Study the reaction. 2AgNO3 (aq)+Cu (s)→Cu(NO3)2 (aq)+2Ag (s)
If the oxidation number of copper changes from 0 to +2, is this a redox reaction?
-No; copper gained 2 electrons and was reduced.
-Yes; copper lost 2 electrons and was oxidized.
-Yes, the products have 2 fewer electrons than the reactants.
-No, the products have as many electrons as the reactants.

If the oxidation number of copper changes from 0 to +2 then yes this is a redox reaction as copper lost 2 electrons and was oxidized.

Explanation:

When an element or atoms loses an electron then it acquires a positive charge.

This charge on an atom represents its oxidation state.

For example,

Here, oxidation number of copper changes from 0 to +2.

A redox reaction in which electrons are removed from an atom or a reaction where an increase in oxidation number of an atom takes place is called oxidation-half reaction. The atom itself gets oxidized.

This means that here copper lost 2 electrons and was oxidized.

Thus, we can conclude that if the oxidation number of copper changes from 0 to +2 then yes this is a redox reaction as copper lost 2 electrons and was oxidized.

you need to take a look at the periodic system in order to sort these elements into pairs that would most likely exhibit similar chemical properties. first, you need to find elements that are found in the same group (groups are vertical in the periodic system), and then to see whether they have the same valence number. if they meet these criteria, they will have similar properties.

the first pair: ne (neon) and kr (krypton), are noble gases found in group 8

the second pair: li (lithium) and k (potassium), found in group 1

the third pair: s (sulfur) and te (tellurium), found in group 6