Atoms, Elements, and Compounds

1. Atoms, Elements, and Compounds Physical Science, Chapter 14, 15, and 16

2. All About Atoms

3. What is an atom? • Basic unit of matter • Definitioncomes from ancient Greek: means indivisible or uncuttable • Can’t be broken down into smaller pieces

4. What are atoms made of? • Protons– positive charge • Neutrons– neutral, no charge • Electrons– negative charge

5. Structure of the Atom • In order to understand atoms, we need to understand the idea of electric charge. • We know of two different kinds of electric charge and we call them positive and negative. • Positive and negative: Attract • Positive and positive: Repel • Negative and negative: Repel

6. Electric charge in matter • We say an object is electrically neutral when its total electric charge is zero. • The charge on a complete atom is ZERO • Atoms are neutral

7. Early model of the atom • In 1897 English physicist J. J. Thomson discovered particles that were too small to be atoms. • These negative particles were eventually called “electrons.” • Thomas thought the electrons were evenly distributed inside the atom, like raisins in bread. (Bread +, raisins – )

8. Testing the model • In 1911, Ernest Rutherford, Hans Geiger, and Ernest Marsden did an experiment to test Thomson’s model. • They discovered that atoms are mostly empty space!

9. Atomic structure video • http://www.teachertube.com/viewVideo.php?video_id=247101

10. Inside an atom • Protons and neutrons are much larger and more massive than electrons. • The mass of the nucleus determines the mass of an atom. • The electrons surround the nucleus and form the electron cloud.

11. Electron Cloud

13. Forces – Electromagnetic Force • Electrons (-) are attracted to the protons (+) in the nucleus because of the difference in electric charge. • Momentum (kinetic energy!) keeps electrons from falling into the nucleus • Electrons “orbit” the nucleus just like Earth orbits the Sun

14. Forces – Strong Nuclear Force • Normally, positively charged protons repel each other • Strong nuclear force holds the protons and neutrons together in the nucleus • Stronger than electromagnetic force.

15. How are elements different? • Atoms of different elements contain different numbers of protons in the nucleus. • Because the number of protons is so important, it is called the atomic number. • Each element has a unique atomic number

16. How to “read” an element Atomic Number Element Symbol Element Name Atomic Mass

17. How to build an element • Atomic number = number of protons • Number of protons = number of electrons • Atomic mass = number of protons + neutrons

18. Let’s practice! 6 (Same as atomic number) 6 (same as protons) 6 (Atomic mass – atomic number: 12 – 6 =6) • Number of Protons? • Number of Electrons? • Number of Neutrons?

19. An exception: Isotopes • Isotopes are atoms of the same element that have different numbers of neutrons. • Remember: the mass numbertells you the number of protons plus the number of neutrons. How are these carbon isotopes different?

20. Isotopic Symbol X = element symbol A = mass number (# protons + # neutrons) Z = atomic number (# proteins) N = # neutrons A – Z = N A typical isotopic symbol takes this form: Ex: The isotopic symbol for carbon would be: A Z 12 6 X C

21. Radioactivity • Almost all elements have one or more isotopes that are stable. • “Stable” means the nucleus stays together. • Carbon-14 is radioactive because it has an unstable nucleus.

22. Carbon dating • Unstable Carbon-14 is used in carbon dating • Carbon-14 breaks down at a predictable rate • Scientists can use this to estimate the age of organic matter (matter which was once alive)

23. More about electrons • The colors of clothes, paint, and everything else around you comes from electrons! • Each element has its own characteristic pattern of colors called a spectrum.

24. Electrons in atoms • Each individual color in a spectrum is called a spectral line because each color appears as a line in a spectroscope. • A spectroscope is a device that spreads light into its different colors.

26. Bohr model of the atom • Danish physicist Neils Bohr proposed the concept of energy levels to explain the spectrum of hydrogen. • When an electron moves from a higher energy level to a lower one, the atom releases the energy difference between the two levels. • The energy comes out as different colors of light.

27. Energy levels • Electrons can absorb or emit energy only at specific wavelengths • Like going up or down steps. • http://www.youtube.com/watch?v=kJBcXFsFa7Y (to about 5 min) • Fireworks are different colors depending on what element they are made with 14.2 Clicker

28. Rules for energy levels Inside an atom, electrons always obey these rules: • The energy of an electron must match one of the energy levels in the atom. • Each energy level can hold only a certain number of electrons, and no more. • As electrons are added to an atom, they settle into the lowest unfilled energy level.

29. Bioluminescence

30. The Periodic Table

31. Remember Physical Properties? • Physical propertiesare seen through direct observation • Physical properties include color, texture, density, and state (solid, liquid, or gas). • Melting point and boiling point are also physical properties. • A physical change does not result in a new substance being formed.

32. Chemical Properties • Properties that can only be observed when one substance changes into a different substance are called chemical properties. • Any change that transforms one substance into a different substance is called a chemical change.

33. Periodic Table • The periodic table organizes the elements according to how they combine with other elements • These are chemical properties. • The periodic table is organized in order of increasing atomic number.

35. Atomic Number • Remember, the atomic number is the number of protons in the nucleus of that element. • If the atom is neutral, it will have the same number of electrons as protons.

36. Periodic Table • The periodic table is divided into periods and groups. • Each horizontal row is called a period. • Each vertical column is called a group.

37. Groups of the Periodic Table • The first group is known as the alkali metals. • The alkali metals are highly reactive. • This group includes the elements lithium (Li), sodium (Na), and potassium (K). • The group two metals include beryllium (Be), magnesium (Mg), and calcium (Ca). • They also bond easily with oxygen.

38. Groups of the Periodic Table – Halogens • The halogens tend to be toxic gases or liquids in their pure form. • Fluorine (F), chlorine (Cl), and bromine (Br) form salts when they bond with alkali metals.

39. Groups of the Periodic Table Noble Gases • The noble gasesgroup includes the elements helium (He), neon (Ne), and argon (Ar). • These elements naturally do not form chemical bonds with other atoms and are almost always found in their pure state.

40. Phases of Elements • Most of the pure elements are solid at room temperature. • Only 11 naturally occurring elements are a gas. • Only 2 elements (Br and Hg) are liquid at room temperature.

41. Electrical conductivity • Electricity is the movement of electric charge, usually electrons. • Some materials allow electrons to flow easily through them. • We call these materials electrical conductors.

42. Thermal conductivity • Like copper, most metals are also good thermal conductors. • That is one reason pots and pans are made of metal.

43. Insulators • Elements on the far right of the table are called non-metals. • Nonmetals make good insulators. • An insulator is a material which slows down or stops the flow of either heat or electricity.

44. Metals and metal alloys • An alloy is a solid mixture of one or more elements. • Most metals are used as alloys and not in their pure elemental form. • Example: Titanium combines the strength of steel with the light weight of aluminum. It is used in racing bicycles and airplanes.

45. Nitrogen, oxygen and phosphorus • Nitrogen is the most abundant element in the atmosphere! • Oxygen and nitrogen are crucial to living animals and plants. • Phosphorus is a key part of DNA, the molecule responsible for carrying the genetic code.

46. Carbon and carbon-like elements • Almost all the molecules that make up plants and animals are constructed around carbon. • Carbon is essential to life! • The chemistry of carbon is so important it has its own name, organic chemistry.

47. Carbon and carbon-like elements • Pure carbon is found in nature as either graphite or diamond. • Silicon is the second most abundant element in the Earth’s crust, second only to oxygen. Why are carbon and silicon important?

48. What’s so great about silicon? • It makes up sand, glass, gemstones like amethyst and opal, and most of the rocks on the surface of the Earth. • Pure silicon is used to make microchips, also known as computer chips. • These microchips can be found in many devices, including computers, cell phones, and microwaves. • Ever heard of Silicon Valley?

49. Creating Compounds

50. What is a compound? • Two or more elements combined to make something new • For example, when hydrogen and oxygen combine to make water • This is a chemical change • A chemical bond forms when atoms transfer or share electrons.