How to get reference table data in Laravel 5.1

I created a Model Account with accountgroup_id which refer from Account_group Model. Then I call it from route like this

 Route::get('test', function () {
    return \App\Account::get()->account_group;
  });

Account Model has belogsto Relationship with Account_group

 class Account extends Model
 {
     protected $fillable = ['accountgroup_id', 'accountno', 'accountname','address','contactno'];

   public function account_group()
   {
         return $this->belongsTo('App\Account_group');
   }
 }

Account_group Model has hasMany relationship with Account

class Account_group extends Model
{
   protected $fillable =['name','under'];
   public function account()
   {
    return $this->hasMany('App\Account','accountgroup_id');
   }
 }

But after calling the route; I got following error.

Undefined property: Illuminate\Database\Eloquent\Collection::$account_group

via Chebli Mohamed