Properties of Atoms and the Periodic Table

1. Chapter 17 Properties of Atoms and the Periodic Table

2. Structure of the Atom Chemical symbols are used as a shorthand to write the name of an element. Ex: Aluminum (Al), Calcium (Ca), Carbon (C), Oxygen (O), Gold (Au), Iron (Fe), Mercury (Hg), etc. Chemical symbols consist of one capital letter or a capital letter plus one or two small letters. Some symbols are derived from either Latin or German Names. Ex: Silver = Argentum = Ag, Mercury = Hydragyrum = Hg, Gold = Aurum = Au. Elements are named in a variety of ways: To honor scientists, places, properties, planets, Gods, etc.

3. Structure of the Atom An element is matter that is composed of one type of atom. Atom: The smallest piece of matter that still retains the property of the element. Atoms are composed of particles called protons, neutrons, and electrons. Protons and neutrons are found in a small, positively charged center of the atom called the nucleus that is surrounded by a cloud containing electrons.

4. Structure of the Atom Protons are particles with an electrical charge of +1. Neutrons are neutral particles that do not have an electrical charge. Electrons are particles with an electrical charge of -1. Atoms of different elements differ in the number of protons they contain.

5. Structure of the Atom Protons and neutrons are made of smaller particles called quarks. Scientists have confirmed the existence of six uniquely different quarks. Up, Down, Strange, Charm, Bottom, and Top. Scientists theorize that an arrangement of three quarks held together with strong nuclear force produces a proton. Another arrangement of three quarks produces a neutron.

6. Structure of the Atom

7. Structure of the Atom In order to study quarks, scientists accelerate charged particles to tremendous speeds and then force them to collide with protons. This collision causes them to break apart. The particles that result from the collision can be detected by various collection devices. Ex: Bubble Chamber: A vessel filled with a superheated transparent liquid used to detect electrically charged particles moving through it. The charged particle deposits sufficient energy in the liquid that it begins to boil along its path, forming a string of bubbles. Scientists often use multiple collection devices to collect the most possible information about the particles created in a collision. Ex: Bubble Chamber, Cloud Chamber, Photographic plate, Photomultiplier, etc. Scientists use inference to identify the subatomic particles and the reveal information about each particle’s inner structure.

8. Structure of the Atom

9. Structure of the Atom The Fermi National Accelerator Laboratory (FNAL) houses a machine that can generate the forces that are required to collide protons. This machine, the Tevatron, is approximately 6.4-km in circumference. Electric and magnetic fields are used to accelerate, focus, and collide the fast-moving particles. Why is such a long tunnel needed? To accelerate the particles so they will have enough energy to break each other apart when they collide.

10. Structure of the Atom Scientists found five quarks and hypothesized that a sixth quark existed. It took nearly 450 scientists from around the world several years to find the sixth quark. The tracks of the sixth quark were hard to detect because only about 1-billionth of a percent of the proton collisions performed showed the presence of a sixth quark (top quark).

11. Structure of the Atom When did the idea of the atom first come about? About 400-B.C. Greek philosopher named Democritus proposed the idea that atoms make up all substances. He also stated that the atom was smallest particle of matter. Another Greek philosopher, Aristotle, argued against Democritus and said that matter was uniform throughout and continuous. People accepted Aristotle’s idea for about 2000 years.

12. Structure of the Atom In the 1800’s John Dalton was able to provide evidence that atoms exist. Dalton’s model of the atom, a solid sphere, was an early model of the atom. Dalton’s modernization of Aristotle’s idea of the atom provided a physical explanation for chemical reactions. Scientists could then express these reactions in quantitative terms using chemical symbols and equations.

13. Structure of the Atom By 1926, scientists had developed the electron cloud model of the atom that is in use today. An electron cloud is the area around the nucleus of an atom where its electrons are most likely found. The electron cloud is 100,000 times larger than the diameter of the nucleus. Each electron in the electron cloud is much smaller than a single proton(1840 electrons = 1 proton). Because the electron is so small and moving so fast, it is impossible to describe its exact location in an atom. An electron cloud is a blur containing all of the electrons of the atom somewhere within it.

14. Structure of the Atom

15. Masses of Atoms The nucleus contains most of the mass of the atom (protons + neutrons). The electron’s mass is so small that it is considered negligible when finding the mass of an atom. The mass of a proton and the mass of a neutron are nearly identical, so they are both given a mass value of 1-amu (amu = atomic mass unit). AMU = 1/12th the mass of a carbon atom containing six protons and six neutrons (carbon-12 atom).

16. Masses of Atoms The number of protons in an atom tells you what type of atom you have. Examples: If an atom has 12 protons, it is the element Magnesium. If an atom has 20 protons, it is the element Calcium. The number of protons in an atom is called the atomic number. On the periodic table, the atomic number is the whole number that appears above the symbol of the element. The atomic number also gives you the number of electrons in an atom. Example: Oxygen = Atomic Number 8 = 8 protons and 8 electrons. If an atom of an element gains or loses protons, it becomes a different element (radioactive).

17. Masses of Atoms The mass number of an atom is the sum of the number of protons and the number of neutrons in the nucleus of an atom. If you know the mass number and the atomic number of an atom, you can calculate the number of neutrons in an atom. The number of neutrons is equal to the mass number minus the atomic number. # of Neutrons = Mass Number – Atomic Number

18. Masses of Atoms Atoms of the same element with different number of neutrons can have different properties. Example: Carbon-12, the most common form of carbon on Earth is not radioactive, whereas Carbon-14, which is also present on Earth, is radioactive. So Carbon-14 is used in carbon-dating. Carbon Dating: A way to determine the approximate age of once-living organisms. Carbon-14 decays with a half-life of about 5715 years. Half-Life: Amount of time it takes for half of the nuclei in a sample of a radioactive isotope to decay. Each isotope has its own characteristic half-life. Some isotopes have half-life’s of millions of years, while others are only minutes.

19. Masses of Atoms These are called isotopes. Isotope: Atoms of the same element that have different numbers of neutrons. Most elements have at least 2 isotopes. Some elements have up to 10 isotopes (Ex: Tin). Because the number of neutrons are different in isotopes, the mass numbers are also different. To identify isotopes, scientists have given them a short-hand notation by using the name of the element followed by the mass number of the isotope. Ex: Carbon-12, Carbon-14, Boron-10, Boron-11, Uranium-235, Uranium-238.

20. Masses of Atoms Because most elements have more than one isotope, each element has an average atomic mass. Average Atomic Mass: The weighted average mass of the mixture of isotopes. Example: 4 out of 5 atoms or boron are boron-11, and one out of 5 is boron-10. To find the weighted average or the average atomic mass of boron, you would solve the following equation: 4/5(11 amu) + 1/5(10 amu) = Average Atomic Mass Average Atomic Mass = 10.8 amu This is what you see on the periodic table, the average atomic mass.

21. The Periodic Table Periodic means “repeated in a pattern.” In the late 1800’s a Russian chemist, Dmitri Mendeleev, searched for a way to organize the elements. He arranged the elements in order of increasing atomic mass. When he did this, he noticed that a pattern kept reoccurring. Because the pattern repeated, it was considered to be periodic. This arrangement is called the periodic table of elements. Periodic Table: Elements are arranged by increasing atomic number and by changes in physical and chemical properties.

22. The Periodic Table Mendeleev left some blank spaces in his periodic table for elements that had yet been discovered. He predicted the missing elements properties and atomic masses from the elements surrounding the blank spaces. His predictions proved to be accurate. Scientists later discovered these missing elements and found that their properties were extremely close to what Mendeleev had predicted. Example: Germanium (ekasilicon).

23. The Periodic Table

24. The Periodic Table Mendeleev’s periodic table was arranged by increasing atomic mass. In 1913, Henry G.J. Moseley, changed the arrangement of the periodic table from increasing atomic mass to increasing atomic number. This new arrangement corrected some the the problems that had occurred in the old periodic table. The current periodic table uses Moseley’s arrangement of the elements. Examples: Cobalt (Co) and Nickel (Ni) – there masses (Cobalt = 58.9332 amu and Nickel = 58.69 amu) decrease from left to right.

25. The Periodic Table The vertical columns in the periodic table are called groups or families, and are numbered 1 thru 18. Elements in each group have similar chemical and physical properties. Example: Group 11 – Copper, silver, and gold have similar properties; each is a shiny metal and a good conductor of electricity and heat. Example: Group 18 – The Noble Gases – these elements are all completely unreactive and do not bond with any other elements.

26. The Periodic Table Elements in the same group or family are similar because of their electron cloud structure. Scientists have found that electrons within the electron cloud have different amounts of energy. Scientists model the energy differences of the electrons by placing them in energy levels. Energy levels nearer the nucleus have lower energy than those levels that are farther away. Electrons fill these energy levels from the inner levels (closer to the nucleus) to the outer levels (farther from the nucleus).

27. The Periodic Table

28. The Periodic Table Elements that are in the same group have the same number of electrons in their outer energy level. It is the number of electrons in the outer energy level that determines the chemical properties of an element. These energy levels are named using numbers one to seven. The maximum number of electrons that can be contained in each of the first four levels is: 1st Energy Level = 2 electrons 2nd Energy Level = 8 electrons 3rd Energy Level = 18 electrons 4th Energy Level = 32 electrons

29. The Periodic Table A complete or stable outer energy level will contain 8 electrons. Example: Group 18 Elements - The Noble Gases – the reason that they are so unreactive is that their outer energy level is full with 8 electrons.In elements in periods three and higher, additional electrons can be added to inner shell energy levels although the outer energy level contains only 8 electrons.

30. The Periodic Table Remember that the atomic number found on the periodic table is equal to the number of electrons in an atom. The first row has hydrogen with 1 electron and helium with two electrons both in energy level 1. Because energy level one is the outermost level containing an electron, hydrogen has one outer electron and helium has two outer electrons. Recall that energy level one can hold only two electrons, therefore, helium has a full or complete outer energy level.

31. The Periodic Table These outer electrons are so important in determining the chemical properties of an element tat a special way to represent them has been developed. G.N. Lewis, an American chemist, created his method, called an electron dot diagram. Electron Dot Diagram: Uses the symbol of the element and dots to represent the electrons in the outer energy level. Electron dot diagrams are used also to show how the electrons in the outer energy level are bonded when elements combine to form compounds.

32. The Periodic Table The elements in Group 1 have one electron in their outer energy level. This electron dot diagram represents that one electron.

33. The Periodic Table The elements in Group 17 (the halogens) have electron dot diagrams similar to chlorine. Halogens = Salt Forming. All halogens have 7 electrons in their outer energy levels. Since all members of a group have the same number of electrons in their outer energy level, group members will undergo chemical reactions in similar ways.

34. The Periodic Table A common property of the halogens is the ability to form compounds readily with elements in Group 1. Group 1 elements have only 1 electron in their outer energy level. Example: The Group 1 element, sodium, reacts easily with the Group 17 element, chlorine. The result is the compound sodium chloride (NaCl) or ordinary table salt.

35. The Periodic Table Neon, a member of Group 18, has a full outer energy level. Neon has 8 electrons in its outer energy level, making it unreactive.

36. The Periodic Table The horizontal rows in the periodic table are called periods. The elements increase by 1 proton and one electron as you go from left to right in a period. Elements in the same period do not have similar chemical and physical properties.

37. The Periodic Table All of the elements in the blue squares are metals. Most metals exist as solids at room temperature (except Mercury). They are shiny, can be drawn into wires (ductile), can be pounded into thin sheets (malleable), and are good conductors of heat and electricity.

38. The Periodic Table Those elements on the right side of the periodic table, in yellow, are nonmetals. Most nonmetals are gases, are brittle, and are poor conductors of heat and electricity at room temperature.

39. The Periodic Table The elements in green are metalloids or semimetals. They have properties of both metals and nonmetals.

40. The Periodic Table Scientist around the world continue their research to synthesize new elements. In 1994, scientists at the Heavy-Ion Research Laboratory in Darmstadt, Germany, discovered element 111. As of 1998, only one isotope of elements has been found. The isotope had a life span of 0.002-seconds. In 1996, element 112 was discovered at the same laboratory. As of 1998, only one isotope of element 112 has been found. The life span of this isotope was 0.00048-seconds. Both of these elements are produced in the laboratory by joining smaller atoms into a single atom. The search for elements with higher atomic numbers continues. Scientists think that they have synthesized elements 114 and 116, however, it has not yet been confirmed.

41. The Periodic Table Using technology, scientists are finding the same elements throughout the universe. They have been able to study only a small portion of the universe, though, because it is so vast. Many scientists believe that hydrogen and helium are the building blocks of other elements. Atoms join together within stars to produce elements with atomic numbers greater than 1 or 2. Exploding stars, or supernovas, give scientists evidence of this theory. When stars go supernova a mixture of elements, including iron, are flung into the galaxy.

42. The Periodic Table Many scientists believe that supernovas have spread the elements that are found throughout the universe. Promethium, technetium, and elements with an atomic number above 92 are rare or not found on Earth. Some of these elements are found only in trace amounts in Earth’s crust as a result of uranium decay. Others have only been found in stars.