Atoms and the Periodic Table

1. Atoms and the Periodic Table Chapter Three

2. Atoms are the tiny parts of matter that determine it’s properties.

3. Democritus, a Greek philosopher, explained the atom. He said that you could take a pair of shears and cut a piece of copper in two and sometime, you would reach a piece that couldn’t be cut anymore. He named this particle an atom meaning indivisible.

4. Although Democritus offered a good explanation, scientists needed a theory to model their thoughts about the atom.

5. In 1808, an English chemistry teacher named John Dalton proposed the atomic theory. From data gathered in his student’s experiments, he explained the theories mentioned above and laid the foundation for understanding the atom. • All matter is composed of small particles called atoms. • Atoms of the same element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and properties. • Atoms combine to form compounds. • Atoms cannot be subdivided, created, or destroyed.

6. Since Dalton’s theory, two everyday facts emerged about the atom: • Atoms are the smallest part of an element that still retains the properties of that element. • Atoms join together to form molecules of compounds that always have the same chemical composition.

7. Since the atom was defined in theory, scientists set to work to develop a model of it.

8. Some of the first discoveries about the atom were made using a cathode ray tube (Crook’s Tube).

9. Experiments using the tube showed that an object placed in the tube cast a shadow and the cathode ray could cause a paddle wheel to roll along rails through the tube. This indicated that the ray was of a particle nature. Cathode rays are deflected by a magnetic field and the rays were deflected away from a negatively charged object. This would indicated that they carry a negative charge.

10. In 1897, Joseph John Thomson measured the ratio of the charge of the cathode particles to their mass and found it always to be the same. Thomson concluded that cathode rays where made up of identical, negatively charged particles with minimum mass which were later named electrons.

11. In 1911, New Zealander Earnest Rutherford and his associates Hans Geiger and Ernest Marsden bombarded a thin gold foil with alpha particles emitted from a radioactive source. They expected an evenly charged force field in the foil so they planned that the particles would pass straight through. When the detector was studied, they were greatly surprised to find about 1 in 8000 particles bounced straight back! Rutherford exclaimed that this would be like firing a 15 inch artillery shell into a piece of tissue paper and have it bounce back.

12. Rutherford concluded that atoms must contain a very small, dense, positively charged nucleus. The nucleus of the atom is very small. If it were the size of a marble, the atom would be larger than a football field.

13. All atoms have two general regions, a tiny, massive, positively charged nucleus surrounded by a negatively charged electron cloud. The electrons have very little mass and carry a negative charge. The nucleus is made up of positively charged protons and neutrons that have no charge. The protons and neutrons are massive when compared to electrons.

14. Unreacted atoms have no net charge. Otherwise, things around you would constantly be shocking you. To keep this neutral state, atoms have the same number of protons and electrons. The number of protons and electrons is unique for each element.

15. Many scientists asks questions like: What causes the chemical properties of an element and why don’t the electrons crash into the protons?

16. Scientists soon became curious about how protons, neutrons, and electrons were arranged in the atom. In 1913, Neils Bohr proposed a model of an atom. He stated that the electron circled the nucleus in certain allowed orbits like floors on a building. It could be excited to higher floors, but it could not orbit between floors. This gave a picture of an atom similar to the solar system. By 1925, Bohr’s model had to be refined as it did not explain larger atoms.

17. In 1926, Erwin Schrodinger studied electrons as waves. His theory mathematically describes the wave properties of electrons and other sub atomic particles.

18. An electron’s exact location cannot be determined because it moves too fast and the direction is constantly changing like the blades of a spinning fan. We assign electrons energy levels where they are likely to be found because of the amount of energy that they contain. The more energy, the higher the level, and the farther from the nucleus.

19. Different energy levels can hold different numbers of electrons.

20. Orbitals are regions within each energy level where electrons are most likely to be found. They are designated s, p, d, and f.

21. Orbital May Contain:s orbital = 2p orbital = 6d orbital = 10f orbital = 14

22. It is important to remember that every atom has from one to eight valence electrons.

23. A Guided Tour of the Periodic Table Section Two

24. As you may be beginning to see, it is the structure of the atom that indicates it’s properties. This structure can be interpreted from the periodic table. • The atomic number increases chronologically across the periodic table. It identifies the element by giving the number of protons and neutrons. Uranium, number 92, is the largest naturally occuring element. • By rounding off the average atomic mass, one can get the mass number which gives the number of protons and neutrons in the nucleus.

25. An ion is a charged atom. Elements near the left hand side of the table tend to lose electrons to become positive ions. Elements near the right hand side of the table tend to gain electrons to become ions. It is the tendency of each atom to attain the electron structure of Group 18 which is stable.

26. Imagine going into a large super store and the aisles have no labels. In fact, the groceries are mixed with the auto parts and the clothing. The cds and the dvds are with the toys and the outdoor items are displayed with the toys. And, the greenhouse is a bluehouse. What a mess! This was the chemist’s situation in 1869. Over 60 elements had been discovered, but there was no arrangement for them. It was hard to use them because they weren’t organized.

28. In the 1860’s, a Russian, Dimitri Mendeleev , began looking for patterns to organize the elements. He recorded the properties of the elements on cards and then tried arranging the cards in different ways. After much thought and work, he discovered that there was a repeating pattern of properties when the elements were arranged according to their atomic mass.From this work, the periodic law emerged. The Periodic Law states that the chemical and physical properties of elements occur in a regular pattern when arranged in order of their numbers of protons.(Note the Periodic Table on PP 112-113)

29. Notice that hydrogen has one s electron and helium has 2 s electrons. Lithium has a full 1st energy level and 1s electron in the 2nd energy level. These rows across the table are called periods.

30. Vertical columns in the periodic table are called groups. Each element in a group has the same number of valence electrons. This gives them similar chemical properties.

31. As you may be beginning to see, it is the structure of the atom that indicates it’s properties. This structure can be interpreted from the periodic table.

32. Families of Elements Section Three

33. Elements that are in the same group on the periodic table are called families. They have many of the same physical and chemical properties. Families of elements may be classified into larger divisions.

34. Elements that are in the same group on the periodic table are called families. They have many of the same physical and chemical properties. Families of elements may be classified into larger divisions. • Metals • Nonmetals • Metalloids

35. Metals are: • Shiney solids • Dense • Of high tensile strength • Malleble and ductile • Conductors of heat and electricity

36. The metals include several families: • Group 1 (shaded blue) is called the alkali metals. They are soft enough to be cut with a knife and lithium, sodium, and potassium are so low in density that they will float on water. • These elements are the most reactive metals because they only have one electron to involve in a reaction. They react violently with many compounds, even water. Because of this, they are usually stored under oil. Because of this they are never found free in nature. Compounds formed by these elements have many uses. Sodium chloride makes up table salt. Sodium hydroxide is used in drain and oven cleaners. Potassium bromide is important to photography.

37. Metals continued: • Group 2 is called the alkaline-earth metals. These metals are low density, silvery metals. They are not as reactive as Group 1 metals because they have to give away two electrons. But they still react with water and are still not found free in nature. • Calcium is an important component of sea shells, coral, and many of the base rocks of the earth. • Magnesium is an important earth component and is found in epsom salts and milk of magnesia. They are used in light weight alloys and their compounds are in cement, plaster, and chalk.

38. Metals, continued: • Group 3-12 is called the transition metals. This group includes some of the common metals that we encounter each day. • The transition metals are not as reactive as Group 1 and 2 metals. Most of them have the standard metal properties that we described earlier. • Some transition metals such as gold are found in their free or native state. • There are many common uses for transition metals: Copper is found in wiring. Tungsten makes up light bulb filaments. Iron, cobalt, copper, and manganese are important trace minerals in the body. Mercury, the only liquid metal is found in thermometers.

39. Metals, continued: • The lanthanides and actinides are separated from periods six and seven and are placed at the bottom of the table. They are placed at the bottom so that the table will not be too wide. • The elements of each group seems to have similar properties. • The lanthanides are shiny, reactive metals that are used to make various kinds of steel. • The actinides are all radioactive and are unstable. They are components of nuclear energy and americium is used in smoke detectors. Technetium is used in medical applications. Promethium is used in glow in the dark paints.

40. Nonmetals are found near the right side of the periodic table and include parts of group 13-16 and all of groups 17 and 18. Nonmetals lack the characteristics of metals.

41. Carbon is an important nonmetal. Carbon in its pure state can occur as graphite, diamond, or fullerenes with some amorphous forms.

42. Carbon can combine with other elements to form millions of compounds including those found in the human body.

43. Oxygen, nitrogen, and sulfur are other common nonmetals.

44. Group 17 is the most reactive nonmetals. They have seven electrons in their outer orbital and only need one more to fill it. They are called halogens which means “salt former”. Chlorine is an important disinfectant for water supplies. Iodine is an important antiseptic and is important for normal thyroid function. Flourine is important in toothpastes.

45. The Noble Gases • The noble gases of Group 18 are the most non-reactive elements. They have 8 electrons in their outer energy level and will not gain or lose any more. • All the noble gases are found in small amounts in the earth’s atmosphere. • Helium is found with natural gas and is important in filling balloons. • Ordinary light bulbs are filled with argon because it is very non-reactive and will not allow the filament to burn out. • Neon and other noble gases produce color when high voltage electricity is passed through them.

46. The metalloids or semiconductors are found along the zigzag line between the metals and nonmetals. • Boron is an extremely hard element that is often added to steel. • Arsenic is a poisonous white solid. • Antimony is a metallic solid that is sometimes used as a fire retardent. • Tellurium is a white solid whose conductivity is affected by light. • Silicon is one of the earth’s most common elements yet it is one of the most important components of semiconductor devices such as transistors.

47. Using Moles To Count Atoms Section Four

48. People have devised a number of ways to count objects from dozens, to grosses, to bundles. Chemists have devised a way to count atoms so that the masses used in reactions can be predicted.

49. One mole is the amount of substance that contains Avagadro’s Number of atoms.Avagadro’s Number is the number of particles in exactly one mole of a substance. 6.022 x 1023The molar mass of a substance is the mass of one mole stated in grams. It is numerically equal to the atomic mass.