Answer
Please see the work below.
Work Step by Step
Electrons present in the outermost shell (valence shell) are called valence electrons.
Aluminum: Al – Atomic number -13.
Electronic configuration - $1s^{2} 2s^{2}2p^{6} 3s^{2}3p^{1}$
Here the valence shell is the third shell and consists of 3s and 3p orbitals with 2 and 1 electrons respectively. That is 3 electrons in the valence shell and hence it belongs to 3A.
Strontium: Sr – Atomic number -38.
Electronic configuration - $[Kr]5s^{2}$ {atomic number of Krypton is 36}
Here valence shell is the fifth shell and consists of 5s orbital with 2 electrons. That is 2 electrons in the valence shell and hence it belongs to 2A.
Potassium: K – Atomic number -19.
Electronic configuration - $[Ar]4s^{1}$ {atomic number of Argon is 18}
Here valence shell is the fourth shell and consists of 4s orbital with 1 electron. That is 1 electron in the valence shell and hence it belongs to 1A.
Bromine: Br – Atomic number -35.
Electronic configuration - $[Ar]3d^{10}4s^{2}4p^{5}$ {atomic number of Argon is 18}.
Here valence shell is the fourth shell and consists of 4s and 4p orbitals with 2 and 5 electrons respectively. That is 7 electron in the valence shell and hence it belongs to 7A.
Phosphorus: P – Atomic number -15.
Electronic configuration - $[Ne]3s^{2}3p^{3}$ {atomic number of Neon is 10}
Here valence shell is the third shell and consists of 3s and 3p orbitals with 2 and 3 electrons respectively. That is 5 electron in the valence shell and hence it belongs to 5A.
Sulfur: S – Atomic number -16.
Electronic configuration - $[Ne]3s^{2}3p^{4}$ {atomic number of Neon is 10}
Here valence shell is the third shell and consists of 3s and 3p orbitals with 2 and 4 electrons respectively. That is 6 electrons in the valence shell and hence it belongs to 6A.
Carbon: C – Atomic number -6.
Electronic configuration - $1s^{2} 2s^{2}2p^{2}$
Here valence shell is the second shell and consists of 2s and 2p orbitals with 2 and 2 electrons respectively. That is 4 electrons in the valence shell and hence it belongs to 4A.
