School of Elements – Maharashtra Board Class 10 Solutions for Science and Technology (English Medium)
- The formula for chloride of metal M is MCl2. The metal M belongs to IIA group.
- 18 group in the periodic table contains elements that are all gases at room temperature.
- The arrangement of elements in a group of three is known as triad.
- The law used by Newlands to arrange elements is known as Newlands’ Octaves.
- The element eka-aluminium in Mendeleev’s periodic table is known as Gallium in the modern periodic table.
- Elements showing properties of both metals and non-metals are known as metalloids.
- Atomic size is determined by atomic radius.
- Atomic radius is the distance between the centre of the atom and the outermost shell.
- As we move down the group, newer shells are added successively.
- So, the distance between the nucleus and the outermost shell increases, and the electrons in the outermost shell have lesser pull from the nucleus.
- Hence, the atomic size increases down the group.
- Metallic character is the tendency of an atom to lose electrons from its outermost shell to form a positively charged ion.
- In a period, from left to right, electrons are added to the same shell.
- These electrons have greater pull from the nucleus as the atomic size decreases.
- So, it becomes difficult to lose electrons. Hence, metallic character decreases from left to right in a period.
- The valency of an element is the number of valence electrons present in the outermost shell of an atom of that element.
- All the elements in a group have the same number of valence electrons. Therefore, they have the same valency.
- Example: The valency of the entire Group I elements is the same, i.e. 1. Similarly, the valency of the entire Group II elements is 2.
- Hydrogen resembles alkali metals and halogens. So, no fixed position was given to hydrogen, and it was placed in Group 1 and Group 17. However, hydrogen can donate the electron to form a stable cation (H+). Hence, it is better placed in Group 1 of the modern periodic table.
- Elements were arranged in their increasing order of atomic masses, but some elements with higher atomic masses are placed before those having lower atomic masses, e.g. Cobalt with atomic mass 58.93 is placed before Nickel whose atomic mass is 58.71. In the modern periodic table, elements are placed in the increasing order of their atomic numbers. The atomic number of Co is 27 and that of Ni is 28. So, nickel is placed after cobalt.
- Isotopes occupy the same place in the modern periodic table as they have the same atomic number.
- Elements are classified into four blocks according to electronic configuration: s-block, p-block, d-block and f-block.
Thus, the various anomalies seen in Mendeleev’s periodic table were overcome by the arrangement of elements based on their atomic numbers instead of their atomic masses.
In the modern periodic table, among the first twenty elements, the following are metals, non-metals and metalloids:
Metals: Lithium, Beryllium, Sodium, Magnesium, Aluminium, Potassium, Calcium
Non-metals: Carbon, Nitrogen, Oxygen, Fluorine, Phosphorus, Sulphur, Chlorine, Argon, Neon, Hydrogen, Helium
Metalloids: Boron, Silicon
- Hydrogen resembles alkali metals and halogens. So, it is placed in Group 1 and Group 17. Hence, no fixed position could be given to Hydrogen.
- Isotopes of the same element have different atomic masses, so they should be given different positions in the periodic table. On the other hand, as isotopes are chemically similar, they had to be given the same position in the periodic table.
- Some elements placed in the same sub-group had different properties; e.g. Manganese (Mn) is placed with halogens which totally differ in properties.
- An element of higher atomic mass has been placed before an element of lower atomic mass; e.g. Cobalt (Co = 58.93) is placed before Nickel (Ni = 58.71).
Atomic radius is the distance between the centre of the atom and the outermost shell of the electrons of that atom.
- In a period, the atomic radius decreases from left to right because the electrons are added to the same shell resulting in a greater nuclear pull on electrons. Thus, the outer shell contracts.
- Atomic radius increases in a group from top to bottom as new shells are added which increases the distance between the centre of the atom and the outermost shell.
Hydrogen, Lithium, Sodium
Helium, Neon, Argon
Fluorine, Chlorine, Bromine