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Carbon 12.01111/7/2023 ![]() In a typical sample of carbon-containing material, 98.89% of the carbon atoms also contain 6 neutrons, so each has a mass number of 12. The element carbon (C) has an atomic number of 6, which means that all neutral carbon atoms contain 6 protons and 6 electrons. Mass Number(A) = Number of Protons + Number of Neutrons Because different isotopes of the same element haves different number of neutrons, each of these isotopes will have a different mass number(A), which is the sum of the number of protons and the number of neutrons in the nucleus of an atom. All isotopes of an element have the same number of protons and electrons, which means they exhibit the same chemistry. Atoms that have the same number of protons, and hence the same atomic number, but different numbers of neutrons are called isotopes. Unlike protons, the number of neutrons is not absolutely fixed for most elements. Recall that the nuclei of most atoms contain neutrons as well as protons. Hence, the atomic number defines the element in question. If you change the atomic number to 12, you are no longer dealing with sodium atoms, but magnesium atoms. That means that all sodium atoms have 11 protons. For example, the atomic number (z) for sodium (Na) is 11. The symbol for the atomic number is designated with the letter Z. This number is known as the atomic number, which identifies the number of protons in the nucleus of ALL atoms in a given element. When you study the periodic table, the first thing that you may notice is the number that lies above the symbol. ![]() ![]() Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License. We recommend using aĪuthors: Paul Flowers, Klaus Theopold, Richard Langley, William R. Use the information below to generate a citation. Then you must include on every digital page view the following attribution: If you are redistributing all or part of this book in a digital format, Then you must include on every physical page the following attribution: If you are redistributing all or part of this book in a print format, Want to cite, share, or modify this book? This book uses the If the molecular (or molar) mass of the substance is known, it may be divided by the empirical formula mass to yield the number of empirical formula units per molecule ( n): As the name suggests, an empirical formula mass is the sum of the average atomic masses of all the atoms represented in an empirical formula. Molecular formulas are derived by comparing the compound’s molecular or molar mass to its empirical formula mass. Molar mass can be measured by a number of experimental methods, many of which will be introduced in later chapters of this text. Molecular mass, for example, is often derived from the mass spectrum of the compound (see discussion of this technique in the previous chapter on atoms and molecules). These quantities may be determined experimentally by various measurement techniques. Determining the absolute numbers of atoms that compose a single molecule of a covalent compound requires knowledge of both its empirical formula and its molecular mass or molar mass. Recall that empirical formulas are symbols representing the relative numbers of a compound’s elements. The percent composition of this compound could be represented as follows: For example, consider a gaseous compound composed solely of carbon and hydrogen. The results of these measurements permit the calculation of the compound’s percent composition, defined as the percentage by mass of each element in the compound. When a compound’s formula is unknown, measuring the mass of each of its constituent elements is often the first step in the process of determining the formula experimentally. ![]() ![]() The elemental makeup of a compound defines its chemical identity, and chemical formulas are the most succinct way of representing this elemental makeup. But what if the chemical formula of a substance is unknown? In this section, these same principles will be applied to derive the chemical formulas of unknown substances from experimental mass measurements. Given the chemical formula of the substance, one may determine the amount of the substance (moles) from its mass, and vice versa. The previous section discussed the relationship between the bulk mass of a substance and the number of atoms or molecules it contains (moles). Determine the molecular formula of a compound.Determine the empirical formula of a compound.Compute the percent composition of a compound.By the end of this section, you will be able to: ![]()
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