Definition of atom chemistry
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Definition of Atom in Chemistry
Historical Context and Evolution of the Atom Concept
The concept of the atom has evolved significantly since its inception. Initially proposed by ancient Greek philosophers, the idea that matter is composed of indivisible units called atoms gained traction over centuries. By the Enlightenment, three core ideas about atoms were established: all matter is made of atoms, atoms occupy space, and atoms have mass. This foundational understanding was solidified through 19th-century chemical experiments and further validated by Albert Einstein's interpretation of Brownian motion in 1905, which provided empirical support for the atomic hypothesis5.
Atoms in Molecules: Theoretical Frameworks
Hirshfeld Atoms and Information Theory
The concept of atoms in molecules (AIM) is crucial in chemistry, particularly in understanding molecular structures and properties. Hirshfeld's method, derived from information theory, defines atoms within molecules by partitioning the molecule into regions associated with each atom. This approach, known as "stockholders partitioning," uses the minimum entropy deficiency principle to provide a unique definition of an atom in a molecule1 7.
Axiomatic Approach and Transferability
Another significant theoretical framework is the axiomatic approach, which emphasizes the transferability of atomic properties across different molecular environments. This approach seeks to establish a mathematical basis for defining atoms in molecules, focusing on the consistent retention of atomic characteristics regardless of the molecular context. This concept is essential for predicting and understanding chemical behavior2.
Unique Definition and Quantum Mechanical Description
The debate over the definition of an atom in a molecule highlights the need for a unique, consistent description that aligns with quantum mechanics. Some definitions, such as those based on orbital models or isovalued density envelopes, fail to provide a comprehensive quantum mechanical description. A robust definition must predict measurable, additive properties of atoms in molecules, ensuring their operational and predictive utility in chemistry3.
Experimental Validation and Practical Implications
Despite theoretical debates, experimental chemistry consistently supports the concept that the properties of a molecular system are the sum of its atomic contributions. This empirical evidence underscores the practical reality that chemists measure atomic properties during experiments, reinforcing the validity of the AIM concept8.
Modern Theories and Quantitative Analysis
Bader's theory of atoms in molecules offers a modern, quantitative approach to defining atomic volumes based on molecular wave functions. This theory allows for the calculation of atomic properties such as electron populations and energies through numerical integration of charge density functions. It also connects traditional chemical concepts like hybridization and electronegativity to a more rigorous theoretical framework, enabling the identification of bent bonds and their relation to structural changes and steric interactions6.
Conclusion
The definition of an atom in chemistry has evolved from ancient philosophical ideas to sophisticated theoretical and experimental frameworks. Modern approaches, such as Hirshfeld's partitioning and Bader's theory, provide robust, quantitative descriptions of atoms in molecules, essential for understanding and predicting chemical behavior. These definitions are not only theoretically sound but also empirically validated, ensuring their relevance and utility in both academic and practical chemistry.
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What is an atom in a molecule?
The concept of atoms in molecules (AIM) is crucial for chemistry, but it is a noumenon in the sense of Kant.
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This paper proposes an axiomatic basis for the concept of atoms in molecules, focusing on transferability and the Hirshfeld atomic description.
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A unique theoretical definition of an atom in a molecule is essential for chemistry, as other definitions fail to predict measurable, additive properties and play no operational or predictive role.
What Is an Atom?
Retaining the term "atom" but making special distinctions may be the best option for avoiding misunderstandings.
2. Atoms
The discovery of atoms, despite their invisibility, was a key milestone in the understanding of matter and its properties.
Information theory, atoms in molecules, and molecular similarity.
The "stockholders partitioning" of a molecule, using information theory, provides a useful definition for atoms in molecules and helps assess molecular similarity.
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Experimental chemistry has consistently shown that the properties of a total system are the sum of its atomic contributions, and experimentalists are unavoidably measuring atomic properties in their experiments.
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