the valence electron is quite complicated. The transition elements have a ({rm{d}})-subshell which plays a vital role in determining the electronic configuration of these elements.
The table below represents the number of valence electrons present in the different groups of the Periodic Table.
| Periodic Table Group | Valence Electrons |
| Alkali metals – Group (1,left( {rm{I}} right)) | (1) |
| Alkaline earth metals – Group ({rm{2}},left( {{rm{II}}} right)) | (2) |
| Boron group – Group (13,left( {{rm{III}}} right)) | (3) |
| Carbon group – Group (14,left( {{rm{IV}}} right)) | (4) |
| Nitrogen group – Group (15,left( {{rm{V}}} right)) | (5) |
| Oxygen group – Group (16,left( {{rm{VI}}} right)) | (6) |
| Halogens – Group (17,left( {{rm{VII}}} right)) | (7) |
| Noble gases – Group (18,left( {{rm{VIII}}} right)) | (8) |
The transition elements from the group (left( {3 – 12} right)) are not included in the above table as these elements include ({rm{d}})-subshell, which are vital to the valence electron determination.
The valence electrons are indicative of the properties of the atom. The reason for this is that atoms try to attain stability by filling their valence shell with (8) electrons(Octet Rule)
Atom either gain an electron(s), lose an electron(s), or share electrons to form a covalent bond to attain an inert gas configuration.
The representation of valence electrons or outer-shell electrons are explained below:
As valence electrons are significant to an atom’s reactivity, it is important to represent them by simple diagrams. Lewis structures, here, comes into the picture where the outer-shell electrons present in an atom are represented as dots. These structures are also known as electron dot diagrams.
In Electron dot diagrams, an element is represented by its chemical symbol surrounded by dots representing the outer-shell electrons.
Typically, the dots are drawn like a square surrounding the element symbol with up to two dots per side of the imaginary square. An element never has more than eight valence electrons, so there cannot be more than eight dots per atom.
The position of an atom in the periodic table represents the number of valence electrons present in the outermost shell of the atom. Across each row or period of the periodic table, the number of outer-shell electrons in groups (1 – 2) and (13 – 18) increases by one from one element to the next. Thus, within each column or group of the table, all the elements have the same number of outer-shell electrons. That explains why all the elements in the same group have very similar chemical properties.
For elements in groups (1 – 2) and (13 – 18,) the number of valence electrons is easy to tell directly from the periodic table. The group number represents the number of outer-shell electrons present in the valence shell of the atom. For elements in groups (3 – 12,) determining the number of outer-shell electrons is more complicated as the electrons are present in the ({rm{d}}) shell.
Carbon: The atomic number of carbon is (6.) It is present in group (4) of the Periodic Table of Elements. It has (6) electrons distributed in (2) energy shells. The electronic configuration of carbon is (2,,4.) Carbon has (4) electrons present in its valence shell.
Oxygen: The atomic number of oxygen is (8.) It is present in group 6 of the Periodic Table of Elements. It has (8) electrons distributed in (2) energy shells. The electronic configuration of oxygen is (2,,6.) Oxygen has (6) electrons present in its valence shell.
Potassium: The atomic number of Potassium is (19.) It is present in group (1) of the Periodic Table of Elements. It has (19) electrons distributed in (4) energy shells. The electronic configuration of Potassium is (2,,8,,8,,1.) Potassium has (1) electron present in its valence shell.
Nitrogen: The atomic number of nitrogen is (7.) It is present in group (5) of the Periodic Table of Elements. It has (7) electrons distributed in (2) energy shells. The electronic configuration of nitrogen is (2,,5.) Nitrogen has (5) electrons present in its valence shell.
Hydrogen: The atomic number of hydrogen is (1.) It is present in group (1) of the Periodic Table of Elements. It has only (1) electron distributed in only (1) energy shell.
Chlorine: The atomic number of chlorine is (17.) It is present in group (7) of the Periodic Table of Elements. It has (17) electrons distributed in (3) energy shells. The electronic configuration of nitrogen is (2,,8,,7.) Chlorine has (7) electrons present in its valence shell.
Valence electrons or outer-shell electrons are indicative of an atom’s reactivity. The presence of eight electrons in the valence shell of an atom imparts stability to that particular atom. To attain stability, atoms transfer or share electrons within themselves in such a way that they can satisfy the octet rule and attain noble gas configuration.
The key characteristics exhibited by outer-shell electrons are as follows:
Table salt is composed of sodium and chlorine in a (1:1) ratio. While one is a strong alkali metal, the other is a toxic gas. Sodium and chlorine are so reactive that they undergo chemical reactions with other elements to form compounds and are rarely present in the free state
The reaction occurs when an atom of Sodium (left( {{rm{Na}}} right)) gives up an electron to become a positively charged ion (left( {{rm{N}}{{rm{a}}^{rm{ + }}}} right),) and an atom of chlorine (left( {{rm{Cl}}} right)) accepts an electron to become a negatively charged chloride ion (left( {{rm{Cl – }}} right){rm{.}}) The two ions being oppositely charged are attracted to each other and form a matrix of interlocking sodium and chloride ions, forming a salt crystal.
Sodium easily loses an electron because it belongs to group ({rm{1}},left( {rm{A}} right)) of the periodic table. It has only one valence electron present in the outermost shell, which it readily loses to attain the stable noble gas electronic configuration of Neon (,left( {2,,8} right).)
Meanwhile, an atom of a Group (17) element such as chlorine has seven valence electrons in its outermost shell. As it needs only one electron in its valence shell to complete the octet and attain the noble gas configuration of Argon (left( {2,,8,,8} right),) it readily accepts the electron given by the sodium atom. Atoms of group 18 elements have eight outer-shell electrons (or two in the case of helium). These elements already have a full outer energy level, so they are very stable. Elements in other groups vary in their reactivity but are generally less reactive than elements in groups (1,,2,,16,) or (17.)
All metals behave as good conductors of electricity. The presence of (1,,2) and (3) electrons in the outermost shell of the atoms enables metals to lose them easily with the little application of energy. These electrons now become free from the effect of their nucleus and are available as free electrons. The presence of free electrons makes metal a good conductor of electricity. The most popularly known electrical conductor is copper, which has only one valence electron.
When the number of valence electrons in an element is (4,) the element will possess properties in between metallic and non-metallic. Neither do these elements or materials conduct electric current efficiently as a conductor, nor do they block the flow of current through them. Such elements or materials are called as the semiconductor. Carbon, silicon and germanium are semiconductor elements, and these have precisely fourouter-shell electrons in their atoms.
An element behaves as a non-metal when the number of valence electrons in its atom is more than four. The non-metals are a bad conductor of electricity. These elements are called insulators. The three well-known examples of the insulator are nitrogen, sulphur and Neon.
1. Nitrogen has (5)
2. Sulphur has (6)
3. Neon has (8)
Broadly, chemical bonding is classified into two categories based on how the valence electrons of an atom interact with the outer-shell electrons of other atoms. These are
The chemical bond formed between (2) atoms through the transfer of one or more outer-shell electrons from the electropositive or metallic element to the atom of an electronegative or non-metallic element is called an ionic or electrovalent bond.
The chemical bond formed between (2) atoms through the mutual sharing of one or more valence electrons between non-metallic elements is called a covalent bond. The sharing of outer-shell electrons between atoms takes place to attain the noble gas configuration of the participating individual atoms.
According to Neils Bohr, electrons are arranged around the nucleus in energy orbits or shells. The electrons present in the outermost orbit are known as valence electrons or outer-shell electrons. Elements possessing eight electrons in their outer orbit or valence orbit are said to possess octet configuration.
These elements are called noble gases and do not react with other elements exhibiting very low reactivity. These elements are said to have zero combining capacity. On the other hand, elements with incomplete outermost shells tend to complete the octet configuration by combining with the same or different kinds of elements.
The capacity of an atom to combine with other atoms is called its valency. Elements combine by losing, gaining or sharing their electrons.
For example, the electronic configuration of sodium is (2,,8,,1,) and chlorine is (2,,8,,7.,{rm{Na}}) finds it easier to lose one electron from its outermost shell rather than gaining seven electrons to complete its octet resulting in the formation of ({rm{N}}{{rm{a}}^ + }) Ion.
While ({rm{Cl}}) finds it easier to gain one electron to its outermost shell rather than losing seven electrons to complete its octet resulting in the formation of chloride ion (left( {{rm{C}}{{rm{l}}^ – }} right).) The electrovalency for sodium and chlorine in ({rm{NaCl}}) is (1.)
On the other hand, in ({rm{C}}{{rm{l}}_{rm{2}}},) both the chlorine atoms are deficient in one electron. In order to attain octet configuration, the individual chlorine atoms neither lose nor gain electrons. They attain stability by sharing electrons; each chlorine atom contributes one electron to the situation. Here, the covalency of the chlorine atom in the chlorine molecule is (1.)
1. Atoms consisting of more than (4) electrons in their valence shell.
Valency ( = 8 – ) Valence electrons.
Example- Fluorine has (9) electrons with an electronic configuration of (2,,7.) It has seven valence electrons. Hence,
Valency ( = 8 – 7 = 1.)
2. Atoms consisting of less than (5) electrons in their valence shell.
Valency ( = ) Valence electrons
Example- Sodium has (11) electrons with an electronic configuration of (2,,8,,1.) It has only one valence electron. Hence,
Valency ( = ) Number of valence electrons ( = 1.)
Note- Both sodium and chlorine have valency as one. However, sodium loses an electron, whereas chlorine gains an electron. Hence, sodium has a positive valency (left( {{rm{N}}{{rm{a}}^{rm{ + }}}} right)) but fluorine has a negative valency (left( {{{rm{F}}^ – }} right).)
3. Valencies of Ions
Ions that carry positive charge are known as cations. For example – ({rm{N}}{{rm{a}}^ + },,{rm{C}}{{rm{u}}^{2 + }},,{rm{A}}{{rm{l}}^{3 + }}) etc carry ( + 1,, + 2,, + 3) valencies respectively.
Ions which carry negative charge are known as anions. For example – ({rm{C}}{{rm{l}}^ – },,{{rm{O}}^{2 – }}) etc., which carry a valency of ( – 1,, – 2,) respectively.
Frequently asked questions related to valence electrons are listed as follows:
Q.1: How do you determine valence electrons?
Ans. The easiest way to determine valence electrons is by checking out the element’s place in the periodic table.
Another way to find or determine outer-shell electrons is through the atomic number.
The atomic number of an element ( = ) number of electrons ( = ) number of protons.
Q.2: What is a valence electron?
Ans. A valence electron is the number of electrons present in the outermost shell or orbit of an atom. It is equal to the group number of a particular element in the Periodic Table.
Q.3: How do valence electrons work?
Ans. Valence electrons play an important role in chemical bonding. Atoms either lose, gain or share their outer-shell electrons to form bonds.
Q.4: What are the (7) valence electrons?
Ans. When only seven electrons are present in the outermost shell of an atom, then the atom is said to possess seven valence electrons. Elements having seven outer-shell electrons are present in group (17) of the Periodic Table. This group of elements consists of Halogens. These elements easily accept an electron to complete its valence shell.
Q.5: Helium atom has (2) electrons in its valence shell, but its valency is not (2.) Explain.
Ans. Helium is an inert gas. It hardly undergoes any chemical reaction. Its outermost shell has a duplet, which is stable. Hence, it has zero combining capacity.
Q.6: What are two valence electrons?
Ans. When only two electrons are present in the outermost shell of an atom, then the atom is said to possess two outer-shell electrons. Elements having two valence electrons are present in group ({rm{II}}left( {rm{A}} right)) of the Periodic Table. These elements are called Alkaline Earth Metals.
Q.7: What is the main function of valence electrons?
Ans. Because they are in the highest energy level, they are generally the most involved in chemical reactions since they are the easiest to transfer. The outer-shell electrons are transferred between the atoms to bring stability to the atom.
Q.8: What is another name for valence electrons?
Ans. Valence electrons are also called outer-shell electrons.
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