Ground the object to only jump once in 2D platformer game.

Hi.. I am new to Unity and C# and i decided i want to make a simple platform game in 2D, where the player only moves in one direction, but i cannot seem to figure it out, how to ground the player so that when i press Space multiple times it won't jump midair. Please help me.. :)

using UnityEngine; using System.Collections;

public class PlayerController : MonoBehaviour { public float maxSpeed = 10f;

 bool facingRight = true;

 bool grounded = false;

 public Transform floorCheck;
 float floorRadious = 0.2f;
 public LayerMask Floor;
 public float jumpForce;
 private Rigidbody2D rb2d;



  void Awake()
 {
     rb2d = GetComponent<Rigidbody2D> ();

 } 


 void FixedUpdate()
 {
     grounded = Physics2D.OverlapCircle (floorCheck.position, floorRadious, Floor);

     float move = 1;


     rb2d.velocity = new Vector2 (move * maxSpeed, rb2d.velocity.y);

     if (move > 0 && !facingRight)
         Flip ();
     else if (move < 0 && facingRight)
         Flip ();
 }

 void Update ()
 {
     if (Input.GetKeyDown (KeyCode.Space) && grounded) 
     {
         rb2d.AddForce (new Vector2 (0, jumpForce));
     } 
 
 }

 void Flip()
 {
     facingRight = !facingRight;
     Vector3 theScale = transform.localScale;
     theScale.x *= -1;
     transform.localScale = theScale;
 }

}