The Periodic Table — ICSE Class 9 Chemistry Important Questions

Answer: (b) Atomic number.

Moseley's work established that atomic number $(Z)$(Z), not atomic mass, is the fundamental property; the modern periodic law states properties are a periodic function of $Z$Z.

Answer: (c) The arithmetic mean of Li and K.

In a Dobereiner triad the middle element's atomic mass is approximately the average of the other two: $\dfrac{7+39}{2}=23$7+39/2=23, the mass of $Na$Na.

Answer: (b) Ar and K.

Argon (mass $39.9$39.9) is placed before potassium (mass $39.1$39.1) so that Ar sits with the noble gases and K with the alkali metals; this anomaly is resolved only by ordering by atomic number.

Answer: (b) Nuclear charge increases while electrons enter the same shell.

With the same outermost shell, the rising positive nuclear charge pulls the electron cloud inward, so the atomic radius decreases across the period despite more electrons.

Answer: (a) Chemical properties depend on valence electrons; members of a group all have the same number of valence electrons, so R correctly explains A.

Newlands' law of octaves: When elements are arranged in order of increasing atomic mass, every eighth element has properties similar to the first, like the eighth note of a musical octave.

Limitation: It held good only up to calcium; heavier elements did not fit, and it left no room for elements discovered later.

Group 17 elements have $7$7 valence electrons, so element X has valency $1$1 (it gains one electron to complete its octet).

Group 17 (the halogens) are non-metals; the period 3 member is chlorine, a reactive non-metal.

Modern periodic law: The properties of elements are a periodic function of their atomic numbers.

Anomaly 1 - reversed pairs: In Mendeleev's table Ar $(39.9)$(39.9) came before K $(39.1)$(39.1) against increasing mass. By atomic number, $Ar (Z=18)$Ar (Z=18) naturally precedes $K (Z=19)$K (Z=19), so no reversal is needed.

Anomaly 2 - isotopes: Isotopes have different atomic masses but the same atomic number, so they occupy a single place in the modern table, which classifies by $Z$Z.

(i) Atomic size: The distance from the centre of the nucleus to the outermost shell of electrons of an isolated atom.

(ii) Ionisation energy: The minimum energy required to remove the most loosely bound electron from an isolated gaseous atom to form a positive ion.

Down a group: Atomic size increases because a new electron shell is added at each step. Ionisation energy decreases because the outer electron is farther from the nucleus and better screened, so it is held less tightly and removed more easily.

P $(Z=11)$(Z=11): $2,8,1$2,8,1 - one valence electron, so group 1, valency $+1$+1.

Q $(Z=17)$(Z=17): $2,8,7$2,8,7 - seven valence electrons, so group 17, valency $-1$-1.

P loses one electron and Q gains one electron; they combine in a $1:1$1:1 ratio.

Formula of compound: $PQ$PQ (an ionic compound, e.g. $NaCl$NaCl).

Mendeleev's classification: Mendeleev arranged the then-known $63$63 elements in a table in order of increasing atomic mass, grouping together elements with similar properties. His periodic law stated: the properties of elements are a periodic function of their atomic masses. The table had horizontal rows (periods) and vertical columns (groups).

Merits:

1. He left gaps for undiscovered elements and predicted their properties (eka-aluminium = gallium, eka-silicon = germanium), which were later found to match closely.

2. He corrected doubtful atomic masses of some elements (e.g. beryllium, indium) to fit them into proper groups.

Defects:

1. Position of isotopes: Isotopes of the same element have different masses but no separate places were provided.

2. Anomalous pairs: Certain pairs such as Ar and K were placed against the order of increasing atomic mass to keep similar elements together.

Across period 2 the outermost shell stays the same (the L shell) while nuclear charge increases from $+3$+3 (Li) to $+9$+9 (F).

(i) Atomic size decreases (Li largest, F smallest). Reason: increasing nuclear charge in the same shell pulls electrons inward.

(ii) Metallic character decreases (Li, Be are metallic; C, N, O, F are non-metallic). Reason: as atoms get smaller and nuclear pull rises, the tendency to lose electrons falls and the tendency to gain electrons rises.

(iii) Electronegativity increases (Li lowest, F highest). Reason: smaller atoms with greater nuclear charge attract the shared bonding electrons more strongly.

(iv) Valency rises then falls: $1$1 (Li), $2$2 (Be), $3$3 (B), $4$4 (C), then $3$3 (N), $2$2 (O), $1$1 (F). Reason: valency w.r.t. hydrogen first increases to $4$4 then decreases as the octet nears completion.

Electronic configurations: A$=2,1$=2,1; B$=2,7$=2,7; C$=2,8,1$=2,8,1; D$=2,8,8$=2,8,8; E$=2,8,8,1$=2,8,8,1.

(i) A $(Z=3)$(Z=3), C $(Z=11)$(Z=11) and E $(Z=19)$(Z=19) all have one valence electron, so they lie in the same group (group 1). (A and C, or A, C and E, are acceptable.)

(ii) D $(Z=18)$(Z=18) has a complete octet $2,8,8$2,8,8, so it is the noble gas (argon).

(iii) Down group 1 size increases with each new shell: A $<$< C $<$< E.

(iv) B $(Z=9)$(Z=9), fluorine, has the highest electronegativity - it is the smallest atom with a strong tendency to attract electrons.