int leds[] = {2, 3, 4, 5, 6,7,8,9};

void setup() {
  for(int i = 0; i <8; i++) {
    pinMode(leds[i], OUTPUT);
  }
}

void loop() {
  for(int i = 0; i < 8; i++) {
    digitalWrite(leds[i], HIGH);
    delay(500);
    digitalWrite(leds[i], LOW);
  }
}