Atoms and the Atomic Theory

Just like chemists today, ancient Egyptians also used symbols as well as words to represent common elements and compounds. Electrum is an alloy of silver and gold. 

Redrawn from Lapsius. Metals In Egyptian Inscriptions. 1860 

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Early in this chapter (Section 2.3) we will introduce a classification system for elements known as the periodic table. It will prove useful in this chapter and throughout the remainder of this text  

An element is composed of tiny particles called atoms. All atoms of a given element show the same chemical properties. Atoms of different elements show different properties. 

In an ordinary chemical reaction, atoms move from one substance to another, but no atom of any element disappears or is changed into an atom of another element. 

Compounds are formed when atoms of two or more elements combine. In a given compound, the relative numbers of atoms of each kind are definite and constant. In general, these relative numbers can be expressed as integers or simple fractions. 

On the basis of Daltons theory, the atom can be defined as the smallest particle of an element that can enter into a chemical reaction. 

Image of silicon atoms that are only 78 pm apart image produced by using a scanning transmission electron microscope (STEM). The hyfxjthesis that all matter is made up of atoms has existed for more than 2000 years. It is only within the last few decades, however, that techniques have been develop ed that can render individual atoms visible. 

We begin this chapter with a brief survey of early chemical discoveries, culminating in Dalton s atomic theory. This is followed by a description of the physical evidence leading to the modern picture of the nuclear atom, in which protons and neutrons are combined into a nucleus with electrons in space surrounding the nucleus. We will also introduce the periodic table as the primary means of organizing elements into groups with similar properties. Finally, we will introduce the 

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In Chapter 2 (Atoms and the Atomic Theory), new material is included to describe the use of atomic mass intervals and conventional atomic masses for elements such as H, Li, B, C, N, O, Mg, Si, S, Cl, Br, and Tl. Atomic mass intervals are recommended by the lUPAC because the isotopic abundances of these elements vary from one source to another, and therefore, their atomic masses cannot be considered constants of nature. 

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