Metallic character is the tendency of an element to lose an electron easily. As we go down the group, the size of atoms increases.
Due to this, the nuclear force of attraction on the electrons in the outermost shell decreases, because of which they can be removed easily. Therefore, metallic character increases down the group.

Number of neutrons = Mass No. - Atomic No.
Since 238 is the mass number,
No. of neutrons = 238 - 92 = 146

Elements try to attain the nearest noble gas configuration by losing or gaining electrons.
Fluorine has 7 electrons in its outer shell. These 7 electrons are bound tightly by the nucleus. It has a tendency to gain an electron to achieve the nearest noble gas configuration.
However, Na, Mg and Al tend to lose electrons to acheive the nearest noble gas configuration.

The term inert means that a substance is chemically non-reactive. Noble gases are considered to be inert. Helium, neon, argon, krypton, xenon and radon are the noble gases that are present in the ${18}^{th}$ group of the periodic table.

According to Dobereiner's law of triads, the atomic mass of the middle element of a triad is equal to the arithmetic mean of the atomic masses of the other two elements.
In the triad of lithium, sodium and potassium. The atomic mass of lithium is 7 and the atomic mass of potassium is 39. The arithmetic mean of atomic masses of lithium and potassium gives atomic mass of sodium i.e. 23.