Questions: Classification of Elements and Periodicity in Properties
Classification of Elements and Periodicity in Properties25 min readLesson 5 of 6
Q1 State the modern periodic law and explain why it is preferred over Mendeleev's periodic law.
Answer (English): The modern law orders the elements by atomic number and says that their properties repeat periodically along that order. Mendeleev had ordered by atomic mass, which produced three problems. Isotopes of an element have different masses but identical chemistry, so a mass-ordered table cannot keep them together. Some pairs, such as argon and potassium and cobalt and nickel, had to be placed out of mass order to keep the chemistry regular. The modern law removes all of these problems because atomic number, the proton count, fixes the element's identity and its outer electronic configuration.
Hinglish: Modern periodic law kehta hai properties atomic number ke periodic functions hain. Mendeleev ne atomic mass use kiya tha, isliye isotopes aur Ar-K, Co-Ni jaise pairs problem dete the. Atomic number se identity aur electronic configuration fix hoti hai, isliye modern law behtar hai.
Q2 Define ionization enthalpy. Why does its definition specify an isolated gaseous atom in the ground state?
Answer (English): The first ionization enthalpy is the energy needed to remove the most loosely bound electron from one mole of isolated gaseous atoms, each in its ground state, to form gaseous ions. The gaseous state removes any interaction between neighbouring atoms, so the measured value reports only the atom's own electronic structure. The ground state removes the possibility that the atom is already excited, which would change the energy required. Without both conditions, values from different elements could not be compared fairly.
Hinglish: Ionization enthalpy ek mole isolated gaseous atoms se electron nikalne ki energy hai. Gaseous state se neighbouring atoms ka effect hatta hai, aur ground state se excited atom ka case nahi aata. Dono conditions ke bina elements ki values compare nahi ho sakti.
Q3 Explain why the first ionization enthalpy of magnesium is higher than that of aluminium.
Answer (English): Magnesium has the configuration [Ne] 3s2 and aluminium has [Ne] 3s2 3p1. Magnesium loses a 3s electron from a filled subshell, which is held relatively tightly. Aluminium loses the single 3p electron, which lies higher in energy and is shielded by the 3s pair, so it comes off more easily. The result is that magnesium's first ionization enthalpy is higher even though aluminium has the larger nuclear charge.
Hinglish: Mg = [Ne] 3s2, Al = [Ne] 3s2 3p1. Mg 3s electron nikalta hai jo filled subshell ka hai, jabki Al ka 3p electron high energy aur zyada shielded hai. Isliye IE(Mg) > IE(Al) hoti hai, chahe Al ka nuclear charge zyada ho.
Q4 Why is the electron gain enthalpy of chlorine more negative than that of fluorine?
Answer (English): Fluorine is the smaller atom, and its incoming electron must enter a compact 2p subshell that already has seven electrons. The electron-electron repulsion in that small volume is strong, so less energy is released. Chlorine's 3p subshell is larger and the added electron is spread over a bigger region, so the repulsion is weaker and the energy released is greater. Fluorine is still the more reactive halogen because of its lower bond dissociation energy in F2 and its small size, but the enthalpy of adding one electron is more negative for chlorine.
Hinglish: F chhota atom hai, uske compact 2p shell mein aathvaan electron daalne par strong repulsion hoti hai, isliye kam energy release hoti hai. Cl ka 3p shell bada hai, repulsion kam hai, isliye EGA(Cl) > EGA(F) (more negative). Fluorine ki reactivity phir bhi zyada hai kyunki F2 ka bond weak hota hai.
Q5 Explain why atomic radius decreases across a period and increases down a group.
Answer (English): Across a period, electrons enter the same principal shell while the nuclear charge grows by one unit at each step. The inner shielding does not grow enough to cancel the extra proton, so the effective nuclear charge rises and the outer electrons are pulled closer, reducing the radius. Down a group, a new shell is added at each step, which places the outer electrons farther from the nucleus, and the extra inner electrons also shield them. The increase of size with the new shell outweighs the increase in nuclear charge, so the radius grows.
Hinglish: Period mein n same rehta hai par Z badhta hai, isliye Z_eff badhta hai aur radius ghatta hai. Group mein naya shell judta hai, outer electrons door ho jaate hain aur inner electrons unhe shield karte hain, isliye radius badhta hai.
Q6 Write the electronic configurations of chromium (Z = 24) and bromine (Z = 35), and state the period, group and block of each.
Answer (English): Chromium is [Ar] 3d5 4s1, with the half-filled 3d subshell that gives extra stability. Its highest occupied shell is n = 4, so it lies in period 4. The last electron enters a d orbital, so it belongs to the d-block, in group 6. Bromine is [Ar] 3d10 4s2 4p5. Its highest shell is n = 4 and the last electron enters a p orbital, so it lies in period 4, group 17, and the p-block. The group number follows from the total valence electrons: 2 plus 5 equals 7, and adding 10 gives group 17 for the p-block.
Hinglish: Cr = [Ar] 3d5 4s1, period 4, group 6, d-block. Br = [Ar] 3d10 4s2 4p5, period 4, group 17, p-block. Group nikaalne ke liye valence electrons (2+5=7) lo aur p-block ke liye 10 add karo.
Q7 Arrange O2-, F-, Na+ and Mg2+ in increasing order of ionic radius and justify the order.
Answer (English): All four species have ten electrons, so they are isoelectronic and the radius depends only on the nuclear charge. The nuclear charges are 8 for O2-, 9 for F-, 11 for Na+ and 12 for Mg2+. A larger nuclear charge attracts the same ten electrons more strongly, so the radius is smallest for Mg2+ and largest for O2-. The order of increasing radius is Mg2+ below Na+ below F- below O2-.
Hinglish: Charo species mein 10 electrons hain, isliye isoelectronic hain. Z: O2- 8, F- 9, Na+ 11, Mg2+ 12. Zyada Z wala same electrons ko zyada kheenchta hai, isliye radius order Mg2+ < Na+ < F- < O2- hai.
Q8 What is electronegativity, and how does it differ from electron gain enthalpy?
Answer (English): Electronegativity is the tendency of an atom in a molecule to attract the shared electron pair of a bond towards itself. It is a relative number on a scale, unitless, and defined only inside a bond. Electron gain enthalpy is the enthalpy change when a gaseous atom accepts an electron, and it has units of energy per mole. Chlorine has a large negative electron gain enthalpy and also high electronegativity, but fluorine has the highest electronegativity while chlorine has the more negative electron gain enthalpy, which shows that the two quantities are not the same.
Hinglish: Electronegativity bond mein shared pair attract karne ki relative tendency hai, unitless scale (F = 4.0). Electron gain enthalpy gaseous atom ke electron lene par enthalpy change hai, unit kJ/mol. Dono alag quantities hain; F ki EN sabse zyada par Cl ki EGA zyada negative hai.
Q9 Why does nitrogen have a higher first ionization enthalpy than oxygen?
Answer (English): Nitrogen has the configuration [He] 2s2 2p3 with a half-filled 2p subshell. The three 2p electrons occupy separate orbitals with parallel spins, and the exchange energy of this arrangement makes it relatively stable. Oxygen is [He] 2s2 2p4, so one 2p orbital is doubly occupied. Removing one of that pair relieves the repulsion and leaves a half-filled 2p subshell, so it costs less energy than removing an electron from nitrogen's stable arrangement. Hence the ionization enthalpy falls from nitrogen to oxygen.
Hinglish: N ka 2p subshell half-filled (2p3) hai aur exchange energy se stable hai. O mein ek 2p orbital doubly occupied hai, aur us paired electron ko nikaalne se repulsion kam hoti hai, isliye kam energy lagti hai. Isliye IE(N) > IE(O) hoti hai.
Q10 The first ionization enthalpy of sodium is 496 kJ mol^-1. Explain why its second ionization enthalpy is much higher.
Answer (English): Removing the first electron from sodium leaves Na+ with the configuration of neon, [Ne], a closed shell. The second electron would have to come from that closed shell, which is much more tightly bound than the single 3s electron of the neutral atom. The nuclear charge also acts on fewer electrons, so the second electron is held strongly. As a result, the second ionization enthalpy is far above the first, and the big jump is the signature of a valence shell that has been emptied.
Hinglish: Na se pehla electron nikalne par Na+ = [Ne] ban jaata hai, yaani closed shell. Doosra electron usi closed shell se nikaalna padta hai, isliye bahut zyada energy lagti hai. Yahi big jump valence shell khatam hone ka signal hai.
That closes the chapter questions. Try the quick quiz below before moving on to the previous year questions.