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) have a complete (stable) outermost shell. Helium has a duplet and the others an octet, so they neither lose nor gain electrons easily; they are called noble or inert gases.

Answer: (b) groups 3 to 12. They sit in the middle block of periods 4 to 7, between the alkaline earth metals (group 2) and group 13.

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.

Inner transition elements are the two series of fourteen elements that follow lanthanum (the lanthanides) and actinium (the actinides). They belong to periods 6 and 7 respectively, in group 3.

They are placed in two separate rows at the bottom of the table, so that the table does not become too wide and the elements with similar properties stay together.

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.

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).

(i) When three elements with similar properties are arranged in order of increasing atomic mass, the atomic mass of the middle element is roughly the average of the other two.

$\dfrac{35.5+127}{2}=\dfrac{162.5}{2}=81.25\approx80$(35.5+127)/2=162.5/2=81.25≈80, the atomic mass of bromine, so Cl, Br and I form a triad.

(ii) Merit: it was the first attempt to link the properties of elements with their atomic masses. Limitation: only a few triads could be found; most known elements could not be grouped this way.

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.

(a) Alkali metals: soft, highly reactive metals; they react with cold water to give an alkali and hydrogen; they form ions with charge $+1$+1. Halogens: reactive non-metals; they exist as diatomic molecules ($Cl_2$Cl₂); they form ions with charge $-1$-1 and salts with metals.

(b) An alkali metal has one electron more than a stable shell and loses it easily; a halogen is one electron short and gains one easily. A noble gas already has a complete outer shell, so it has no tendency to lose or gain electrons.

(c) Two valence electrons and three shells: group 2, period 3, an alkaline earth metal (magnesium), valency $2$2.

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) B ($2,7$2,7): seven valence electrons, so it is in group 17, the halogens (it is fluorine).

(iv) E ($2,8,8,1$2,8,8,1): one valence electron and four shells, so it is the group 1 alkali metal of period 4 (potassium).