Physical Sciences Lesson 3 the Atomic Structure and the Chemical Elements

February 5, 2019 | Author: Justin Bird | Category: Nuclear Physics, Nuclear Reaction, Chemical Elements, Radioactive Decay, Atomic Nucleus
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The Atomic Structure and the Chemical Elements The Ideas of the Ancient Greeks on the Elements

Objective  At the end of the lesson, lesson, you should be able able to describe describe the ideas of the Ancient Ancient Greeks on the elements.  What were the early ideas ideas of the Greeks on the elements?

Last Updated: 08.13.16

Learn about it! Many Greek philosophers tried to answer the question "What are the primordial substances from which everything is made up of?"  Some  Some of them believed that there  was only one element element that made made up all materials. Anaximen materials. Anaximenes es thought  thought that it was air; Heraclitus Heraclitus supposed  supposed it was fire; Thales Thales believed  believed that it was water; and Xenophanus Xenophanus assumed  assumed it was earth. Empedocles  proposed that all four – air, fire, water, and earth, are the primordial Empedocles proposed substances. He called them roots roots..

Plato, the founder of the Academy Plato, the Academy in Athens Athens (the  (the first institution of higher learning in the Western world), first used u sed the term element. The word element came from the Greek word “στοιχε ῖ ον” (stoicheion) ον” (stoicheion) which means smallest division.

Plato treated the four elements geometrically and named them Platonic solids. Air  was an octahedron; fire was a tetrahedron; water was an icosahedron; and earth was a cube. He also added a fifth one, a dodecahedron, which was the shape of the Universe.

Try it!  Ancient Chinese, Japanese, and Indians also had their own lists of elements. Research about their classical elements and their characteristics.

What do you think?  What do you think were the bases of the Greek philosophers’ notions of elements?

Key Points 







Some Greek philosophers believed that there was only one element that made up all materials. Anaximenes thought that it was air; Heraclitus supposed it was fire; Thales believed that it was water; and Xenophanus assumed it was earth. Empedocles proposed that all four – air, fire, water, and earth, were the primordial substances. Plato first used the term element. He treated the four elements geometrically and named them Platonic solids.  Aristotle described each element using the qualities hot, cold, wet, or dry. He also added a fifth element, aether.

The Atomic Structure and the Chemical Elements The Atomic Number and the Synthesis of New Elements

Objective

In this lesson, you should be able to explain how the concept of atomic number led to the synthesis of new elements in the laboratory. How does the concept of the atomic number lead to the synthesis of elements?

Last Updated: 08.13.16

Learn about it! Moseley’s X-ray Spectroscopy 

Henry Gwyn-Jeffreys Moseley  was an English physicist who demonstrated that the atomic number, the number of protons in an atom, determines most of the properties of an element. He began his study of radioactivity in Ernest Rutherford’s laboratory but later decided to explore more on X-rays. In 1913, Moseley published a paper on the arrangement of the elements in the Periodic table based on their atomic numbers. He used X-ray spectroscopy to determine the atomic number of an element. He bombarded a beam of electrons to different elements and measured their X-ray spectral lines. His results clearly showed that frequency of the X-rays given off by an element was mathematically related to the position of that element in the Periodic table. The frequency is proportional to the charge of the nucleus, or the atomic number.  When the elements were arranged according to their atomic numbers, there were four gaps in the table. These gaps corresponded to the atomic numbers 43, 61, 85, and 87. These elements were later synthesized in the laboratory through nuclear transmutations.

Learn about it! The Discovery of the Missing Elements

Recall that in 1925, there were four vacancies in the periodic table corresponding to the atomic numbers 43, 61, 85, and 87. Two of these elements were synthesized in the laboratory using particle accelerators.  A particle accelerator is a device that is used to speed up the protons to overcome the repulsion between the protons and the target atomic nuclei by using magnetic and electrical fields. It is used to synthesize new elements. In 1937, American physicist Ernest Lawrence synthesized element with atomic number 43 using a linear particle accelerator. He bombarded molybdenum (Z=42) with fast-moving neutrons. The newly synthesized element was named Technetium (Tc) after the Greek word "technêtos" meaning “artificial.” Tc was the first man-made element. In 1940, Dale Corson, K. Mackenzie, and Emilio Segre discovered element with atomic number 85. They bombarded atoms of bismuth (Z=83) with fast-moving alpha particles in a cyclotron. A cyclotron is a particle accelerator that uses alternating electric field to accelerate particles that move in a spiral path in the presence of a magnetic field. Element-85 was named astatine from the Greek word “astatos” meaning unstable. The two other elements with atomic numbers 61 and 87 were discovered through studies in radioactivity. Element-61 (Promethium) was discovered as a decay product of the

fission of uranium while element-87 (Francium) was discovered as a breakdown product of uranium.

Learn about it! Synthesis of New Elements

In the 1930s, the heaviest element known was uranium, with an atomic number 92. Early in 1940, Edwin McMillan proved that an element having an atomic number 93 could be created. He used a particle accelerator to bombard uranium with neutrons and created an element with an atomic number 93 which he named neptunium.  At the end of 1940, element-94 was synthesized by Seaborg, McMillan, Kennedy , and Wahl. They bombarded uranium with deuterons (particles composed of a proton and a neutron) in a cyclotron. Element-94 was named plutonium. Elements with atomic numbers greater than 92 (atomic number of uranium) are called transuranium elements. Hence, neptunium and plutonium are both transuranium elements. They are unstable and decay radioactively into other elements.  All of these elements were discovered in the laboratory as artificially generated synthetic elements. They are prepared using nuclear reactors or particle accelerators. In the next lesson, you will learn the nuclear reactions involved in the synthesis of these transuranium elements.

Try it! Research on the latest instruments used in preparing new elements in the laboratory.  What were the instruments used in preparing the newest four elements, nihonium, moscovium, tennessine, and oganesson?

What do you think?  Why do scientists study and synthesize new transuranium elements in the laboratory?  What are the uses of these elements?

Key Points 



The atomic number is the number of protons (positively charged particles) in an atom. Henry Gwyn-Jeffreys Moseley   was an English physicist who demonstrated that the atomic number, the number of protons in an atom, determines most of the properties of an element.









In 1919, Ernest Rutherford successfully carried out a nuclear transmutation reaction — a process of transforming one element or isotope into another element. In 1925, there were four vacancies in the periodic table corresponding to the atomic numbers 43, 61, 85, and 87. Elements with atomic numbers 43 and 85  were synthesized using particle accelerators.  A particle accelerator is a device that is used to speed up the protons to overcome the repulsion between the protons and the target atomic nuclei by using magnetic and electrical fields. It is used to synthesize new elements. Elements with atomic numbers greater than 92 (atomic number of uranium) are called transuranium elements. They were discovered in the laboratory using nuclear reactors or particle accelerators.

The Atomic Structure and the Chemical Elements The Nuclear Reactions Involved in the Synthesis of New Elements

Objective In this lesson, you should be able to write the nuclear reactions involved in the synthesis of new elements. Nuclear reaction is the process where two nuclei collide to produce new nuclei. There are two types of nuclear reaction: nuclear transmutation and nuclear decay reaction.  What happens during nuclear transmutation?

Last Updated: 08.13.16

Learn about it! Nuclear Transmutation

 A nuclear transmutation is a reaction involving the transformation of one element into another element. It happens when a nucleus reacts with a subatomic particle to produce a more massive nucleus. It occurs only on special conditions such as the collision of the target nuclei with a beam of particles with high energies. In 1919, the first successful transmutation was done by Rutherford. He bombarded nitrogen nuclei with alpha particles to form oxygen nuclei. In the next decades, other nuclear reactions were discovered by bombarding other elements with alpha particles. However, since these particles have a strong repulsive force with the target nuclei (both

are positively charged), the progress in discovering elements was slow. Scientists then tried other particles with higher energies. In 1932, major advancements in nuclear reactions took place. Particle accelerators, which use a projectile of high-energy particles, were invented. Transuranium Elements Transuranium elements are elements whose atomic numbers are greater than 92. They are all unstable and undergo radioactive decay. Many of the transuranium elements were prepared using particle accelerators, and much of this work was facilitated by a group of scientists led by the American chemist, Glenn Theodore Seaborg, and later, nuclear scientist Albert Ghiorso in the University of California. The most effective way of preparing transuranium elements specifically in the lower members of the series (elements 93 to 95), was through nitrogen bombardment. As for the heavier transuranium elements (96 to 101), they were generally prepared with highenergy positive ions which include the use of deuterons, carbon nuclei, and ions.

Key Points  



Nuclear reaction is the process where two nuclei collide to produce new ones. There are two types of nuclear reaction: nuclear transmutation and nuclear decay reaction. Nuclear transmutation is a reaction involving the transformation of one element or isotope into another element. This process occurs only in special















conditions such as the collision of target nuclei with a beam of particles with high energies. Transuranium elements are elements whose atomic numbers are greater than 92. They are all unstable and undergo radioactive decay. High-energy positive ions which include the use of deuterons, carbon nuclei, and ions are used for synthesizing massive nuclei. Nuclear decay reaction is a reaction in which the nucleus of an element emits radiation and transforms into another nucleus. The nuclear decay reactions involved in the synthesis of transuranium elements are alpha decay, beta decay, and spontaneous fission.  Alpha decay  produces daughter nucleus with an atomic number reduced by two and a mass number reduced by four compared with the parent nucleus. In beta decay , the atomic number of the nucleus increases by one while its mass number remains the same. In spontaneous fission, several fission products are produced from the parent nucleus. The sum of their mass numbers is equal to the mass number of the parent nucleus. Similarly, the sum of their atomic numbers is equal to the atomic number of the parent nucleus.

The Atomic Structure and the Chemical Elements Understanding the Concept of the Chemical Elements: The Contributions of John Dalton

Objectives In this lesson, you will be able to cite the contributions of John Dalton toward the understanding of the concept of the chemical elements and explain how Dalton’s theory contributed to the discovery of other elements.  Who is John Dalton? What are his contributions to the concept of chemical elements?

Last Updated: 08.13.16

Learn about it!

Dalton’s Atomic Theory 

John Dalton was an English chemist known for his pioneering work in the development of the atomic theory. In his theory, he proposed that elements differ due to the mass of their atoms. The postulates of Dalton’s atomic theory are: 1.  All matter is composed of very small, indivisible particles called atoms. 2.  All atoms of a given element are identical in properties and masses but differ from another element’s. 3.  Atoms cannot be created or destroyed. 4.  Atoms of the same or different elements may combine with each other in a fixed, whole number ratio. 5.  Atoms may combine, separate, or rearrange in chemical reactions.

The atomic theory had been revised over the years with the discovery of isotopes, subatomic particles, and nuclear reactions. However, Dalton’s atomic theory has been  widely recognized because it became the foundation of the modern concept of the atom.

Learn about it! Dalton's Table of Elements

Dalton published his atomic theory in New System of Chemical Philosophy. Back then, he needed to propose a new set of standard symbols for chemical elements  because the Greeks' and alchemists' symbols for elements were not supported by his theory. He first published his table of elements according to increasing relative atomic  weights. The elements were hydrogen, carbon, oxygen, nitrogen, sulfur, and phosphorus. He assumed that the atomic weight of hydrogen was one, and calculated the rest of the elements' atomic weights based on hydrogen. After more experiments, he listed 20 elements with different symbols and atomic weights.

 A binary  compound is formed by two elements. For example, two atoms of azote (nitrogen) may combine to form a binary compound.

 A ternary  compound is formed by three atoms. For example, when one atom of nitrogen combines with two atoms of oxygen, then the resulting compound, nitrogen dioxide, is ternary.

Similarly, quarternary  compounds are made of four atoms; quinquenary  are made of five; sextenary  are made of six; and septenary  are made of seven. Dalton’s system of naming elements and compounds were then replaced with the chemical symbols and formulae by Jons Berzelius. Berzelius’ symbols are the ones we use today.

What do you think?

 Why are the chemical symbols and formulae we use today different with those proposed  by Dalton?

Key Points 





John Dalton developed the atomic theory, which has been widely recognized  because it became the foundation of the modern concept of the atom. Dalton published his atomic theory together with his symbols of chemical elements in the New System of Chemical Philosophy. Dalton developed the law of multiple proportions, which states that atoms of elements combine in small whole numbers, not in fraction numbers, to form a number of possible compounds.

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