How many valence electrons does an atom of lithium possess?
[A] 1
[B] 2
[C] 3
[D] 4
Answer
HINT: Valence electrons are the electrons present in the outermost shell of an atom. To find the number of valence electrons in an atom of lithium you have to know that atomic number of lithium. You can use the atomic number to write down its electronic configuration and then find the electrons in the valence shell.
COMPLETE STEP BY STEP SOLUTION: To answer this question we need to understand the meaning of the term valence.
Electrons in the valence shell of an atom are
known as valence electrons. The valence shell is the outermost shell of an atom. The electrons in this outermost shell are responsible for the bonding of the atoms.
The atoms attain stability by filling their octet (or duplet in case of hydrogen atom as it has only one electron) and attaining a stable configuration.
To find the number of valence electrons in lithium we need to know its atomic number so that we can write down its electronic configuration.
Atomic number of lithium is 3.
It will have two electrons in the first shell and 1 electron in the next shell which is its valence shell.
Therefore, we can write down its electronic configuration as- 2, 1. So, there is one electron in its outermost shell.
We can understand from the above discussion that lithium as one electron in its valence shell which makes it valence electrons.
Therefore, the correct answer is option [A] 1.
NOTE: Lithium has only one electron in its outermost shell. To gain stable configuration it will need to gain seven electrons. Losing one electron is more favourable energetically than gaining seven electrons and due to small size there will be electron-electron repulsion causing instability. Therefore, lithium exists in a stable ion form as $L{{i}^{+}}$ by losing one electron.
Answer
Verified
Hint: The answer to this question is dependent on the fact that the valence electrons are the electrons that are present in the outermost orbit of the atom and it can be found by writing the electronic configuration of the atom.
Complete answer:
In the lower classes, we have studied about the basic topic of general chemistry which tells us about the structure and composition of an atom and also about the periodic table and how the atoms are arranged in it.
Let us now
see the electronic configuration of the beryllium atom according to the atomic number.
- Atoms are composed of nuclei in which the neutrons and protons are concentrated in it and the negatively charged particles called the electrons revolve around the orbit of the nucleus of an atom.
- The number of protons in an atom is nothing but its atomic number and also the total number of protons are always equal to the total number of electrons in an atom.
Based on these facts, we can write the
electronic configuration of an atom. Now, let us write the electronic configuration of lithium atom which has the atomic number 3 with the atomic symbol $Li$ and the configuration is $1{{s}^{2}}2{{s}^{1}}$.
- Here, the outermost shell is 2s and the total number of electrons present in this shell is 1.
Therefore, the correct answer is that Lithium has 1 valence electron.
Note:
Note that the valence electrons are also called as the electrons in the last ring or the outermost
orbit of the atom. This term is not to be confused with valency of an atom as valency is the measure of the combining capacity of the atom with the other atom/s when it forms a molecule.
A chemical reaction involves either electron removal, electron addition, or electron sharing. The path that a specific element will take in a reaction depends on where the electrons are in the atom and how many there are.
Valence Electrons
In the study of chemical reactivity, electrons in the outermost principal energy level are very important and so are given a special name. Valence electrons are the electrons in the highest occupied principal energy level of an atom. In the second period elements, the two electrons in the \(1s\) sublevel are called inner-shell electrons and are not involved directly in the element's reactivity, or in the formation of compounds.
Valence electrons for representative elements
Lithium has a single electron in the second principal energy level, and so we say that lithium has one valence electron.
Li: 1s22s1 (the electron in the 2s energy level is the valence electron)
Beryllium has two valence electrons:
Be: 1s2 2s2 (the two electrons in the 2s energy level are the valence electrons)
How many valence electrons does boron have? Recognize that the second principal energy level consists of both the \(2s\) and the \(2p\) sublevels, and so the answer is three.
B: 1s2 2s2 2p1 (there are three electrons on the highest occupied energy level n=2)
In fact, the number of valence electrons goes up by one for each step across a period, until the last element is reached. Neon, with its configuration ending in \(2s^2 2p^6\), has eight valence electrons.
Valence electrons for transition elements.
Transition elements are a bit trickier. In this case, we also need to consider the electrons in the highest occupied energy level (n) plus the electrons in the (n-1) d orbital. For example, the electron configuration of iron is Fe is 1s2 2s2 2p6 3s2 3p6 4s2 3d6. The total number of valence electrons for iron is 8: 2 electrons in the highest occupied energy level (n=4) plus 6 electrons in the (n-1) d orbital, that is, 3d.
Using abbreviated electron configuration (or noble gas configuration) to identify valence electrons
The best approach to identify the number of valence electrons is to use the abbreviated notation for electron configuration. In this notation, the inner-shell electrons are abbreviated by indicating the noble gas that corresponds to that same number of electrons
For example, the abbreviated electron configuration for Lithium is [He] 2s1, so the number of valence electrons is 1 (2s1)
In the case of Boron, the abbreviated electron configuration is [He] 2s2 2p1, so the number of valence electrons is 1 (2s2 2p1)
In the case of Iron, the abbreviated electron configuration is [Ar] 4s2 3d6, so the number of valence electrons is 8 (4s2 3d6)
Summary
- Valence electrons are the outer-shell electrons of an atom.
- Valence electrons determine the reactivity of an atom.