CHEMISTRY

ATOMS AND MOLECULES

The Greek philosopher Democritus (470–400 BC) suggested that all matter is composed

of tiny, discrete, indivisible particles that he called atoms. His ideas, based entirely on

philosophical speculation rather than experimental evidence, were rejected for 2000 years.

By the late 1700s, scientists began to realize that the concept of atoms provided an explanation

for many experimental observations about the nature of matter.

By the early 1800s, the Law of Conservation of Matter (Section 1-1) and the Law of

Definite Proportions (Section 1-5) were both accepted as general descriptions of how matter

behaves. John Dalton (1766–1844), an English schoolteacher, tried to explain why

matter behaves in such systematic ways as those expressed here. In 1808, he published the

first “modern” ideas about the existence and nature of atoms. Dalton’s explanation summarized

and expanded the nebulous concepts of early philosophers and scientists; more

importantly, his ideas were based on reproducible experimental results of measurements by

many scientists. These ideas form the core of Dalton’s Atomic Theory, one of the highlights

in the history of scientific thought. In condensed form, Dalton’s ideas may be stated

as follows:

1. An element is composed of extremely small, indivisible particles called atoms.

2. All atoms of a given element have identical properties that differ from those of

other elements.

3. Atoms cannot be created, destroyed, or transformed into atoms of another element.

4. Compounds are formed when atoms of different elements combine with one another

in small whole-number ratios.

5. The relative numbers and kinds of atoms are constant in a given compound.

Dalton believed that atoms were solid, indivisible spheres, an idea we now reject. But

he showed remarkable insight into the nature of matter and its interactions. Some of his

ideas could not be verified (or refuted) experimentally at the time. They were based on

the limited experimental observations of his day. Even with their shortcomings, Dalton’s

ideas provided a framework that could be modified and expanded by later scientists. Thus

John Dalton is often considered to be the father of modern atomic theory.

The smallest particle of an element that maintains its chemical identity through all

chemical and physical changes is called an atom (Figure 2-1). In Chapter 5, we shall study

the structure of the atom in detail; let us simply summarize here the main features of

atomic composition. Atoms, and therefore all matter, consist principally of three fundamental

particles: electrons, protons, and neutrons. These are the basic building blocks ofatoms. The masses and charges of the three fundamental particles are shown in Table

2-1.

The masses of protons and neutrons are nearly equal, but the mass of an electron is

much smaller. Neutrons carry no charge. The charge on a proton is equal in magnitude,

but opposite in sign, to the charge on an electron. Because atoms are electrically neutral,an atom contains equal numbers of electrons and protons.

The atomic number (symbol is Z) of an element is defined as the number of protons

in the nucleus. In the periodic table, elements are arranged in order of increasing atomic

numbers. These are the red numbers above the symbols for the elements in the periodic

table on the inside front cover. For example, the atomic number of silver is 47.

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.

Individual oxygen atoms are not stable at room temperature and atmospheric pressure.

Single atoms of oxygen mixed under these conditions quickly combine to form pairs. The

oxygen with which we are all familiar is made up of two atoms of oxygen; it is a diatomic

molecule, O2. Hydrogen, nitrogen, fluorine, chlorine, bromine, and iodine are other examples

of diatomic molecules (Figure 2-2).

Some other elements exist as more complex molecules. One form of phosphorus molecules

consists of four atoms, and sulfur exists as eight-atom molecules at ordinary temperatures

and pressures. Molecules that contain two or more atoms are called polyatomic

molecules (Figure 2-3).

In modern terminology, O2 is named dioxygen, H2 is dihydrogen, P4 is tetraphosphorus,

and so on. Even though such terminology is officially preferred, it has not yet gained

wide acceptance. Most chemists still refer to O2 as oxygen, H2 as hydrogen, P4 as phosphorus,

and so on.

Molecules of compounds are composed of more than one kind of atom. A water molecule

consists of two atoms of hydrogen and one atom of oxygen. A molecule of methane

consists of one carbon atom and four hydrogen atoms. The shapes of a few molecules are

shown in Figure 2-4.

Atoms are the components of molecules, and molecules are the stable forms of many

elements and compounds. We are able to study samples of compounds and elements that

consist of large numbers of atoms and molecules. With the scanning tunnelling microscope

it is now possible to “see” atoms (Figure 2-5). It would take millions of atoms to

make a row as long as the diameter of the period at the end of this sentence.

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Titanium's history

Titanium is one of the most widespread element, the average content of it in the earth"s crust is 0.57% by mass (among construction metals it takes 4th place, cede to iron, aluminum and manganese). Titanium is mainly in base rock so called “basaltic sell? (0.9%), less in rocks of “granite shell? (0.23%) and less in ultrabasic rock (0.03%) etc.

There are known 67 minerals of titanium, mainly with abyssal origin, the most important of which are rutile and ilmenite. Ilmenite (43.7-52.8% ТiO2); rutile, anatase and brookite (94.2-99.0%); titanite (33.7-40.8%), perofskite (38.7-58.9%), titaniferous magnetite.

Titanium as dioxide was discovered by English dilettante mineralogist U. Gregor in 1791 magnetic ferriferous sands of Manacan (England); in 1795 German chemist M.G. Claprot found out that mineral rutile is the natural oxide of this metal, named “titanium? by him [in Greek mythology titans are the children of Uranium (Sky) and Hea (Earth)].

Technical titanium, used in industry, content oxygen, nitrogen, silicon and carbon admixtures increasing its strength, decreasing its plasticity and impacting on the temperature of its polymorphic conversion.

Titanium has good corrosion stability in atmosphere, sea water and oceanic atmosphere, in various technical solutions and reagents used in chemical, petrol chemical, paper-making and other branches of industry, as well as in hydrometallurgy.

The main advantages of titanium against other construction materials are the combination of lightness, strength and corrosion stability. Titanium alloys by absolute and specific durability are excel in the majority of other alloys, based on another metals (e.g. iron or nickel) in temperature range from -250 till 550 centigrade. According to corrosion stability it can be compared with alloys of noble metals. Yet, as construction metal titanium has started being used in 50th of 20th century because of technical hardship of its extraction from ores and treatment, that’s why the titanium has conditionally considered as rare metal. The main part of titanium is used for the necessities of air and rocket technique and shipbuilding. Titanium and iron alloys, known as “ferrotitanium? (20-50% Тi), is used as inoculant and deoxidizing component in the metallurgy of special alloys and high quality steels.

Technical titanium is used for chemical reactors, pipelines, control valves, pumps and other equipment production, which are working in aggressive mediums, e.g. chemical machine building. The equipment made of titanium alloys is used in non ferrous metallurgy; it’s used for covering of items made of steel. The usage of titanium in major cases brings noticeable technical and economical effect not only because of increasing of equipment’s working period, but due to process’ intensification possibility (e.g. in nickel’s hydrometallurgy). Biological safety of titanium makes it perfect material for producing of food and surgical equipment. In deep cold the hardness of titanium is increasing but keeping its good plasticity, which allow its usage as construction material in cryogenic technique. Titanium is well for polishing, anodic coating and other methods of surface treatment, so that is used for various art items’ and statues’ creation.

Titanium carbide has high hardness and used in hard alloys production used in tools and abrasive grains manufacturing.