Here are the solutions to the questions: 25. The energy change accompanying the addition of an electron to a gaseous atom is called This process is defined as the electron affinity*. It is the energy change when an electron is added to a neutral atom in the gaseous state to form a negative ion. a) first ionization energy: Energy to remove* the first electron. b) second ionization energy: Energy to remove* the second electron. c) electron affinity: Energy change when an electron is added*. d) electronegativity: A measure of the tendency of an atom to attract* a bonding pair of electrons. The correct option is (c)*. 26. Four elements P, Q, R and S have atomic numbers of 4, 10, 12 and 14 respectively. Which of these elements is a noble gas? Let's identify the elements based on their atomic numbers: P: Atomic number 4 is Beryllium (Be). Q: Atomic number 10 is Neon (Ne). R: Atomic number 12 is Magnesium (Mg). S: Atomic number 14 is Silicon (Si). Noble gases are elements in Group 18 of the periodic table, characterized by a full outermost electron shell. Neon (Ne) is a noble gas. The correct option is (b)*. 27. $X_{(g)} \rightarrow X_{(g)}^{+}$ The type of energy involved in the above transformation is This equation shows a gaseous atom ($X_{(g)}$) losing an electron to form a gaseous positive ion ($X_{(g)}^{+}$). This process is known as ionization. a) ionization energy*: The energy required to remove an electron from a gaseous atom or ion. b) sublimation energy*: Energy required to change a substance from solid to gas. c) lattice energy*: Energy released when gaseous ions combine to form an ionic solid. d) electron affinity: Energy change when an electron is added* to a gaseous atom. The correct option is (a)*. 28. Which of the following is arranged in order of increasing electronegativity? Electronegativity generally increases* from left to right across a period in the periodic table. The elements listed (Sodium, Magnesium, Aluminum, Phosphorus, Chlorine) are all in Period 3. Their order in Period 3 is: Na (Group 1) < Mg (Group 2) < Al (Group 13) < P (Group 15) < Cl (Group 17). Therefore, the order of increasing electronegativity for these elements is Na < Mg < Al < P < Cl. Let's check the options: (a) chlorine, aluminum, magnesium, phosphorus, sodium: This is not in increasing order. (b) sodium, aluminum, phosphorus, chlorine: This sequence (Na < Al < P < Cl) correctly shows increasing electronegativity for the listed elements. (c) chlorine, phosphorus, magnesium, aluminum, sodium: This is not in increasing order. (d) sodium, chlorine, phosphorus, magnesium, aluminum: This is not in increasing order. The correct option is (b)*. 29. Four elements P, Q, R, and S, have 1, 2, 3, and 7 electrons in their outermost shells respectively. The element which is unlikely to be a metal is Metals typically have 1, 2, or 3 valence electrons and tend to lose these electrons to form positive ions. Non-metals typically have 4, 5, 6, 7, or 8 valence electrons (excluding hydrogen and helium) and tend to gain or share electrons. Based on the number of outermost electrons: P: 1 electron (likely a metal, e.g., alkali metal). Q: 2 electrons (likely a metal, e.g., alkaline earth metal). R: 3 electrons (likely a metal, e.g., aluminum). S: 7 electrons (likely a non-metal, e.g., halogen). Therefore, element S, with 7 valence electrons, is most unlikely to be a metal. The correct option is (d)*.