Helium has the lowest boiling point among all noble gases because it is the smallest atom, resulting in the weakest inter atomic forces between its particles making it easier to overcome and transition to a gas state. Essentially, weaker intermolecular forces lower the boiling point.

Reaction of alkali metals with chlorine becomes more vigorous as we go down the group.

2Li(s)+Cl₂​(g) ----Heat ----> ​2LiCl(s)

2Na(s)+Cl₂​(g)----HeatVigorous reaction​-----> 2NaCl(s)

Most metals are solid at room temperature because of strong metallic bonds between their atoms, which create a tightly packed lattice structure. This structure requires a significant amount of energy to break.
While non-metals often have weaker intermolecular forces, leading to gaseous states and lower melting points.

Three elements in the periodic table which exist as liquids are:

• Mercury (Hg)
• Bromine (Br)  
• Gallium (Ga)

Transition elements are different from normal elements because they have incomplete d-orbitals in their electronic configuration. This allows them to exhibit multiple oxidation states, form coloured compounds, readily participate in complex ion formation, act as good catalysts Unlike normal elements which only have valence electrons in their outermost shell.

Chlorine is a stronger oxidizing agent than bromine. Chlorine is more reactive and readily accepts electrons, so it can oxidize more substances than bromine can.

Reason:

 Oxidizing power of halogens decreases down the group.

Cl₂ >Br₂ ​>I₂​

The reactivity of halogens decreases as we move down the group in the periodic table. Order from most to least reactive:

Fluorine > Chlorine > Bromine > Iodine > Astatine

This trend is due to decreasing electronegativity and increasing atomic size as we move down the group.

 A catalytic converter is a device used in the exhaust system of an automobile.
Role: It converts harmful gases produced in the engine into less harmful gases which do not pollute the atmosphere. Platinum (Pt), Palladium (Pd) and Rhodium (Rh) are used as catalysts in catalytic converters. Nickel (Ni) is used as a catalyst for the hydrogenation of vegetable oils to give solid margarine. Margarine is less likely to spoil than butter.

For Alkali Metals:  
The chemical reactivity of alkali metals increases down a group because as the atomic radius increases, the outermost electron is further from the nucleus. Nuclear attraction kam ho jati hai, is liye electron lose karna easy ho jata hai. So they react more readily.

For Halogens:  
The reactivity decreases down the group because of increasing atomic radius. Bara size hone ki wajah se nucleus ki additional electrons ko attract karne ki power kamzor ho jati hai. Larger the halogen atom, less readily it can accept an electron.

Metals are defined as the elements which can generally form cations easily. They also tend to form strong metallic bonds. Metals cannot be cut easily because they are hard substances. Due to the presence of strong metallic bonds, metals generally have high melting and boiling points. Their densities are also very high.
Example:

 Copper, Silver and Iron etc.

Due to non-ductile and non-malleable properties, non-metals are not strong at all. Their bonds are weak, so they break easily. All non-metals have low melting and boiling points. Non-metals have low densities as compared to metals. This means that in non-metals atoms are not strongly bonded with each other.
Example:

Oxygen, Nitrogen, Chlorine etc.

Alkali metals and halogens are both highly reactive elements, but they are considered "opposite" to each other because alkali metals readily lose electrons to achieve stability while Halogens readily gain electrons to achieve stability, making them naturally attracted to each other and they readily react to form ionic compounds when they come into contact. Essentially, one wants to give up an electron and the other desperately wants to take one.

Bond length between hydrogen and halogen increases away from the halogen nucleus as we go down the group. The bond between hydrogen and halogen therefore gets weaker. The weaker the bond, less heat energy it will need to break it. Hence, the thermal stability of hydrogen halides decreases down the group.

Order of Thermal Stability:

HF >  HCl  > Hr  >  HI

Platinum was originally used as a catalyst in the contact process for the manufacture of sulphuric acid. This expensive catalyst is, however, rendered inactive by impurities. Now Vanadium pentoxide (V₂O₅) is preferred as a catalyst

Equations of Contact Process:

2SO₂​(g) + O₂​(g) ---V₂O₅ -----> ​​2SO₃(g)

SO₃​(g) + H₂O(l)   ⟶   H₂SO₄(aq)

Sodium can be used as a heat transfer agent in atomic nuclear power plant beacuse sodium absorb excessive heat produced by reactor and transfer it into another place. 

Following properties of sodium are utilized in this role:

i). High thermal conductivity

ii). High boiling point

As lithium is the first element of the metal group so its behaves differently from the other alkali metals. Also lithium has high charge density, high ionization energy and a strong power to pull electrons from other atoms.

Reason:

A protective layer of aluminum oxide is formed on the surface of aluminum which prevents it from reacting with acid and water. On the other hand, magnesium is more reactive, that's why it reacts with food and damages the utensil.