When the absolute value of the difference in electronegativity of two adjacent atom in a compound is 1, they are most likely bonded by .

polar covalent bond

Explanation:

Compounds are formed ether by donating, receiving or sharing of electrons making the bonds between them to  be ionic, or covalent .

Now  according to the rule  of type of bond assignment due to electronegativity difference of atoms

---When the  electronegativity difference is less than 0.5,  the bond formed is nonpolar covalent.

---When   the  electronegativity difference is between 0.5 and 1.6, the bond is considered polar covalent

---When  the  electronegativity difference greater than 2.0, then the bond is ionic.

Therefore, When the absolute value of the difference in electronegativity of two adjacent atom in a compound is 1, they are most likely bonded by  polar covalent bond eg HCl bond , CCL4 bond

out of these options:

ability to burn paper

pure sodium reacting violently with water

the boiling point of water

helium does not tend to react with anything

i would say this is a physical property:

helium does not tend to react with anything

why? :

i said helium does not tend to react with anything because well, it reacts to ! but ability to burn paper, pure sodium reacting violently with water, & the boiling point of water are chemical or

physical properties: the properties of a substance that can be observed without changing the chemical makeup of the substance

chemical properties: properties of a substance relating to the chemical nature and reactivity of the substance

c-14 is carbon-14, or radiocarbon.   it is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons.