Unit of which elements are composed

Atom the fundamental unit of which elements are composed. 

Crystals of macromolecules, like those shown in Figure 3.1, are like crystals of all other kinds. They are precisely ordered three-dimensional arrays of molecules that may be characterized by a concise set of determinants that exactly define the disposition and periodicity of the fundamental units of which they are composed. The set of parameters is comprised of three elements. These define the symmetry properties, the repetitive and periodic features, and the distribution of atoms in the repeating unit. The properties may be separated and understood by considering how a crystal can be developed as a three-dimensional form from a basic building block (the asymmetric unit), by the application of symmetry (the space group), and translation (the unit cell, or lattice). As illustrated in Figure 3.2, this can be accomplished in four stages. 

The number of atoms of each element that make up a molecule or formula unit can be seen in the formulas. The numbers written as subscripts in the formulas, such as the 2 in the formula for water, H2O, and the 2 and the 4 in the formula for potassium chromate, K2Cr04, represent the number of atoms of the elements immediately to the left of each subscript in the formula. If there is no subscript immediately to the right of an element symbol, then there is only one atom of that element in the molecule or formula unit. Thus, in a molecule of water, H2O, there are 2 atoms of hydrogen and one atom of oxygen that are chemically combined. In a formula unit of salt, NaCl, there is one atom (ion) of each element, sodium and chlorine. In a formula unit of potassium chromate (also an ionic compound), K2Cr04, there are two atoms of potassium, one atom of chromium, and four atoms of oxygen. Thus chemical formulas are reasonable representations of the molecules and formula units of which compounds are composed. 

An atom is the smallest unit quantity of an element that is capable of existence, either alone or in chemical combination with other atoms of the same or another element. The fundamental particles of which atoms are composed are the proton, electron and neutron. 

To gain a proper understanding of the behaviour of a complex system we must first appreciate the structure and properties of the elementary units of which it is composed. In the study of ice this means that we must begin with a study of the water molecule, for it is from the individuality of the structure of that molecule that most of the unusual properties of ice and water arise. Without such a relation back to the fundamentals of molecular structure, the study of a particular material becomes simply a catalogue of its properties—useful, no doubt, but not very illuminating. In this book we shall try, at all stages, to show this relation so that a coherent picture emerges. Similar pictures can be built up for all solids the outlines, it is true, have many variations but they all follow in the same sort of way from the basic elements of which they are built. 

Atom The basic unit of an element that retains all the element s chemical properties. An atom is composed of a nucleus (which contains one or more protons and neutrons) and one or more electrons in motion around it. Atoms are electrically neutral because they are made of an equal number of protons and electrons. 

The formula of a compound tells us which elements it is composed of and how many atoms of each element are present in a formula unit. For example, a unit of sulfuric acid is composed of two atoms of hydrogen, one atom of sulfur, and four atoms of oxygen. We could express this compound as HHSOOOO, but this is cumbersome, so we write H2SO4 instead. The formula may be expressed verbally as H-two-S-O-four. Numbers that appear partially below the line and to the right of a symbol of an subscripts element are called subscripts. Thus, the 2 and the 4 in H2SO4 are subscripts (see Figure 3.9). Characteristics of chemical formulas are as follows ... 

All ofthe literally millions of different substances are composed of only around 100 elements. Each atom of a particular element is chemically identical to every other atom and contains the same number of protons in its nucleus. This number of protons in the nucleus of each atom of an element is the atomic number of the element. Atomic numbers are integers ranging from 1 to more than 100, each of which denotes a particular element. In addition to atomic numbers, each element has a name and a chemical symbol, such as carbon, C potassium, K (for its Latin name kalium) or cadmium, Cd. In addition to atomic number, name, and chemical symbol, each element has an atomic mass (atomic weight). The atomic mass of each element is the average mass of all atoms of the element, including the various isotopes of which it consists. The atomic mass unit, u (also called the dalton), is used to express masses of individual atoms and molecules (aggregates of atoms). These terms are summarized in Figure 1.2. 

Sigma (a) bonds Sigma bonds have the orbital overlap on a line drawn between the two nuclei, simple cubic unit cell The simple cubic unit cell has particles located at the corners of a simple cube, single displacement (replacement) reactions Single displacement reactions are reactions in which atoms of an element replace the atoms of another element in a compound, solid A solid is a state of matter that has both a definite shape and a definite volume, solubility product constant (/ p) The solubility product constant is the equilibrium constant associated with sparingly soluble salts and is the product of the ionic concentrations, each one raised to the power of the coefficient in the balanced chemical equation, solute The solute is the component of the solution that is there in smallest amount, solution A solution is defined as a homogeneous mixture composed of solvent and one or more solutes. 

Materials and substances are composed of particles such as molecules, atoms and ions, which in turn consist of much smaller particles of electrons, positrons and neutrons. In electrochemistry, we deal primarily with charged particles of ions and electrons in addition to neutral particles. The sizes and masses of ions are the same as those of atoms for relatively light lithiiun ions the radius is 6 x 10 m and the mass is 1.1 x 10" kg. In contrast, electrons are much smaller and much lighter them ions, being 1/1,000 to 1/10,000 times smaller (classical electron radius=2.8 x 10 m, electron mass = 9.1 x 10" kg). Due to the extremely small size and mass of electrons, the quantization of electrons is more pronoimced than that of ions. Note that the electric charge carried by an electron (e = -1.602 X 10 C) is conventionally used to define the elemental unit of electric charge. 

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