Task: Question Answering
Input (English): Context: In a chemical formula, the elements in a compound are represented by their chemical symbols, and the ratio of different elements is represented by subscripts. Consider the compound water as an example. Each water molecule contains two hydrogen atoms and one oxygen atom. Therefore, the chemical formula for water is: H2 O The subscript 2 after the H shows that there are two atoms of hydrogen in the molecule. The O for oxygen has no subscript. When there is just one atom of an element in a molecule, no subscript is used in the chemical formula. The Table 1.1 shows four examples of compounds and their chemical formulas. The first two compounds are ionic compounds, and the second two are covalent compounds. Each formula shows the ratio of ions or atoms that make up the compound. Name of Compound Type of Compound Sodium chloride ionic Calcium iodide ionic Hydrogen peroxide covalent Carbon dioxide covalent Ratio of Ions or Atoms of Each Element 1 sodium ion (Na+ ) 1 chloride ion (Cl ) 1 calcium ion (Ca2+ ) 2 io- dide ions (I ) 2 hydrogen atoms (H) 2 oxygen atoms (O) 1 carbon atom (C) 2 oxy- gen atoms (O) Chemical Formulas NaCl CaI2 H2 O2 CO2 There is a different rule for writing the chemical formula for each type of compound. Ionic compounds are compounds in which positive metal ions and negative nonmetal ions are joined by ionic bonds. In these compounds, the chemical symbol for the positive metal ion is written first, followed by the symbol for the negative nonmetal ion. Click image to the left or use the URL below. URL: Q: The ionic compound lithium fluoride consists of a ratio of one lithium ion (Li+ ) to one fluoride ion (F ). What is the chemical formula for this compound? A: The chemical formula is LiF. Covalent compounds are compounds in which nonmetals are joined by covalent bonds. In these compounds, the element that is farther to the left in the periodic table is written first, followed by the element that is farther to the right. If both elements are in the same group of the periodic table, the one with the higher period number is written first. Click image to the left or use the URL below. URL: Q: A molecule of the covalent compound nitrogen dioxide consists of one nitrogen atom (N) and two oxygen atoms (O). What is the chemical formula for this compound? A: The chemical formula is NO2 . 
 Question: correct rules for writing the chemical formulas of covalent compounds include which of the following?
Output (English): the element that is farther to the left in the periodic table is written first.