Unit 2 continued-chemical Foundations Atoms, Ions, &Elements

Transcription

1 Unit 2 continuedchemical Foundations Atoms, Ions, &Elements The Elements Most abundant elements in/on Earth: Oxygen 49.2% Silicon25.7% Most abundant in the human body: Oxygen65.0% Carbon18.0 % Hydrogen10.0% Microscopic vs. Macroscopic Chemists use the word element for macroscopic (things we can see with the eye) meaning an amount that we would mass on a balance Chemists also use the word element to describe microscopic form of an element when we are talking about a single atom of that element. Element Defined Element a substance that cannot be decomposed into a simpler substances by chemical or physical means. It consists of atoms all having the same atomic number. Element Symbols Symbols are used to represent the element. They usually consist of the first letter of the first two letters of the element names. The first letter is always capitalized, and the second is not. Most Common Elements You will need to memorize the elements name and symbol of the elements (on your sheet) Polyatomic ions included. 1

2 The Language of Chemistry CHEMICAL ELEMENTS Aluminum The Language of Chemistry The elements, their names, and symbols are given on the PERIODIC TABLE How many elements are there? Sodium Bromine The Periodic Table The Atom An atom consists of a nucleus (of protons and neutrons) electrons in space about the nucleus. Electron cloud Nucleus Dmitri Mendeleev ( ) Known as the Father of the Periodic Table. An atom is the smallest particle of an element that has the chemical properties of the element. Distance across = 1.8 nanometer (1.8 x 10 9 m) Copper atoms on silica surface. ATOM COMPOSITION The atom is mostly empty space protons and neutrons in the nucleus. the number of electrons is equal to the number of protons. electrons in space around the nucleus. extremely small. One teaspoon of water has 3 times as many atoms as the Atlantic Ocean has teaspoons of water. 2

3 ATOMIC COMPOSITION Protons (p ) electrical charge Located in the nucleus mass = x g relative mass = atomic mass units (amu) we round to 1 Electrons (e ) negative electrical charge Located outside of nucleus relative mass = amu we round to 0 Neutrons (n o ) no electrical charge Located in the nucleus mass = amu We round to 1 Quarks component of protons & neutrons 6 types 3 quarks = 1 proton or 1 neutron Subatomic Particles He You do not have to know the types, just that quarks exist! Scientists are learning more everyday! CHEMICAL COMPOUNDS are two or more elements chemically combined. They can be broken back down into the elements by a chemical process. (Usually requires much energy!) The red compound is composed of nickel (Ni) (silver) carbon (C) (black) hydrogen (H) (white) oxygen (O) (red) nitrogen (N) (blue) Compounds composed of 2 or more elements in a fixed ratio properties differ from those of individual elements EX: table salt (NaCl) A MOLECULE is the smallest unit of a compound that retains the chemical characteristics of the compound. (Covalent compounds are called molecules) Composition of molecules is given by a MOLECULAR FORMULA H 2 O C 8 H 10 N 4 O 2 caffeine ELEMENTS THAT EXIST AS DIATOMIC MOLECULES Remember: BrINClHOF These elements only exist as PAIRS. Note that when they combine to make compounds, they are no longer elements so they are no longer in pairs! 3

4 How do we know? How do we know all of these things about the atom? As part of the chemistry curriculum, you will need to know about the development of the model of the atom. In the next few days, we will explore and see what each scientist learned as the current model of the atom developed. Atomic Models Models are used to help us visualize something that cannot be seen Democritus This Greek philosopher is credited with coining the term atomos This was the term for the indivisible units. The idea ONLY He had no Proof! Another Greek Aristotle All substances are made of 4 elements Fire Hot Air light Earth cool, heavy Water wet Blend these in different proportions to get all substances He was better at debating, this was believed until 1700 salchemy Late 1700 s John Dalton An English Scientist who studied the nature of materials. He developed what is known as the Atomic Theory. The brown box on page 52 of text.. You will also need to know the main ideas of his theory. 4

5 Dalton s Atomic Theory John Dalton ( ) proposed an atomic theory While this theory was not completely correct, it revolutionized how chemists looked at matter and brought about chemistry as we know it today instead of alchemy Thus, it s an important landmark in the history of science. Dalton s Atomic Theory 1. matter is composed, indivisible particles (atoms) 2. all atoms of a particular element are identical 3. different elements have different atoms 4. atoms combine in certain wholenumber ratios 5. In a chemical reaction, atoms are merely rearranged to form new compounds; they are not created, destroyed, or changed into atoms of any other elements. Problems with Dalton s Atomic Theory? 1. matter is composed, indivisible particles True, but Atoms Can Be Divided, but only in a nuclear reaction 2. all atoms of a particular element are identical **WRONG** Does Not Account for Isotopes (atoms of the same element but a different mass due to a different number of neutrons)! 3. different elements have different atoms True! 4. atoms combine in certain wholenumber ratios True! Called the Law of Definite Proportions 5. In a chemical reaction, atoms are merely rearranged to form new compounds; they are not created, destroyed, or changed into atoms of any other elements. True, except for nuclear reactions that can change atoms of one element to a different element J.J. Thomson Deflected a beam of particles with magnets Determined that particles must have a negative charge 27 Thomson s Experiment Thomson s Experiment Voltage source Voltage source Metal Disks Passing an electric current makes a beam appear to move from the negative to the positive end 5

6 Thomson s Experiment Voltage source Thomson s Experiment Voltage source By adding an electric field By adding an electric field he found that the moving pieces were negative Thomson s Model Found the electron. Couldn t find positive (for a while). Said the atom was like plum pudding. A bunch of positive stuff, with the electrons able to be removed. About Millikan Millikan determined the charge of the electron. Millikan s Experiment Atomizer Millikan s Experiment Atomizer Oil droplets Microscope Metal Plates Oil Microscope Oil

7 Millikan s Experiment Millikan s Experiment Xrays Xrays give some drops a charge by knocking off electrons Millikan s Experiment Millikan s Experiment They put an electric charge on the plates Some drops would hover Millikan s Experiment Millikan s Experiment Some drops would hover From the mass of the drop and the charge on the plates, he calculated the charge on an electron

8 The modern view of the atom was developed by Ernest Rutherford ( ). Rutherford s Experiment Ernest Rutherford English physicist. (1910) Believed the plum pudding model of the atom was correct. Wanted to see how big electrons were. Used radioactivity. Alpha particles positively charged pieces given off by uranium. Shot them at gold foil which can be made a few atoms thick. 44 Rutherford Rutherford s experiment. A stream of positive alpha particles were directed at foil Thomson s model predicted that particles should pass through neutral material 45 Rutherford s experiment When the alpha particles hit a florescent screen, it glows. Lead block Uranium Flourescent Screen Gold Foil

9 He Expected The alpha particles to pass through without changing direction very much. Because The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles. What he expected Because Because, he thought the mass was evenly distributed in the atom Because, he thought the mass was evenly distributed in the atom Results of foil experiment if Plum Pudding model had been correct. 53 9

10 What he got How he explained it Atom is mostly empty. Small dense, positive piece at center. Alpha particles are deflected by it if they get close enough What Actually Happened 57 Rutherford Rare deflections indicated two things: A positive mass existed in the atom The positive center is very small Density and the Atom Since most of the particles went through, it was mostly empty. Because the pieces turned so much, the positive pieces were heavy. Small volume, big mass, big density. This small dense positive area is the nucleus

11 Modern View Which Model is this? Is it correct? The atom is mostly empty space. Two regions. Nucleus protons and neutrons. Electron cloud region where you might find an electron A Z X Atomic Number, Mass Number, and Element Symbol 23 11Na There are 3 different parts to identify an element. Atomic Number The number of protons in the nucleus of an atom. Symbolized by Z Mass Number The sum of the protons and neutrons. Symbolized by A Element Symbol The one or two letter symbol given to every element. {Plus} If you see an element written this way: Carbon14 mass number is 14 Atomic Number, Z All atoms of the same element have the same number of protons in the nucleus, Z 13 Al Atomic number Atom symbol AVERAGE Atomic Mass Mass Number, A C atom with 6 protons and 6 neutrons is the mass standard = 12 atomic mass units Mass Number (A) = # protons # neutrons NOT on the periodic table (it is the AVERAGE atomic mass on the table) A boron atom can have A = 5 p 5 n = 10 amu A Z 10 5 B 11

12 Isotopes Figure 3.10: Two isotopes of sodium. Atoms of the same element (same Z) but different mass number (A). Boron10 ( 10 B) has 5 p and 5 n Boron11 ( 11 B) has 5 p and 6 n 11 B 10 B Isotopes & Their Uses Isotopes & Their Uses Bone scans with radioactive technetium99. The tritium content of ground water is used to discover the source of the water, for example, in municipal water or the source of the steam from a volcano. Atomic Symbols Show the name of the element, a hyphen, and the mass number in hyphen notation sodium23 Show the mass number and atomic number in nuclear symbol form mass number atomic number Na Isotopes? Which of the following represent isotopes of the same element? Which element? 234 X 234 X 235 X 238 X

13 Answer: Counting Protons, Neutrons, and Electrons 234 U 234 Np 235 U 238 U Np is not an isotope of Uranium. 93 Protons: Atomic Number (from periodic table) Neutrons: Mass Number minus the number of protons (mass number is protons and neutrons because the mass of electrons is negligible) Electrons: If it s an atom, the protons and electrons must be the SAME so that it is has a net charge of zero (equal numbers of and ) If it does NOT have an equal number of electrons, it is not an atom, it is an ION. For each negative charge, add an extra electron. For each positive charge, subtract an electron (Don t add a proton!!! That changes the element!) Proton Electron Neutron Learning Check Counting charge charge 0 charge In nucleus Outside nucleus in specific levels In nucleus Naturally occurring carbon consists of three isotopes, 12 C, 13 C, and 14 C. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12 C 13 C 14 C #p 1 1/ #n o #e Answers 12 C 13 C 14 C #p #n o #e Learning Check An atom has 14 protons and 20 neutrons. A. Its atomic number is 1) 14 2) 16 3) 34 B. Its mass number is 1) 14 2) 16 3) 34 C. The element is 1) Si 2) Ca 3) Se D. Another isotope of this element is 1) 34 X 2) 34 X 3) 36 X

14 Solution An atom has 14 protons and 20 neutrons. A. It has atomic number 1) 14 B. It has a mass number of 3) 34 C. The element is 1) Si D. Another isotope of this element would be 3) 36 X 14 IONS IONS are atoms or groups of atoms with a positive or negative charge. Taking away an electron from an atom gives a CATION with a positive charge Adding an electron to an atom gives an ANION with a negative charge. To tell the difference between an atom and an ion, look to see if there is a charge in the superscript! Examples: Na Ca 2 I O 2 Na Ca I O Forming Cations & Anions PREDICTING ION CHARGES A CATION forms when an atom loses one or more electrons. An ANION forms when an atom gains one or more electrons In general metals (Mg) lose electrons > cations nonmetals (F) gain electrons > anions Mg > Mg 2 2 e F e > F Learning Check Counting Learning Check Counting State the number of protons, neutrons, and electrons in each of these ions. 39 K 16 O 2 41 Ca #p #n o #e State the number of protons, neutrons, and electrons in each of these ions. 39 K 16 O 2 41 Ca #P #N #E

15 One Last Learning Check Answers Write the nuclear symbol form for the following atoms or ions: A. 8 p, 8 n, 8 e B. 17p, 20n, 17e C. 47p, 60 n, 46 e Write the nuclear symbol form for the following atoms or ions: A. 8 p, 8 n, 8 e 16 O 8 B. 17p, 20n, 17e 37 Cl 17 C. 47p, 60 n, 46 e 107 Ag 47 Charges on Common Ions By losing or gaining e, atom has same number of e s as nearest Group 8A atom. AVERAGE ATOMIC MASS 10 B 11 B Because of the existence of isotopes, the mass of a collection of atoms has an average value. Boron is 20% 10 B and 80% 11 B. That is, 11 B is 80 percent abundant on earth. For boron atomic weight = 0.20 (10 amu) 0.80 (11 amu) = 10.8 amu Isotopes & Average Atomic Mass Because of the existence of isotopes, the mass of a collection of atoms has an average value. 6 Li = 7.5% abundant and 7 Li = 92.5% Avg. Atomic mass of Li = 28 Si = 92.23%, 29 Si = 4.67%, 30 Si = 3.10% Avg. Atomic mass of Si = The Periodic Table Honors only 15

16 Periods in the Periodic Table Groups in the Periodic Table Elements in groups react in similar ways! Regions of the Periodic Table Group 1A: Alkali Metals Reaction of potassium H 2 O Cutting sodium metal Group 2A: Alkaline Earth Metals Magnesium Group 7A: The Halogens (salt makers) F, Cl, Br, I, At Magnesium oxide 16

17 Group 8A: The Noble (Inert) Gases He, Ne, Ar, Kr, Xe, Rn Lighter than air balloons Neon signs Very Unreactive because they have full electron levels XeOF 4 Transition Elements Lanthanides and actinides Iron in air gives iron(iii) oxide 17