CHEMICAL REACTIONS

A chemical change is a process in which at least some of the matter involved changes its chemical identity. Bonds are broken and/or formed. A physical change is one in which chemical identities remain the same. That's it.

It's really just a matter of semantics whether or not nuclear reactions count as "chemical changes." It's not wrong to say they do.
Simply put, a chemical reaction occurs when two or more reactants react to form a product. 

For e.g,
Two atoms of hydrogen and one atom of oxygen combine to form water.
A chemical reaction is usually accompanied by easily observed physical effects, such as the emission of heat and light, the formation of a precipitate, the evolution of gas, or a color change.  Absolute confirmation of a chemical change can only be validated by chemical analysis of the products! There are many different types of chemical reactions.  Chemists have classified the many different reactions into general categories.  The chemical reactions we will explore are a representation of the types of reactions found in each group.
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei (no change to the elements present), and can often be described by a chemical equation. Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes may occur.Chemical reactions happen at a characteristic reaction rate  at a given temperature and chemical concentration. Typically, reaction rates increase with increasing temperature because there is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms.

How do Chemical Reaction occur ?

Atoms have electrons and protons.  Atoms prefer to have a full octet, meaning 8 electrons around its shell.  Not all atoms have 8 electrons, so they bond with other atoms that don't have 8 electrons.  

When you mix a solution of atoms/molecules that are unstable (have less than an octet) with other atoms/molecules that are unstable but complementary, you have a chemical reaction.

A chemical reaction is the formation and breaking of bonds.
Nuclear forces are involved in nuclear reactions, strong electrical forces are involved in chemical reactions, and weak electrical forces are involved in mechanical forces. 

In chemistry a system of strong mutual attractive and repulsive electrical forces between positive nuclei and negative electrons rearranges into an overall more stable configuration. 

Several posts point to thermodynamics and although it applies of course, and is very useful, it should be understood that there is  nothing specific to chemistry about thermodynamics.

But certainly chemical thermodynamics predicts the extent of a reaction and chemical kinetics predicts  the rate of a reaction.

Compounds are formed from elements. Elements in their "elemental" state contain some amount of energy. When energy is high in atoms it causes them to be "agitated". It is a natural tendency for every system to lose energy and stabilise itself. When elements combine to form compounds they lose some amount of energy. Thus the formed compound has lesser energy than the total energy of its elements. Simple example is of carbon burning in oxygen to form carbon dioxide and heat. Since heat (energy) is given out during burning carbon dioxide will have lesser energy than carbon and oxygen combined. The same is the case with compounds. If two compounds can react to form another compound which has lesser energy then the two compounds will react. Like an acid and a base reacting to form salt and water. 
At times the compounds or elements need a kick for them to start reacting. Like a piece of wood wouldn't automatically start burning. This kick varies from reaction to reaction. Its called activation energy.
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Source - Quara