int led_pin[] =  {2,3,4,5,6};
int num_led = sizeof(led_pin)/sizeof(int);

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  for ( int i=0; i < num_led; i++ ) {
    // set the direction of the i-th LED pin
    pinMode( led_pin[i], OUTPUT ); 
    // turn on the first LED (i=0) and the rest off. 
    digitalWrite( led_pin[i], (i==0) ? HIGH : LOW );
  }
}

void loop() {
  // put your main code here, to run repeatedly:
  for (int i=0;i<num_led;i++){
    digitalWrite(led_pin[i], HIGH);
    delay(500);
    digitalWrite(led_pin[i], LOW);
  }
}