Similar Properties of Elements

a) 13 and 31
- These elements are aluminum (Al) and gallium (Ga).
- Both elements belong to Group 13 of the periodic table, also known as the boron group.
- They have similar chemical properties due to the presence of three valence electrons.
- Aluminum and gallium both exhibit metallic properties such as high electrical conductivity and ductility.
- They also have similar reactivity, forming ionic compounds with halogens and oxygen.
- However, gallium is more reactive than aluminum and has a lower melting point.

b) 11 and 20
- These elements are sodium (Na) and calcium (Ca).
- Sodium belongs to Group 1 (alkali metals) while calcium belongs to Group 2 (alkaline earth metals) of the periodic table.
- Both elements are metals and have similar metallic properties such as high electrical conductivity and shiny appearance.
- Sodium and calcium also have similar reactivity, readily forming positive ions by losing electrons.
- They react vigorously with water and oxygen, forming hydroxides and oxides respectively.
- However, calcium is less reactive than sodium due to its higher ionization energy.

c) 12 and 10
- These elements are magnesium (Mg) and neon (Ne).
- Magnesium belongs to Group 2 (alkaline earth metals) while neon belongs to Group 18 (noble gases).
- Magnesium is a metal, whereas neon is a non-metal.
- They have different properties and reactivity.
- Magnesium is an active metal, readily reacting with acids and other non-metals to form compounds.
- Neon, on the other hand, is inert and does not readily react with other elements.
- Magnesium has a lower ionization energy and is more likely to form positive ions, while neon has a high ionization energy and tends to remain as a stable, non-reactive atom.

d) 21 and 33
- These elements are scandium (Sc) and arsenic (As).
- Scandium belongs to Group 3 (transition metals) while arsenic belongs to Group 15 (pnictogens).
- Scandium is a metal, whereas arsenic is a metalloid.
- They have different properties and reactivity.
- Scandium is a relatively reactive metal and forms stable compounds with oxygen and halogens.
- Arsenic is a semi-metal with both metallic and non-metallic properties.
- Arsenic compounds are toxic and have been used historically as poisons.
- Overall, scandium and arsenic have distinct properties and differ in terms of their reactivity and chemical behavior.

A)13 and 31 is the answer since they have same valence electron ( i.e ) 3