Properties of Matter: Composition
The composition of matter refers to the fundamental components that make up various substances, which are primarily atoms. Each atom is characterized by its atomic number, which identifies it as a specific chemical element. The arrangement and types of these atoms create molecules, whose unique combinations define different chemical compounds. The empirical formula of a compound can be determined by analyzing its percent composition, allowing for the calculation of the proportions of each element present.
Matter can exist in various forms, including homogeneous mixtures where different molecules are uniformly distributed. Despite this uniformity, the individual chemical identities of the molecules remain intact unless a chemical reaction occurs. Analyzing the composition of a pure compound involves determining the weight percentages of its elements, which facilitates the derivation of its empirical formula. To establish a compound's molecular formula, further analysis is often necessary, such as assessing the molecular weight through various experimental methods. Understanding these concepts is essential to grasping the properties of matter and how they interact at both macroscopic and microscopic scales.
Properties of Matter: Composition
FIELDS OF STUDY: Physical Chemistry; Organic Chemistry; Inorganic Chemistry
ABSTRACT
The principles of chemical identity and composition are discussed. The atomic number defines the identity of the atom as an element, and the differing amounts and types of elements in a molecule define the identity of a compound. The empirical chemical formula of a compound can be calculated using percent composition.
The Nature of Matter
All matter is composed of atoms, which are in turn composed of electrons, protons, and neutrons. Each atom has a unique chemical identity as one of the chemical elements, determined by the number of protons in its nucleus. Each atom also has a specific mass that is essentially determined by the combined masses of the protons and neutrons that it contains. The elemental identity of an atom is maintained in normal, nonnuclear chemical reactions, since all such reactions take place only at the level of the outermost electrons of the atoms as they undergo chemical bonding and molecular formation. The specific combination of different atoms in a molecule is unique to the identity of the compound in the same way that atoms are unique to the identity of an element. Adding, removing, or even just changing one single atom in a molecule of a specific compound changes the chemical identity of the compound, no matter how large and complex the molecule may be. The different types of atoms and the number of each type in the molecule are given in the standard form of the chemical formula.
Matter and Composition
Any number of molecules with as many different chemical identities can be mixed together, even to the extent that no adjacent molecules are of the same compound. Macroscopically, such a homogeneous mixture would have consistent physical properties throughout its mass due to the uniform distribution of its components. This is a matter of scale, depending on how broadly the mass is defined. Concrete, for example, consists of water, cement, and innumerable sand and stone components of different sizes and shapes, yet it has a uniform consistency and distribution of components throughout, making it a homogeneous mixture. At the molecular level, however, the different component molecules each retain their specific, unique chemical identities. This is not affected by the combination of components, unless a reaction can occur between two or more of the components.
The composition of matter, whether as bulk mixtures or discrete molecules, is typically described in terms of the percent of each component. For a molecule, this corresponds to the proportion of each type of atom in the molecule’s unique structure. Analysis of a sample of a pure compound can identify the proportion by weight of each individual element present in its molecular structure. This allows calculation of the molecular formula. For example, a sample of a compound may be found to consist of x percent by weight of one type of atom, y percent by weight of a second, and z percent by weight of a third. Since the percentage by weight of each element must add up to 100 percent, the easiest way to solve a problem like this is to assume a starting weight of one hundred grams to simplify the calculations. In one hundred grams, there are therefore x grams of the first type of atom, y grams of the second, and z grams of the third. Having made this assumption, the next step is to use the appropriate atomic weights to determine the number of moles of each element present. Next, use the molar quantities of each element to determine their lowest whole-number ratios. It is helpful to use a table to track the different values. This procedure returns the empirical formula of the compound, giving the proportions, but not the actual numbers, of each element present.
Deriving the Molecular Formula
The actual molecular formula of a compound will be a whole-number multiple of its empirical formula, but because every multiple will produce exactly the same proportions by weight, it requires more information than this to determine the molecular formula. A second analysis, such as a freezing-point depression study, is needed to determine the number of moles in a specific weight of the compound and return the molecular weight of the compound. Comparison between the molecular and empirical weights easily reveals the molecular formula.
PRINCIPAL TERMS
- bonding: the formation of a link between two atoms as a result of the interaction of their valence electrons.
- chemical formula: the combination of symbols and numerical coefficients that specifies the number and identity of the different atoms involved in a chemical reaction or molecular structure.
- element: a form of matter consisting only of atoms of the same atomic number.
- homogeneous mixture: a physical combination of different materials that has a generally uniform distribution of composition, and therefore properties, throughout its mass; called a solution when liquid and an alloy when solid metal.
- molecule: the basic unit of a compound, composed of two or more atoms connected by chemical bonds.
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