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Encyclopedia about Chemistry


The idea of the atom

Models and mechanisms of how particles and other materials behave have been proposed for thousands of years. Especially in the last few centuries, however, these models have been constantly improved and specified. In the following chapters, a cross section of these developments will be presented, leading all the way to our present model of the atom, which will be explained along with all of the laws that govern its behaviour.

The first model of matter which included elements and atoms was proposed in ancient times. The Greek philosopher Leukippos (around 500-400 B.C.) and his student Democritus (around 460-370 B.C.) were the first to describe the matter present in our world as a collection of atoms (Greek: indivisible). Their theory was based on the idea that if any body is divided into its smallest constituent parts, at some point the parts are so small that they can no longer be divided. They used the word indivisible to describe this remaining matter. According to this theory, atoms are small bodies which are not able to be divided.

Atoms of different materials must differ in their composition and size. The characteristics of materials must therefore be determined by differences in their individual atoms: differences in their size, grouping and mutual arrangement. At the beginning of the 19th century, the Greek atomic model was expanded upon and specified by J. Dalton (1766-1844). According to his theory, elements are composed of small particles called atoms

Atoms of individual materials differ in their mass and size. During chemical reactions, atoms themselves remain unchanged. Of course, the number and position of individual atoms in the reactant compounds can and does change. They are combined in certain proportions, only to change those combinations and proportions during a reaction. In more advanced atomic models, atoms are composed of a nucleus and electrons.

The atom, of course, is composed of elementary particles. In an atomís nucleus are neutrons (uncharged) and positively charged protons. Atoms of the same element always contain the same amount of protons. Only the number of neutrons can differ slightly (in isotopes). Isotopes are actually different atoms of the same element differing only in the number of neutrons they contain and their atomic weight. Otherwise, isotopes of one element generally have the same chemical and physical characteristics as the element itself.

The average atomic nucleus is relatively small compared to the atom itself, but it makes up the greatest part of an atomís mass. The mass of protons and neutrons has been designated with the relative number 1. The number of protons in an atom determines its atomic number. This number is also used to symbolize the atom, or element, in the periodic table of the elements. (hydrogen (H)=1, Helium (He)=2, etc.). Electrons (negatively charged particles) revolve around the nucleus of an atom in electronic orbitals, designated areas where they can be found. Their mass is relatively small Ė 1/1836 the mass of protons and neutrons. There is the same amount of electrons as the number of protons in the nucleus. For this reason, every atom, in its natural state, is neutral.

Atoms can lose one or more of their electrons. When they do, they become positively charged. Or, atoms can gain electrons, which makes them negatively charged. When an atom gains or loses electrons, it is called an ion. The outer reaches of an atom, its shell, away from the inner nucleus and where electrons are found, makes up the greatest part of its size. This area is mostly empty space. Electrons move in certain designated areas around the atomic nucleus. Some electrons are closer to the nucleus than others (inner orbital, or shielded electrons). Others are further away from the nucleus (outer orbital electrons).

The nucleus of an atom does not change during a chemical reaction. For this reason, it does not appear to be very important. Of course, an atomís electrons determine its chemical behaviour (mostly these are outer orbital electrons).

The energy of a specific electron is defined with the help of both letters and numbers, according to the orbital where the electron is found. Of great importance is an electronís distance from the nucleus. The exact placement of an atomís electrons at any one time is impossible to determine, because location and direction of an individual electron are not able to be calculated (The Heisenberg Uncertainty Principle).

The more accurately we try to determine the location of a specific electron, the less accurate is our ability to determine its direction. Why? Because it is impossible to tell which direction that electron will move in the moment we have determined its location. Unfortunately, only the probability of where an electron might be found can be calculated. On the other hand, if we know the direction an electron is moving, its exact location becomes impossible to locate. The spacial limitation, more simply the area where an electron of a certain energy can be found with greatest probability, is called the atomic orbital.


Because atoms and their electrons cannot be directly investigated, reality at the atomic level is more or less unknown. From atomic characteristics which can be observed, however, atomic models can be made. The accuracy of these models is seen in their ability to explain certain phenomena. Often, these incredibly small particles show characteristics that are not usual in the macro world we live in. Electrons themselves are capable of a certain principle of duality Ė as is light: the duality of waves and particles. This means that on the one hand, an electron can behave as a sort of particle beam, a bit like a ray gun. On the other hand, electrons also show a purely wave-like character. Electrons are not, however, one or the other, because these two characteristics are contradictory. Yet we need both concepts to be able to describe an electronís behaviour. The wave-like mechanical atomic model comes from the description of the outer shell of an atom and the wave-like characteristics of electrons.

Quantum numbers

In the atomic model of Niels Bohr (Danish physicist), an electron cloud swarms around the nucleus of an atom. Electrons are allowed to move only in certain orbitals around the nucleus. The individual orbitals represent a certain amount of energy. All of the electrons in one orbital are seen as containing the same amount of energy.

The energy of an electron is given by a quantum number n. The larger this number is, the more energy an electron contains, and the further away it is from the nucleus.

When an electron is excited to a more distant orbital from the nucleus, one with a higher energy, a certain energy must be added to the electron (a quantum). When an electron moves from a higher energy orbital to a lower energy orbital, closer to the nucleus, energy must be omitted in the form of radiation (heat, light or in the form of a different type of electromagnetic energy. With the help of the main quantum number, we are able to figure the maximum number of electrons in the outer shell of an atom.

The number of an atomís electrons can be calculated using the formula 2n2, where n is the main quantum number. More recent atomic models use other quantum numbers to describe an atom and its electrons. A secondary quantum number, designated as l, represents the spin of an electron, or its angular momentum. That means its geometric spatial orientation. This quantity is decisively important in order to explain the arrangement of certain chemical bonds in the atoms of a compound.

The energy of a specific electron is defined mainly by the main quantum number n, and to a lesser degree by its secondary quantum number l. From the position of the energy level of an electron, from its orbital (where the electron moves) compared to the outer magnetic field, the magnetic quantum number m (also called the direction quantum number) can be determined. According to the value of m, orbitals can be divided on the basis of their energy.

There is one s orbital (spherical symmetrically placed around the nucleus), three p orbitals (which look like three dumb-bells protruding from the nucleus in their centers and and pointing out in three directions), five d orbitals (four-leaf structures lying between the p orbitals) and seven f orbitals. Within the individual types (s, p, d, f) are individual orbitals of the same energy. If we take the electron to be a small particle, we can imagine it to be spinning on its own axis, to the left or to the right. The direction of its rotation is termed its spin, and is determined by the quantum number s, for spin. With the help of these four quantum numbers, each and every electron can be exactly described.

Stable electron orbitals

The assignment of electrons to their individual orbitals is termed electron configuration. According to the Pauli principle (Swiss-American physicist), no more than two electrons can be found in one orbital at one specific time.

Orbitals are occupied by electrons from lowest energy orbital to highest energy orbital (in the order s, p, d, f). First of all, every orbital of a specific energy is occupied by one electron. Then, an orbital of opposite spin moves into an orbital to join the first electron. Once there are two electrons in one orbital, it is filled completely. The two electrons are called an electron pair. Individual electrons are called unpaired electrons. In each element of the main group, all s and p orbitals are filled gradually, as electrons are added. For the elements of other groups, the d orbitals are filled.

Ionisation energy

Electrons have a certain amount of energy associated with them, and this energy determines their distance from the nucleus. If energy is added to an electron, an electron can increase its distance from the nucleus, or can even escape from the nucleus. In the latter case, an atom becomes a positively charged ion. The amount of energy which is necessary for an electron to leave the atom is called its ionization energy. Therefore, the ionisation energy necessary to free an electron in an outer orbital from an atom is less than for an electron which is closer to the nucleus.

The density of an element is a relative number given by how the matter of an element is arranged around its atoms, on average. The density of different elements can only be compared given the same volume. Density is a function of both mass and volume.

Density units are often given as kg/m3 or g/cm3. The densities of a number of materials are included in tables.

At first glance, many elements share a number of characteristics. A closer comparison of those characteristics, including colour, state of matter (solid, liquid, gas), odour, flammability and density, allow substances to be distinguished one from another. When substancesí characteristics are compared and contrasted, they can be divided into groups. The most important groups that chemistry deals with are: acids, bases, oxides, salts, metals, hydrocarbons and polymers (materials with a great number of atoms which repeat their patterns in a periodic way.

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Molecules and Moles

The smallest possible chemical unity is formed by the union of a number of atoms Ė a compound Ė also called a molecule. If we want to produce a certain amount of a material, we choose whether to produce that certain amount as a function of its mass, volume or even amount of individual particles.

In chemistry, we use the variable (n) very often as a measure of the amount of a certain substance. One unit of a material is called a mole. We can imagine this amount of a substance as a chemical dozen, an even unit, so to speak. and just like a dozen, or 12, one mole is always equal to a certain number of particles. Of course, this number is more than 12, because of the minute size of atoms and molecules. It would indeed be difficult to count in multiples of 12.

One mole is given as 6.022 x 10 23 particles. This seemingly arbitrary amount of particles is actually based on a chemical truth, using carbon (chemical symbol C), because this element plays one of, if not the, most important role in chemistry. Twelve grams (g) of the element carbon contains exactly 1 mole of atoms. Why is the number of smallest particles so important in chemistry? The answer to this question has to do with the nature and types of chemical reactions. During a chemical reaction, particles interact with one another, often combining to form a new substance. For example, water is actually the combination, or a compound, of two atoms, two atoms of hydrogen and one atom of oxygen. The mass of the two reacting elements would not be enough to ensure a sufficient amount of each element for combination, because oxygen atoms are significantly heavier than hydrogen atoms.

In the laboratory, a chemist cannot determine the amount of a substance by deduction, or by some type of instinct. The amount of a substance can, however, be determined by its mass, which directly relates to the amount of particles a certain amount of substance contains. The quotient of a certain amount of mass (m) and an amount of substance (n) is given by the molar mass (M), with the unit number of grams per one mole.

Molar mass is determined by the sum of the masses of the individual atoms in a molecule. Atomic masses are easily attainable, from the periodic table of the elements. (Hydrogen (H) 1g/mol, Helium (He) 4 g/mol, Lithium (Li) 7g/mol, Beryllium (Be) 9 g/mol, etc.). See the periodic table for more atomic masses.

The molar mass of water (H2O) is 18 grams per mole: 1g/mol for each hydrogen atom (H) and 16 g/mol for the one oxygen atom (O). The molecule is composed of three atoms (2H + 1 O), or more simply: three parts, or atoms, join to make one larger compound, or molecule. The amount of particles corresponding to 1 mole of water is 6.022 . 10 23 molecules of water.

Individual atoms of each element have the same mass. The variable masses of individual molecules is a function of the bonding capabilities of those moleculesí constituent atoms, and their atomic masses.

Matter, or mass, is neither created nor destroyed. If during a chemical reaction a compound, or other products of that reaction have less mass than the original reactant materials, most likely one of the products is not easily detectable Ė possibly an invisible, odourless gas, or some other byproduct of the reaction. If a scientist accurately compares the mass of all reactant materials with the mass of all products produced, the same amount is always present on both sides. Matter is neither created nor destroyed; it can only change form.

A mixture of a solid material dissolved in a liquid is called a solution. These mixtures can be measured by their volumes. The amount of a material dissolved in the same volume of a solution can vary from one mixture to another, however. To determine the amount of a dissolved substance in a solution, we use the chemical formula concentration (symbol: c), a measure of its variable "strength". The units of concentration of a solution are amount of moles dissolved in one litre of solution. Substance concentration is indicated as the concentration of a substance in solution. It is the quotient equal to the amount of a material dissolved in a certain volume of a solution (12 g of carbon (C) in one liter of water has a concentration of 1 mol/l). We call this amount of solution a one molar solution of carbon, and abbreviate it as 1 M.

In order to determine the molar concentration of a solution, or in the case that a chemist might need to prepare a solution of a given molar concentration, it is necessary to calculate the mass of each material. The mass of the dissolved substance is calculated from the necessary material mass and mass of one mole of the material. The amount of a substance in a solution can be calculated from the concentration of a substance and the volume of the solution.

For example, for a 1 molar solution of table salt we need 58.5 g of table salt in 1 l of water. Table salt is made of one part sodium and one part chlorine. The chemical formula of this compound is NaCl. The mass of one mole of NaCl is 58.5 g, because sodium (Na) has a molar mass of 23 g and chlorine (Cl) an atomic mass of 35.5 g. Add the two together (23 + 35.5 = 58.5). The mass of one mole is easily attainable from the periodic table of the elements.

A certain molar concentration does not tell how much volume a certain solution contains. That is, a 1 molar solution does not guarantee that there is 1 liter of solution. Rather, a 1 M solution implies that the ratio of dissolved substance (solute) to volume of substance dissolved in (solvent). In our example with table salt, then, rather than use 58.5 g of NaCl with 1 l of water, we could have just as easily used 29.25 g of NaCl with 0.5 l of water, or 117 g of salt with 2 l of water.

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Chemical symbols

Substances and chemical reactions can be denoted in a simple and straightforward way in chemistry. A system of symbols, abbreviations and chemical formulas is used, and these are all internationally recognised Ė thanks to a committee of international experts who have agreed upon these symbols. At first, however, somewhat abstract symbols were used. Eventually, circular symbols to denote compounds were used. Todayís system was introduced by J. J. Berzeliem (Swedish chemist 1779-1848). According to this system, each element was assigned a chemical symbol, usually taken from its Latin or Greek equivalent (for example Magnesium - Mg or oxygen = Oxygenium - O).

Elements are made up of small particles of one and only one kind. We call these particles atoms. In some elements, atoms combine in their natural state, in twos or even more, to form a compound of the given element. In this case, the atoms of one element are joined tightly together, thereby attaining an increased chemical stability. We call these combinations molecules and molecular substances. Molecules are often the smallest building blocks of

gaseous or fluid substances. For example, atoms of hydrogen, nitrogen and oxygen are always joined together, in pairs, two each. There are molecules, however, that are made of different elements. The compound " water " is made of one atom of oxygen and two atoms of hydrogen.

One important foundation of chemical terminology is the concept of using small numbers after a chemical element symbol to indicate number of atoms, called stoichiometry. In the language of chemical symbols, an element symbol is often combined with these numbers, and is called a chemical formula. A formula, then, is made up of the element symbols that a certain compound is composed of. and, after each element symbol, the number of atoms of that element contained in the compound is given. This number is smaller than the element symbol. Ones, as in one atom of an element, are understood, and therefore not written, as in the chemical formula of water, H2O, understood as two atoms of hydrogen, and one atom of oxygen. Water is therefore not written as H2O1.

The formula of a compound characterises the material it represents and denotes its constituent elements, the elements it is made of. At the level of individual particles, the formula symbolises the molecule and gives the amount of all atoms in the molecule, and their ratio to one another. The ratio of the number of individual atoms in a molecule can be calculated for example with the help of the mass ratio of the individual elements and their atomic masses.

Stoichiometry says that the atoms in a compound are mutually bonded in unchanging ratios.

1. Daltonís law: The ratio of the masses of two elements which are bonded together in one molecule can be given as the ratio of one whole number to another.

2. The law of definite proportions: Every compound contains elements in a certain specific and constant mass ratio.

3 The law of consistent proportions: Elements combine together in certain specific ratios of masses or in whole number amounts.

How many atoms of one element join together with how many atoms of another element can be determined by experiment and calculation. The true chemical formula of a number of compounds can be determined rather simply, however, if we know the bonding possibilities of individual elements (their valence). This is the deciding factor for individual elements. For example, once we know the bonding possibilities of an element, we can figure out quickly how many hydrogen atoms could conceivably bond to it. The valence of an element when bonding with hydrogen is given by the amount of unpaired electrons in the outer shell of its electron cloud (the cloud made up of electrons moving at certain levels or in certain orbitals around the nucleus). For example: in water (H2O) one oxygen atom (O) bonds with two hydrogen atoms (H) and therefore has a valence of 2.

In chemical bonds, elements, or their atoms, are not only joined in whole numbers, but their mass ratios also remain constant. For example, in the chemical reaction of iron (Fe) se sulfur (S) iron sulfide (FeS) is formed. The ratio of the number of individual atoms is 1:1. The ratio of masses of the individual atoms is determined from the atomic masses of sulphur and iron, and is 1.45 (7:4).

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