Science Answer the questions.[Che-1M]

Six elements.

Noble gases were missing from Mendeleev’s original periodic table.

Mendeleev’s basis for the classification of elements was the atomic mass of elements.

The element fluorine is in group 17 of the periodic table and has a valency of 1. So, all the elements in the same group of periodic table as fluorine will have a valency of 1.

All elements in the same column of the periodic table as boron have 3 valence electrons.

8.

Usual number of valence elec trons is 8; Valency is 0 (zero).

1.

Metalloids.

The first elements in the periods of the periodic table have 1 valence electron. Such elements are called alkali metals.

The number of valence electrons remains the same.

On moving from left to right in a period, the metallic character of elements decreases.

On going down in a group of the periodic table, the metallic character of elements increases.

Scandium, Gallium, Germanium.

Oxygen (X is monovalent so Y has to be divalent to form the compound X Y).

Order of atomic numbers of elements:
X < Z < Y. Because as the atomic number increases in a period from left to right, the size of atoms goes on decreasing.

Fluorine and chlorine have been placed in the same group because both of them have 7 valence electrons.

The modern periodic law states that the properties of elements are a periodic fu nction of their atomic numbers.

Periods have elements with consecutive atomic numbers.

Right side.

Left side.

Mendeleev arrange the element in his periodic table on the basis of atomic masses.

Now they are arranged on the basis of atomic numbers.

Gallium and Scandium.

Eka-Aluminium (gallium) and Eka - Silicon (germanium).

Magnesium, Calcium.

Helium, Neon, Argon.

Lithium, Sodium, Potassium.

The modern periodic table was prepared by Bohr.

Mendeleev said that the properties of elements are a periodic function of their atomic masses.

Atomic number is always a simple whole number. It can either be 1 or 2. There can be no element with atomic number 1.5.

Lithium, Beryllium are metals.

Mendeleev was guided by two factors:

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

On moving from left to right in a period, the tendency of atoms to lose electrons decreases.

On moving from left to right in a period, the tendency of atoms to gain electrons increases.

There are seven periods and eighteen groups in the long form of periodic table.

On moving from left to right in a period, the atomic size decreases.

All the elements in a group have the same valency while going down the group.

The valency of elements increases from 1 to 4 and then decreases to zero in the 3rd period.

On going down in a group of the periodic table, the electropositive character of elements increases.

When elements are arranged according to increasing atomic numbers, there is a periodicity in the electronic configurations of the elements. The elements in a period have consecutive atomic numbers. The elements having same number of valence electrons in their atoms are placed in a group. All the elements in a group have similar electronic configurations and show similar properties.

In group 17 of halogen elements, the chemical reactivity decreases from fluorine to iodine.

In group 1 of alkali metals, the chemical reactivity increases from lithium to francium.

2, 1; 2, 2; 2, 3; 2, 4; 2, 5; 2, 6; 2, 7; 2, 8.​​​​