Python, error with web driver (Selenium)

    import time
from selenium import webdriver
from selenium.webdriver.common.action_chains import ActionChains
from selenium.webdriver.common.keys import Keys

driver = webdriver.Chrome()
driver.get('http://arithmetic.zetamac.com/game?key=a7220a92')
element = driver.find_element_by_link_text('problem')
print(element)

I am getting the error:

FileNotFoundError: [Errno 2] No such file or directory: 'chromedriver'

I am not sure whythis is happening, because I imported selenium already.