How to add a force to a Rigidbody2D?

Hello,

I'm working on my first project and I would like to get some help with using the Rigidbody2D.addForce() command.

This is what i have so far and I would like it to add a force to the bullets actually fly out of the gun instead of dropping down instantly. I am sure I am using the addForce method wrong (last line) so could anyone set me straight?

 using UnityEngine;
 using System.Collections;
 
 public class fireBullets : MonoBehaviour
 {
     public GameObject projectileType; //the prefab that should be spawned.
     
     void Update ( )
     {
         if (Input.GetMouseButtonDown(0)) //when left mousebutton is pressed.
         {
             Fire(); //fire projectile.
         }
     }
     
     void Fire( )
     {
         Transform spawn = transform.Find( "Spawn" ); //find the posotion where the projectile should spawn.
         GameObject projectile = Instantiate( projectileType, spawn.position, spawn.rotation ) as GameObject; //create projectile.
         Vector2 force = new Vector2 (10, 0); //create a force.
         projectile.Rigidbody2D.addForce(force, ForceMode2D.Impulse); //add it to the newly created projectile.
     }    
 }