The structure of Atoms and Subatomic Particles in Chemistry from HelpWithAssignment.com

The structure of Atoms and Subatomic Particles

The study of chemistry begins with atoms, the basic building blocks of matter. Early theories of atom considered atoms to be invisible, but we know now that this idea is wrong. Elements differ from one another because of differences in the internal structure of their atoms. Under the right conditions, smaller particles within atoms – known as subatomic particles – can be removed or rearranged. The term atomic structure refers to the identity and arrangement of these subatomic particles in the atom. An understanding of how atoms combine to form compounds and are rearranged in chemical reactions. Atomic structures also accounts for the properties of materials.

Atomic Structure and Subatomic particles

Electrical charges played an important role in many of the experiments from which the theory of atomic structure was derived. Two types of electrical charge exist: positive and negative charge. Electrical charges of the same type repel one another and charges of the opposite type attract one another. A positively charged particle repels another positively charged particle. Likewise, two negatively charged particles repel each other. In contrast, two particles with opposite signs attract each other.

Radioactivity

In 1896, Henri Becquerel discovered that a sample of a uranium ore emitted rays that darkened a photographic plate, even though the plate was covered by a protective black paper. In 1898, Marie and Pierre Curie isolated the new elements polonium and radium, which emitted the same kind of rays. Marie suggested that atoms of such elements spontaneously emit these rays and named the phenomenon radioactivity.

Atoms of radioactive elements can emit three types of radiation: alpha (α), beta (β) and gamma (γ) rays. These radiations behave differently when passed between electrically charged plates. Alpha and beta rays are deflected, but gamma rays are not. These events occur because alpha rays and beta rays are composed of charged particles that come from within the radioactive atom. Alpha rays have 2+ charge and beta rays have a 1 – charge. Alpha rays and beta rays are particles because they have mass – they are matter. Experiments revealed that alpha particles were deflected less so must be heavier than beta particles. Gamma rays did not have any detectable charge or mass as they behaved like light rays. If radioactive atoms can break apart to produce subatomic alpha and beta particles, then there must be something smaller inside the atoms.

Electrons

Further evidence that atoms are composed of subatomic particles came from experiments with specially constructed glass tubes called cathode-ray tubes. Most of the air was removed from these tubes and metal electrode sealed into each end. When a sufficiently high voltage is applied to the electrodes, a beam of rays flows from the negatively charged electrode (the cathode) to the positively charged electrode (the anode). These rays, known as cathode rays, come directly from the metal atoms of the cathode. The cathode rays travel in straight lines, are attracted toward positively charged plates, can be deflected by a magnetic field, can cast sharp shadows, can heat metal objects red hot and can cause gases and fluorescent materials to glow. When cathode rays strike a fluorescent screen, the energy transferred causes light to be given off as tiny flashes. Thus, the properties of cathode ray are those of a beam of negatively charged particles, each of which produces light flash when it hits a fluorescent screen. Sir Joseph John Thomson suggested that cathode rays consist of the same particles that had earlier been named electrons and had been suggested to be the carriers of electricity. He also observed that cathode rays were produced from electrodes made from different metals. This implied that electrons are constituents of the atoms of those elements.

In 1897, Thomson used a specially designed cathode ray tube to simultaneously apply electric and magnetic fields to a beam of cathode rays. By balancing the electric field against the magnetic field and using the basic laws of electricity and magnetism, Thomson calculated that the mass of charge for the electrons in the cathode ray beam: 5.60 × 10^-9 grams per Coulomb (g/C).

Protons

When atoms loose electrons, the atoms become positively charged. When atoms gain electrons, the atoms become negatively charged. Such charged atoms, or similarly charged group of atoms are known as ions. From experiments with positive ions, formed by knocking electrons out of atoms, the existence of a positively charged fundamental particle was deducted. Positively charged particles with different mass-to-charge ratios were formed by atoms of different elements. The variation in masses showed that atoms of different elements must contain different numbers of positive particles. Those from hydrogen atoms had the smallest mass-to-charge ratio, indicating that they are the fundamental positively charged particles of atomic structure. Such particles are called protons. The mass of a proton is known from experiment to be 1.67262129 × 10^-24 g, which is about 1800 times the mass of an electron. The charge of a proton is 1.602176462 × 10^-19 C, equal in size, but opposite in sign, to the charge on an electron. The proton’s charge is represented by 1+. Thus, an atom that has lost two electron has a charge of 2+.

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This article is in continuation with our previous articles on Chemistry which include Classification of matter in Chemistry, Periodic Table in Chemistry

Periodic Table in Chemistry from HelpWithAssignment.com

Periodic Table in Chemistry

The table in which the elements are arranged in the increasing order of their atomic number is called periodic table. The similar properties which recur after a regular interval are called the periodic properties. Several attempts were made to classify the elements so as to make their study simple and systematic. J.W. Dobereiner arranged the elements in the group of three called triads. According to this, the chemically similar elements were arranged in the increasing order of their atomic weights in the group of three to make a triad. It was noted that the atomic weight of middle element present in a triad is the mean of the other two elements.

Triad of alkali metals – lithium, sodium and potassium have atomic weights 7, 23 and 39 respectively. By taking the arithmetic mean of lithium and potassium we get the atomic weight of sodium as 7+39/2 = 46/2 = 23.

Triad of halogens – chlorine, bromine and iodine have atomic weights 35.5, 80.4 and 126.5 respectively. By taking the arithmetic mean of chlorine and iodine, we get the atomic weight of bromine as 35.5 + 126.5/2 = 81

Triad of alkaline earth metals – calcium, strontium and barium have atomic weights 40, 88 and 137 respectively. By taking the arithmetic mean of calcium and barium, we get the atomic weight of strontium as 40+ 137/2 = 88.

This classification was not accepted as many elements could not be grouped in triads. For very low mass or for very high mass elements, the law was not holding good, eg. F, Cl and Br. Atomic mass of Cl is not the arithmetic mean of atomic masses of F and Br.

In periodic table the vertical lines are called groups and the horizontal lines are called periods. Mendeleev suggested the periodic law, ie., the physical and the chemical properties of elements are the periodic functions of their atomic weights.

Mendeleev’s periodic table had certain defects:

Position of hydrogen: Hydrogen had a controversial position, i.e. it was placed in two groups, group 1 (alkali metals) and group 17 (halogens).

• Position of isotopes:
• Position of transition elements:
• Position of rare earth metals:

When Mendeleev presented his periodic table inert gases were not discovered, he easily placed these elements at the end of the table without disturbing the table.

The elements which were discovered later could be easily placed in the gaps provided in the table.

The most accepted form of periodic table is the long form of the periodic table. In 1913 Mosley modified the previous periodic law as it states that the physical and the chemical properties of the elements are the periodic functions of their atomic number. As Mosley found that the atomic number (i.e. the number of protons) is more fundamental property of an atom than its atomic weight.

On the basis of the modern periodic law proposed by Mosley, the long form of Modern or Bohr’s periodic table was prepared, which consists of 18 vertical lines (groups) and 7 horizontal lines (periods). In the long form of periodic table the elements having same number of valence electrons are placed under the same vertical columns called groups.

• The different characteristics of the long form of the periodic table are given below.
• The highly metallic elements are placed on the left hand side of the periodic table.
• The highly non-metallic elements are placed on the right hand side of the periodic table.
• The transition elements are accommodated between metals and non-metals.
• The noble gases are placed in group 18.
• The elements present in the left and right side of the vertical columns are called normal elements or responsive elements. The elements having complete penultimate shells are called normal elements or the representative elements.
• Lanthanides and Actinides are placed outside the main body of the periodic table.
• The seventh period is an incomplete period.

The elements present in different groups are given specific names like

Group Number	Name
1	Alkali Metals
2	Alkaline earth metals
17	Halogens
18	Inert Gases or Noble Gases

The horizontal rows are called periods.

• First period is the shortest period having only 2 elements.
• Second period is a short period consisting of 8 elements.
• Third period is a short period consisting of 8 elements.
• Fourth period is a long period consisting of 18 elements. Ten elements are of transition series and eight elements are normal elements.
• Fifth period is a long period consisting of 18 elements. Eight are normal elements or representative elements and ten are transition elements.
• Sixth period is a very long period consisting of 32 elements. It includes eight normal or representative elements, 10 transition elements and 14 inner transition elements of lanthanide series.
• Seventh period is an incomplete period.
• As we move down the group, the basic character of oxides increases.
• As we move across the period, the acidic character of oxides increases.
• At the end of every period there is a presence of an inert gas with octet configuration.
• Inert gases have zero valency as they can neither lose electron nor they can gain electron as they have a complete octet.
• Metals have 1,2 or 3 electrons in their valence shell.
• Metals can easily lose their outermost 1, 2 or 3 electrons to complete their outermost octet hence they act as reducing agents.
• Non-metals have 5, 6 or 7 electrons in their valence shell.
• Non metals lose electrons to form positively charged particles called cations where as non metals gain electrons to form negatively charged particles called anions.

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Classification and types of Matter in Chemistry from HelpWithAssignment.com

Classification and types of Matter in Chemistry

Matter is anything that has mass and occupies space. It exists in three phases: solids, liquid and gas. A solid has a fixed shape and volume. A liquid has a fixed volume but is not rigid in shape, it takes the shape of the container. A gas has neither a fixed volume nor a rigid shape, it takes on both the volume and the shape of the container.

Matter can be classified into two categories.

Pure substances, each of which has a fixed composition and a unique set of properties, mixtures, composed of two or more substances.

Pure substances are either elements or compounds, where as mixtures can be either homogeneous or heterogeneous.

Elements

An element is a type of matter that cannot be broken down into two or more pure substances. There are 115 elements of which 91 occur naturally.

Many elements are familiar to all of us. The charcoal used in outdoor grills is nearly pure carbon. Electrical wiring, jewelry and water pipes are often made from copper, a metallic element. Another such element, aluminum, is used in many household utensils.

Some elements come in and out of fashion, so to speak. Sixty years ago, elemental silicon was a chemical curiosity. Today ultra pure silicon has become the basis for multibillion dollar semiconductor industry. Lead, on the other hand, is an element in the other direction. A generation ago, it was widely used to make paint pigments, plumbing connections and gasoline additives. Today, because of the toxicity of lead compounds, all of these applications have been banned in the United States.

In chemistry, an element is defined by its symbol. This consists of one or two letters, usually derived from the name of the element. Thus, the symbol for carbon is C. That for aluminum is Al. Sometimes the symbol comes from the Latin name of the element or one of its compounds. The two elements copper and mercury, which were known in ancient times, have the symbols Cu (cuprum) and Hg (Hydrargyrum).

Curiously, several of the most familiar elements are really quite rare. An example is mercury, which has been known since at least 500 B.C., even though its abundance is only 0.00005%. It can easily prepared by heating the red mineral cinnabar. Beyond that, its unique properties make it extremely useful. Because of its high density, mercury is the liquid used in barometers and manometers. Mercury dissolves many metals, forming solutions (amalgams). A silver-mercury-tin amalgam is used in filling teeth.

In contrast, aluminum (abundance 7.5%), despite its usefulness, was little more than a chemical curiosity until about a century ago. It occurs in combined form in clays and rocks, from which it cannot be extracted. In 1886 two young chemists, Charles Hall in the United States and Paul Herroult in France, independently worked out a process for extracting aluminum from relatively rare ore, bauxite. That process is still in use today to produce the element.

Compounds
A compound is a pure substance that contains more than one element. Water is a compound of hydrogen and oxygen. The compounds methane, acetylene and naphthalene all contain the elements carbon and hydrogen, in different proportions.

Compounds have fixed compositions. That is, a given compound always contains the same elements in the same percentage by mass. A sample of pure water contains precisely 11.19% of hydrogen and 88.81% oxygen. In contrast, mixtures can vary in composition. For example, a mixture of hydrogen and oxygen might contain 5, 10, 25 or 60% hydrogen, along with 95, 90, 75 and 40% oxygen.

The properties of compounds are very different from those of the elements that they contain. Ordinary table salt, sodium chloride, is a white, unreactive solid. As one can guess from its name, it contains two elements sodium and chlorine. Sodium (Na) is a shiny, extremely reactive metal. Chlorine (Cl) is a poisonous, greenish-yellow gas. Clearly, when these two elements combine to form sodium chloride, a profound change takes place. Sodium, a metallic element that is soft enough to be cut with a knife and chlorine, a nonmetallic element that is a gas. Both sodium and chlorine combine to form the crystalline compound.

Mixtures

A mixture contains two or more substances combined in such a way that each substance retains its chemical identity. When you shake copper sulfate with sand, the two substances do not react with one another.

There are two types of mixtures

Homogenous or uniform mixtures are ones in which the composition is the same throughout. Another name for a homogenous mixture is solution, which is made up of a solvent, usually taken to be the substance present in largest amount, and one or more solutes. Most commonly, the solvent is liquid, whereas solutes may be solids, liquids or gases. Soda water is a solution of carbon dioxide (solute) and water (solvent). Seawater is a more complex solution in which there are several solid solutes, including sodium chloride; the solvent, is water. It is also possible to have solutions in the solid state. Brass is a solid solution containing the two metals copper and zinc in proportions of 67-90% and 10-33% respectively.

Heterogeneous or nonuniform mixtures are those in which the composition varies throughout. Most rocks fall into this category. In a piece of granite, several components can be distinguished, differing from one another in color.

Many different methods can be used to separate the components of a mixture from one another. A couple of methods that you may have carried out in the laboratory are

Filtration, used to separate a heterogeneous solid liquid mixture. The mixture is passed through a barrier with fine pores, such as filter paper. Copper sulfate, which is water soluble, can be separated from sand by shaking with water. On filtration the sand remains on the paper and the copper sulfate solution passes through it.

Distillation, used to resolve a homogenous solid-liquid mixture. The liquid vaporizes, leaving a residue of the solid in the distilling flask. The liquid is obtained by condensing the vapor. Distillation can be used to separate the components of a water solution of copper sulfate.

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