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# Chemical Formula

This lesson aligns with Next Generation Science Standards (NGSS) PS1.B

Introduction
Chemical reactions occur around us all the time. During a chemical reaction, substances are broken, and new substances are formed. Atoms grouped in a molecule rearrange in the reactants and form different molecules as new products. The properties of the products and the reactants are entirely different. A chemical equation is a model that represents a chemical reaction in which reactants and products are expressed in terms of their respective chemical formulas. A Chemical formula identifies each constituent element in a compound and its relative proportion in it.  This lesson will discuss chemical formulas, types of chemical formulas, and how they can describe a chemical reaction.

Chemical Formula
Everything in this universe is composed of different chemical substances. Atoms are arranged in different ways to form these chemical substances. Each element has one, two, or more chemical symbols associated with it. A chemical formula is an expression that describes a single unit of substance by using a combination of chemical symbols and numbers. This single unit represents a molecule for many substances, while for other substances, this single unit is part of a repeating, larger pattern of atoms. It tells the exact atoms in a molecule or a unit.

A Chemical formula provides information about the chemical composition of a compound and represents the ratios of the combined constituent elements to form that compound. The chemical formula for water is H2O which includes 2 hydrogen atoms and one oxygen atom in a 2:1 ratio. A chemical formula also uses other symbols such as parentheses, dashes, brackets, plus (+) and minus (-) signs, and commas.

Subscript
A subscript that is written below and right to the symbol shows the number of atoms of that element that are taking part. If no subscript is mentioned, then only one atom of that element is present. The chemical formula for sulfuric acid is $H_2SO_4$. A molecule of sulfuric acid consists of 2 hydrogen atoms, one sulfur atom, and 4 oxygen atoms.

Types of Chemical Formula
The term “chemical formula” basically describes the molecular formula of a compound, but the composition of chemical compounds can be shown in several ways, as mentioned below:

Molecular Formula
A molecular formula is a chemical formula that expresses the total number of atoms of constituent elements in a compound. In a molecular formula, the elements are denoted by their respective chemical symbols, and the number of atoms of each element present in a molecule is written in subscripts. For example, glucose is an important simple sugar, its molecular formula is $C_6H_12O_6$. It contains 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Benzene $C_6H_6$ and Ammonia $NH_3$ are other examples of molecular formulas.

Empirical Formula
A compound's empirical formula represents a compound's elements in their lowest whole-number ratio. It is the simplest formula of a compound. An empirical formula does not necessarily show the arrangement of atoms in the actual molecule. Taking the example of glucose ($C_6H_12O_6$), it can be observed that each subscript is divisible by 6. Therefore, the empirical formula for glucose is $CH_2O$. The empirical formula can be written from the molecular formula. Other examples are ribose ($C_5H_10O_5$) and ethylene ($C_2H_4$) their empirical formulas are $CH_2O$, and $CH_2$, respectively.

Structural Formula
The structural formula of a chemical compound provides information about the arrangement of atoms in a molecule. For example, the structural formula for water is given below:

Summary
• The chemical formula of a compound is a symbolic representation of the elements present in a compound and their chemical compositions.
• The number of elements can be written as a subscript below and right to the chemical symbol.
• The molecular formula of a chemical compound represents the kinds and the total number of elements present in a compound.
• The empirical formula of a unit substance shows the ratio of the elements present in a compound.
• A chemical compound's structural formula tells the molecule's arrangement of atoms.

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