Atomic Foundations of MatterClass 9 Science NCERT Solutions

Element A has one valence electron. To achieve a stable octet in its second shell, it will tend to lose (give) one electron.

After losing one electron, element A will have more protons than electrons, resulting in a net positive charge. It would form a cation with a charge of +1, represented as $A^+$.

Element B has six valence electrons. To achieve a stable octet, it will tend to gain (take) two electrons.

After gaining two electrons, element B will have more electrons than protons, resulting in a net negative charge. It would form an anion with a charge of -2, represented as $B^{2-}$.

The combination involves the transfer of electrons from element A (a metal) to element B (a non-metal). This transfer of electrons results in the formation of an ionic bond.

To form a neutral compound, the total positive charge must balance the total negative charge. We have ions $A^+$ and $B^{2-}$. To balance the charges, two cations of A ($2 \times +1 = +2$) are required for every one anion of B ($-2$). Using the criss-cross method, the formula for the compound formed would be $A_2B$.

Element X has six electrons in its outer (valence) shell. To achieve a stable octet, it needs two more electrons. By forming a diatomic molecule (a molecule with two atoms, $X_2$), each atom of X can share two of its valence electrons with the other atom. This allows both atoms to complete their octets.

The bond is formed by the sharing of electrons between the two atoms of X. This type of bond is a covalent bond. Since each atom contributes two electrons to be shared, a total of two pairs of electrons are shared, forming a double covalent bond.

The structure of the molecule would be similar to an oxygen molecule ($O_2$), as oxygen also has six valence electrons. The two X atoms are joined by a double bond. Each atom has two shared pairs (in the bond) and two unshared pairs (lone pairs) of electrons. The structure can be represented as: $X=X$

Element Y has two electrons in its second shell, so its electronic configuration is 2, 2. It is a metal and will tend to lose these two valence electrons to form a stable cation with a +2 charge ($Y^{2+}$). Element X needs to gain two electrons to form a stable anion with a -2 charge ($X^{2-}$). The bond formed between X and Y will be an ionic bond due to the transfer of two electrons from Y to X.

The structure of the compound formed would be an ionic lattice of $Y^{2+}$ cations and $X^{2-}$ anions. The formula unit would be YX. The formation can be shown as: $Y \rightarrow Y^{2+} + 2e^-$ $X + 2e^- \rightarrow X^{2-}$ Overall: $Y + X \rightarrow Y^{2+}X^{2-}$ (or simply YX)

$2 \, Al^{3+}$ and $3 \, Cl^{-}$: Total positive charge = $2 \times (+3) = +6$ Total negative charge = $3 \times (-1) = -3$ The charges do not balance. This is incorrect.

$3 \, Mg^{2+}$ and $1 \, PO_{4}^{3-}$: Total positive charge = $3 \times (+2) = +6$ Total negative charge = $1 \times (-3) = -3$ The charges do not balance. This is incorrect.

$2 \, Fe^{3+}$ and $3 \, O^{2-}$: Total positive charge = $2 \times (+3) = +6$ Total negative charge = $3 \times (-2) = -6$ The total positive charge (+6) and total negative charge (-6) are equal and opposite, resulting in a neutral compound. This combination satisfies the condition.

$3 \, Ca^{2+}$ and $2 \, SO_{4}^{2-}$: Total positive charge = $3 \times (+2) = +6$ Total negative charge = $2 \times (-2) = -4$ The charges do not balance. This is incorrect.

Therefore, the correct combination is (iii) $2 \, Fe^{3+}$ and $3 \, O^{2-}$.

False. Correction: Elements are made up of atoms. Compounds are formed when atoms of two or more different elements combine chemically.

False. Correction: The molecule of a compound is always made up of two or more atoms of different kinds, combined in a fixed ratio. A molecule made up of atoms of the same kind is a molecule of an element (e.g., $O_2$, $N_2$).

False. Correction: Nitrogen gas is a diatomic molecule. One molecule of nitrogen gas ($N_2$) contains two nitrogen atoms.

True. This statement is correct. A molecule of water ($H_2O$) consists of one oxygen atom that forms single covalent bonds with two separate hydrogen atoms.

$Ca^{2+}$ and $Br^{-}$ Two bromide ions are needed to balance the charge of one calcium ion. The formula is $CaBr_2$.

$Al^{3+}$ and $CO_{3}^{2-}$ Two aluminium ions (total charge +6) and three carbonate ions (total charge -6) are needed. The formula is $Al_2(CO_3)_3$.

$K^{+}$ and $SO_{4}^{2-}$ Two potassium ions are needed to balance the charge of one sulfate ion. The formula is $K_2SO_4$.

$NH_{4}^{+}$ and $Cl^{-}$ The +1 charge of the ammonium ion is balanced by the -1 charge of the chloride ion. The formula is $NH_4Cl$.

Reactivity Element B is more reactive. Both A and B are non-metals that tend to gain electrons to achieve a stable octet. Element B (with 7 valence electrons) needs to gain only one electron, which is energetically more favorable and easier than for element A (with 5 valence electrons) to gain three electrons. Therefore, element B has a stronger tendency to react.

Type of Bond Since both A and B are non-metals, they will form a covalent bond by sharing electrons. An ionic bond (transfer of electrons) is unlikely because a large amount of energy would be required for element A to lose 5 electrons or for element B to lose 7 electrons.

Formula of the Compound The valency of A (based on sharing) is 3, and the valency of B is 1. Using the criss-cross method for covalent compounds, the formula of the compound they would form is $AB_3$.