Molecule The smallest particle of an element

Molecule The smallest particle of an element or compound that exists on its own and still retains its properties... 

Atom. The hundred or so elements that make up matter are composed of particles called atoms. The atom is unique for each element and has consistent properties, including weight, number of neutrons, electrons, and protons. The atom is the smallest particle of an element that can exist alone or combine with similar or dissimilar atoms to form compounds (molecules) having unique characteristics. 

A molecule is the smallest particle of an element or compound that can have a stable independent existence. In nearly all molecules, two or more atoms are bonded together in very small, discrete units (particles) that are electrically neutral. 

Elements are fundamental substances that cannot be broken down into smaller chemical components. The smallest unit of an element is an atom, a term based on the Greek word atomos, meaning indivisible. But atoms are divisible—they consist of a nucleus containing positively charged particles called protons and electrically neutral particles called neutrons, surrounded by a swarm of electrically negative particles called electrons. In chemical reactions, atoms interact and combine to form a molecule of a compound. (Chemical reactions also occur when the atoms in molecules interact and combine to form even bigger com-... 

But this idea contradicts the very heart of Dalton s notion that the smallest particles of any element are supposed to be indivisible. Dalton s own reaction to Gay-Lussac s law was to question the data and to repeat the experiments. This led to his claiming that the ratios were not in fact as simple as reported by von Hum-bolt and Gay-Lussac. Nevertheless, the simple ratio continued to be reproduced by others and has passed the test of time. Dalton could have accepted the existence of molecules of elements composed of two or more atoms of an element while stiU holding that such a body represented the simplest chemical unit that retains the properties of the element in question, but he failed to do so. 

It is nsefnl to note a fact here that is discnssed in more detail later in Section 4.11. The smallest particles of some elements and componnds are individual atoms rather than molecules. However, in elements of this type, the individual atoms are all of the same kind, whereas in componnds, two or more kinds of atoms are involved. Thus, the classification of a pure substance as an element or a componnd is based on the fact that only one kind of atom is fonnd in elements and two or more kinds are found in compounds. In both cases, the atoms may be present individnally or in the form of homoatomic molecules (elements) or heteroatomic molecules (compounds). Some common household materials are pure substances (elements or componnds), snch as alnminnm foil, baking soda, and table salt. 

Matter comes in many forms. The fundamental building blocks of matter are atoms and molecules. These particles make up elements and compounds. An atom is the smallest unit of an element that maintains the chemical identity of that element. An element is a pure substance that cannot be broken down into simpler, stable substances and is made of one type of atom. Carbon is an element and contains one kind of atom. The model of diamond in Figure 2.1a consists of carbon atoms. 

A compound is a substance that can be broken down into simple stable substances. Each compound is made from the atoms of two or more elements that are chemically bonded. Sucrose, in Figure 2.1b, is an example of a compound. It is made of three elements carbon, hydrogen, and oxygen. The atoms are chemically bonded to form a molecule. You will learn more about the particles that make up compounds when you study chemical bonding. For now, you can think of a molecule as the smallest unit of an element or compound that retains all of the properties of that element or compound. 

Definitions. Discuss the inadequacies and limitations of die following definitions. Supply better ones, (a) A molecule is the smallest particle of matter that retains the specific properties of the matter, (b) A solid is matter that retains its shape, (c) A liquid is matter that takes the shape of its container. (Is this true of a liquid in a vehicle in interplanetary space ) (d) An element is a substance that cannot be decomposed by chemical means, (e) A compound is a substance that contains more than one element. 

For the purposes of photophysics and photochemistry it is therefore sufficient to keep in mind the simple picture of an atom as a heavy, positively charged nucleus around which move light, negatively charged electrons. In the smallest atom, that of hydrogen, there is a single electron, whereas in the uranium atom, which is the heaviest natural element known on Earth, there are 92 electrons. It is the motion of these electrons which determines the chemical properties of an atom or a molecule so that it is now necessary to consider in a qualitative way the structure of these elementary particles of matter. 

Atoms are the smallest particle within elements. They are made up of protons and neutrons (in the nucleus) and electrons (in orbits around the nucleus). Each orbit represents an energy level and these give the atom stability. Electrons in the outer orbit, or valence shell, control how the atom bonds. When atoms are linked together by chemical bonds they form molecules. 

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