Structures and Shapes of Molecules
Counting Valence Electrons
The Periodic Table of Elements
Valence electrons are electrons located at the outermost shell of the atom of a given
element. These are the electrons involved in chemical bonding.
The periodic table provides various information about the elements, and some of these
information can be used to determine the number of valence electrons in the atom.
These are representative elements, transition metals, lanthanides, and actinides.
The s- and p-blocks
The representative (or main group) elements are those in the s- and p-blocks.
The valence electrons of representative elements are those present in the electron shell
having the highest principal quantum number, n.
Silicon (Si), its outermost shell has an n of 3, you can count all electrons contained in
shells with n = 3 (i.e. 3s23p2).
On the same hand, Beryllium (Be) has a condensed electron configuration of 1s 22s2. its
outermost shell has an n of 2, and contains only two valence electrons.
The number of valence electrons of representative elements is equal to their group
numbers provided by the Chemical Abstracts Service (CAS).
o Elements in group 1A always have one valence electron, while elements in group
8A have eight.
CAS Group IUPAC group Number of valence
Number number electrons
1A 1 1
2A 2 2
3A 13 3
4A 14 4
5A 15 5
6A 16 6
7A 17 7
8A 18 8
, The d-block
The transition metals are those in the d-block. Elements in this block have their d
subshells being filled.
They are the elements belonging to groups 3 to 12.
Their electrons behave differently, such as the electrons present in the largest d-block
for the atom may add in the total number of valence electrons from the largest orbitals
of s and p.
CAS group IUPAC group First element of Number of valence
number number the group electrons
3B 3 scandium 3
4B 4 titanium 4
5B 5 vanadium 5
6B 6 chromium 6
7B 7 manganese 7
8B 8 iron 3
8B 9 cobalt 3
8B 10 nickel 3
1B 11 copper 2
2B 12 zinc 2
The f-block
These elements, which are found under the main body of the periodic table, are
referred to as the rare earth metals.
These elements vary in oxidation states and are all radioactive. They are classified into
two.
o Lanthanides are those elements that have their 4f orbitals being filled.
o Actinides are those elements having their 5f orbitals filled.
Determining the number of valence electrons of compounds
To determine the number of valence electrons in a compound, simply add the number
of valence electrons in each constituting atoms or elements.
No. of valence electrons = ∑ valence electrons of each element
How many valence electrons does NO2 has?
Solution
Counting Valence Electrons
The Periodic Table of Elements
Valence electrons are electrons located at the outermost shell of the atom of a given
element. These are the electrons involved in chemical bonding.
The periodic table provides various information about the elements, and some of these
information can be used to determine the number of valence electrons in the atom.
These are representative elements, transition metals, lanthanides, and actinides.
The s- and p-blocks
The representative (or main group) elements are those in the s- and p-blocks.
The valence electrons of representative elements are those present in the electron shell
having the highest principal quantum number, n.
Silicon (Si), its outermost shell has an n of 3, you can count all electrons contained in
shells with n = 3 (i.e. 3s23p2).
On the same hand, Beryllium (Be) has a condensed electron configuration of 1s 22s2. its
outermost shell has an n of 2, and contains only two valence electrons.
The number of valence electrons of representative elements is equal to their group
numbers provided by the Chemical Abstracts Service (CAS).
o Elements in group 1A always have one valence electron, while elements in group
8A have eight.
CAS Group IUPAC group Number of valence
Number number electrons
1A 1 1
2A 2 2
3A 13 3
4A 14 4
5A 15 5
6A 16 6
7A 17 7
8A 18 8
, The d-block
The transition metals are those in the d-block. Elements in this block have their d
subshells being filled.
They are the elements belonging to groups 3 to 12.
Their electrons behave differently, such as the electrons present in the largest d-block
for the atom may add in the total number of valence electrons from the largest orbitals
of s and p.
CAS group IUPAC group First element of Number of valence
number number the group electrons
3B 3 scandium 3
4B 4 titanium 4
5B 5 vanadium 5
6B 6 chromium 6
7B 7 manganese 7
8B 8 iron 3
8B 9 cobalt 3
8B 10 nickel 3
1B 11 copper 2
2B 12 zinc 2
The f-block
These elements, which are found under the main body of the periodic table, are
referred to as the rare earth metals.
These elements vary in oxidation states and are all radioactive. They are classified into
two.
o Lanthanides are those elements that have their 4f orbitals being filled.
o Actinides are those elements having their 5f orbitals filled.
Determining the number of valence electrons of compounds
To determine the number of valence electrons in a compound, simply add the number
of valence electrons in each constituting atoms or elements.
No. of valence electrons = ∑ valence electrons of each element
How many valence electrons does NO2 has?
Solution
