Understanding Atoms and the Periodic Table Structure

1. Chapter 3: Atoms & the Periodic Table

2. These are uranium atoms. What do they look like? What actually are you seeing?

3. Section 1: Inside an Atom • Structure of an Atom • An atomconsists of a nucleus surrounded by one or more electrons. • The nucleus is the tiny, central core of an atom

5. An atom has 3 types of particles: • (subatomic particles) • Protons -- have a positive electric charge & are located in the nucleus • Neutrons-- have a neutral electric charge & are located in the nucleus • Electrons -- have a negative electric charge & are located outside of the nucleus

6. The atom’s nucleus contains protons and neutrons. The high energy electrons move in the space around the nucleus called the electron cloud.

7. Atomic Number = the number of protons in a nucleus • Is a unique property that identifies the element • Ex: Every Carbon atom has 6 protons • In an atom, the # of protons & the # of electrons are equal, making the atom neutral.

8. Atomic Mass = the average mass of one atom of an element • Since atoms are so small, they are measured in atomic mass units (amu). • The mass of a proton or a neutron is about one amu. • Electrons are much smaller.

9. It takes almost 2000 electrons to equal one amu! • Therefore, most of an atom’s mass is in the nucleus of an atom. • Ex: An atom that has 6 protons, 6 neutrons, and 6 electrons has a mass of about 12 amu’s

10. The number of protons in an element does not change • the number of neutrons can change. (Isotopes of an atom) • Ex: Carbon atoms always have 6 protons. But they may have 5,6,7,or 8 neutrons. (Isotopes of carbon) • This means that the amu will vary.

11. Since neutrons don’t play a role in chemical reactions, the chemical properties of each element are the same despite having different masses.

12. The Role of Electrons • Electrons move around the nucleus so fast that it is impossible to know exactly where any electron is at a particular time! • It’s like a spherical cloud of negatively charged electrons.

13. Imagine the blades of a moving fan.

14. Little Particles, Big Spacesvideo • The space in which electrons move is HUGE!!! • Ex: Imagine standing at the pitcher’s mound in a baseball stadium. If the nucleus were the size of a pencil eraser, the electrons could be in the outfield or the top row of seats!

15. So, what composes the majority of an atom’s mass? What composes the majority of an atom’s volume?

16. Valence Electrons • Electrons in an atom are not all the same distance away from the nucleus. • Valence electrons are those that are the farthest away.

17. A chemical bond forms between 2 atoms when valence electrons move between them. • Valence electrons may be transferred from one atom to another, or they may be shared between atoms.

18. The number of valence electrons can vary from 1 to 8. • Each element has a typical # of valance electrons. • Ex: Oxygen has 6, Carbon has 4, & Hydrogen has 1

19. Electron Dot Diagrams • Used to represent valence electrons. • A symbol for an element is surrounded by dots. Each dot stands for one valence electron.

20. When atoms have 8 or 0 valence electrons, the atom becomes more stable- or less reactive than they were before.

21. Models of Atoms • 1808- Dalton Model • each element is made of small atoms & different elements have atoms of different masses • atoms similar to tiny, solid balls

23. Models of Atoms • 1897-Thomson Model • Atom is a positively charged sphere with electrons embedded in it. • Similar to a muffin w/ berries scattered through it

24. THOMPSON’S MODEL

25. Models of Atoms • 1904- Nagaoka Model • Atom had a large sphere in the center with a positive charge. • Electrons revolved around the sphere like planets revolve around the sun.

27. Models of Atoms • 1911-Rutherford Model • Atom is mostly empty space. • Electrons orbit randomly around a small, positively charged nucleus.

28. RUTHERFORD’S MODEL

29. Rutherford’s Experiment - showed that particles were deflected by something in atoms. He deduced that this was a positively charged nucleus.

30. Models of Atoms • 1913-Bohr Model • Electrons move in specific layers, or shells • Atoms absorb or give off energy when the electrons move from one shell to another

31. BOHR’S MODEL

32. Models of Atoms • 1932-Chadwick Model • Discovered the neutron. • The existence of the neutron explained why atoms were heavier than the total mass of their protons and electrons.

33. Models of Atoms • 1920’s to Present- Modern Model • Electrons form a negatively charged cloud around the nucleus. • It’s impossible to determine the exact location of an electron

35. Section 2: Organizing the Elements

36. The Periodic Table • Mendeleev: patterns appeared when the elements were arranged in order of increasing atomic mass. • Sometimes this method didn’t work, so he would put the elements in a “best fit” location.

37. Modern Periodic Table • Based on atomic number (discovered in the 1900’s) rather than atomic mass • the properties of the elements repeat in each period (row) of the table

38. This is Henry Mosley, a British scientist. He created the modern periodic table of elements by placing the elements in order of their atomic number instead of mass. (He was only 27 years old when he was killed in WWI.)

40. Reading the Periodic Table • Each square of the table usually includes the element’s atomic number, chemical symbol, name, and atomic mass.

41. Element’s Name Atomic Number Element’s Symbol Atomic Mass

42. An element’s properties can be predicted from its location in the periodic table!

43. Organization of the Periodic Table • Main body of table has 18 vertical columns & 7 horizontal rows • The elements in a column are called a group • Groups are also known as families

44. Group Numbers

45. The elements in each group, or family, have similar characteristics. • Ex: elements in group 1A are all metals that react violently with water. Elements in group 18 rarely react at all

46. Each horizontal row across the table is called a period • Elements in a period have very different properties • As you move across a period from left to right, those properties change according to a pattern

47. Periods

48. Why the Periodic Table Works • Because it is based on the structure of atoms, especially the valence electrons. • Elements in a family all have the same number of valence electrons • This is a reason why the elements in a particular group have similar properties

49. As you move from left to right across a period, the atomic number increases by one • This means that an element has one more valence electron than the element to its left

50. Section 3: Metals