Science [2 Mark Questions]

1. No, because all these elements do not have similar properties although the atomic mass of silicon is average of atomic masses of sodium (Na) and chlorine (Cl).
2. Yes, because they have similar properties and the mass of magnesium (Mg) is roughly the average of the atomic mass of Be and Ca.

The vertical columns in a periodic table are called groups.

1. The greatest metallic character is found in the elements in the lowest part of the group.
2. The largest atomic size is found in the lowest part of the group.

1. 2.
2. Group 2.
3. XCl 2.

The main difference between Mendeleev's and modern periodic law is that:Mendeleev’s Periodic Law states that the physical and chemical properties of elements are periodic functions of their atomic weights. On the other hand, the Modern periodic Law states that the physical and chemical properties of elements are periodic functions of their atomic numbers.

Now valency of B is Si and it has a valency of 4. The element C is Chlorine and it has a valency of 1. So the formula of compound when B reacts with C is SiCl4.

1. Covalent bond is formed between two non-metals (A and B).
2. AB 2.

The elements are classified into groups so that the elements with similar properties fall in the same group and hence the study of a large number of elements is reduced to the study of a few group of elements.

Noble gases. Since they are chemically unreactive, so they got a place in the periodic table in the form of a separate group and hence did not disturb the original order of Mendeleev’s periodic table.

1. All the elements of a group have similar chemical properties because they have same no. of valence electrons in their outermost shell.
2. All the elements of a period have different chemical properties because they have different no. of valence electrons in their atoms.

Limitation of Dobereiner's classification:
All known elements could not be classified into groups of triads on the basis of their properties.

Position of isotopes:All the isotopes of an element have the same number of protons, so their atomic number is also the same. Since, all the isotopes of an element have the same atomic number; they can be put at one place in the same group of the periodic table.

1. Elements in the same group have similar properties because they have the same number of valence electrons in their outermost shell.
2. Elements in the same period have different properties because the number of valence electrons in their outermost shells differ.

A belongs to Group 13, B belongs to Group 14 and C belongs to Group 2. Valency of A is 3, that of B is 4 and that of C is 2.

1. XY.
2. Ionic bond.

1. The vertical columns in a periodic table are called groups.
2. The horizontal rows of elements in a periodic table are called periods.

In the given set of elements, i.e., calcium, magnesium, sodium and beryllium, sodium does not belong to the set. This is because sodium is a group 1 element and the remaining three elements​belong to group 2. Elements of the same group show similar chemical properties.

Valency of X = 2.

1. XCl 2.
2. XO.

Helium and neon atoms have completely filled outermost electron shells (containing the maximum number of electrons which can be accommodated in them).

In the given set of elements, i.e., oxygen, nitrogen, carbon, chlorine and fluorine, chlorine does not belong to the set. The element chlorine belongs to the third period while the remaining elements belong to the second period.

Mendeleev created his Periodic Table on the basis of their fundamental property the atomic mass and also on the similarities of chemical properties. Among chemical properties he concentrated on the compounds formed by elements with oxygen and hydrogen.

Out of eight groups in the original periodic table of Mendeleev, first seven groups are of normal elements and eighth group is of transition elements. Noble gases were not known at that time. So, there was no group of noble gases in Mendeleev’s table.

N (2, 5 ); P (2, 8, 5); Nitrogen will be more electronegative because its atom has small size due to which the attraction of its nucleus for the incoming electron is more.

1. Yes, The atoms of all the three elements lithium. sodium, and potassium have one electron in their outermost shells.
2. Both helium (He) and neon (Ne) have filled outermost shells. Helium has a duplet in its K shell, while neon has an octet in its L shell.

1. Na has the smaller size than K because number of shells increases down a group. Hence K is bigger and Na is smaller.
2. C has smaller size than B because effective nuclear charge increases from left to right in a period. Due to this effective nuclear charge, the electronegativity increases. Hence C has smaller size.

According to Mendeleev’s periodic law:The properties of elements are a periodic function of their atomic masses. It was the discovery of atomic number which led to a change in Mendeleev’s periodic law which was based on atomic mass.

1. The elements that have similar properties belong to the same group. From the list of elements available, elements which belong to same group are:

Na, K (Alkali metals); F, Cl (Halogens)

2. The sequence is according to Newland’s law of octaves.

Modern periodic table arranges the elements according to increasing atomic numbers. So, the atomic number of cobalt (27) comes first whereas the atomic number of nickel (28) comes later.

Anomalies of Mendeleev’s classification of elements:

1. The position of isotopes could not be explained: If the elements are arranged according to atomic masses, the isotopes should be placed in different groups of the periodic table. But, the isotopes were not given separate places in Mendeleev’s periodic table. They were placed at the same place in the table. This placing of the isotopes at same place could not be explained by Mendeleev’s periodic law.
2. Wrong order of atomic masses of some elements could not be explained: In Mendeleev’s periodic table, when certain elements were put in their correct group on the basis of their chemical properties, it was found that the element with higher atomic mass comes first and the element with lower atomic mass comes later. Mendeleev’s periodic law could not explain this abnormal situation of wrong order of atomic masses.

1. Increasing atomic masses.
2. Grouping together of elements having similar properties.

1. All the chemically similar elements will have same valence electrons.
2. The 1st element in a period is determined by the no. of valence electrons in its atoms. The 1st element of every period has 1 valence electron.

Eka - aluminium and gallium are the two names of the same element as Eka - Aluminum has almost exactly the same properties as the actual properties of the gallium element. The properties: atomic mass, density, melting point, formula of chloride and formula of oxide are almost the same.

The properties of elements are repeated after 2, 8, 18 and 32 elements in the periodic table because the electronic configurations of the elements are repeated in this manner.

Valency is the number of electrons an element has to lose or gain in order to become stable and achieve noble gas configuration.
Silicon, has atomic number 14, electronic configuration 2, 8, 4. It has 4 valence electrons. It can gain or lose 4 electrons to achieve noble gas configuration and stability. Thus the valency of silicon is 4.
Chlorine, has atomic number 17, electronic configuration 2, 8, 7. It has to gain one more electron, to complete its outermost shell and achieve noble gas configuration. Thus valency is 1.

The isotopes of chlorine, Cl - 35 and Cl - 37 are placed in the same slot because they have similar chemical properties and same atomic number.

The 1st period has two elements because the 1st electron shell of an atom c a n take a maximum of two electrons only. The 2nd period of the periodic table has 8 electrons because the maximum no. of electrons which can be put in the 2nd shell of an atom is 8.

1. Mg since atomic size decreases from left to right in a period.
2. K since atomic size increases on going down a group.