Form a chemical bond

# Table: Elements
# 
# +-------------+---------+
# | Column Name | Type    |
# +-------------+---------+
# | symbol      | varchar |
# | type        | enum    |
# | electrons   | int     |
# +-------------+---------+
# symbol is the primary key (column with unique values) for this table.
# Each row of this table contains information of one element.
# type is an ENUM (category) of type ('Metal', 'Nonmetal', 'Noble')
# - If type is Noble, electrons is 0.
# - If type is Metal, electrons is the number of electrons that one atom of this element can give.
# - If type is Nonmetal, electrons is the number of electrons that one atom of this element needs.
# 
#  
# 
# Two elements can form a bond if one of them is 'Metal' and the other is 'Nonmetal'.
# 
# Write a solution to find all the pairs of elements that can form a bond.
# 
# Return the result table in any order.
# 
# The result format is in the following example.
#
# Example 1:
# Input:
# Elements table:
# +--------+----------+-----------+
# | symbol | type     | electrons |
# +--------+----------+-----------+
# | He     | Noble    | 0         |
# | Na     | Metal    | 1         |
# | Ca     | Metal    | 2         |
# | La     | Metal    | 3         |
# | Cl     | Nonmetal | 1         |
# | O      | Nonmetal | 2         |
# | N      | Nonmetal | 3         |
# +--------+----------+-----------+
# Output:
# +-------+----------+
# | metal | nonmetal |
# +-------+----------+
# | La    | Cl       |
# | Ca    | Cl       |
# | Na    | Cl       |
# | La    | O        |
# | Ca    | O        |
# | Na    | O        |
# | La    | N        |
# | Ca    | N        |
# | Na    | N        |
# +-------+----------+
# Explanation:
# Metal elements are La, Ca, and Na.
# Nonmeal elements are Cl, O, and N.
# Each Metal element pairs with a Nonmetal element in the output table.

import pandas as pd

def form_bond(elements: pd.DataFrame) -> pd.DataFrame:
  metals = elements[elements['type'] == 'Metal']['symbol']
  nonmetals = elements[elements['type'] == 'Nonmetal']['symbol']
  result = metals.to_frame().merge(nonmetals.to_frame(), how='cross')
  result.columns = ['metal', 'nonmetal']
  return result