Code

 Header...
#define NDIM 2
typedef struct particle {
  float x[NDIM];
  float mass;
  struct particle *next;
}Particle;

// Allocate a new particle and add it to the END of the list
 struct particle* newparticle(Particle *first){
   ... 

»  Particle *p, *last=first;

»  p=xmalloc(sizeof *p);
»  printf("Please enter the mass and x and y values.\n");
»  scanf("%f %f %f", &p->mass, &p->x[0], &p->x[1]);
»  p->next=NULL;

»  if (first==NULL) 
    return p;// New list

  // Find the last element
»  while (last->next!=NULL) 
    last=last->next;

»  last->next=p;
»  return first;
}

 int main(){
   ... 

»  int i, n;
»  Particle *first=NULL;

»  printf("How many structures would you like?\n");
»  scanf("%d", &n);

»  for (i=0; i<n; ++i) 
    first=newparticle(first);

»  for ( Particle *p=first;p!=NULL; p=p->next) 
    printf("Mass: %f position: (%g, %g)\n", p->mass, p->x[0], p->x[1]);

»  return 0;
}