Chapter 2 Atoms, Molecules, and Ions

Transcription

1 Chapter 2 Atoms, Molecules, and Ions 1

2 Self study: The history of the development of atomic theory. 9 th Ed: pp or 10 th Ed: pp

3 The Atomic Theory of Matter John Dalton ( ), began teaching at age 12! He later discovered the ATOMIC THEORY! Each element is composed of. All atoms of a specific element are identical. Atoms of different elements are different, and have differing properties. In chemical reactions, the atoms are not changed into other atoms. Atoms are not created or destroyed. Compounds are formed when. ATOMS are therefore the of matter! 3

4 The Atomic Theory of Matter Law of Constant Composition: In a given compound the relative and of atoms are constant. Law of Conservation of Mass: Total mass of materials a chemical reaction is the same as the mass of the materials. Law of Multiple Proportions: When two elements form different compounds, the mass ratio of the elements in one compound is related to the mass ratio in the other by a small whole number. 4

5 Law of Multiple Proportions When elements combine they do so in a ratio of small whole numbers. For example, carbon and oxygen react to form CO or CO 2, but not CO 1.3. Also, if two elements form more than one compound between them then the ratios of the masses of the second element combined with a fixed mass of the first element will also be in ratios of small whole numbers. For example: Water and hydrogen peroxide are both made up from only oxygen and hydrogen. So the ratio for oxygen between water and hydrogen peroxide is 1:2. 5

6 The Modern View of Atomic Structure Atoms are composed of even smaller particles called particles. These are made up of positive, negative, and neutral entities: protons, p + electrons, e - neutrons, n o (uncharged) Protons and neutrons are located in. The nucleus is very small compared to the size of the atom, but carries most of the mass of the atom. 6

7 Relative Size of an Atom s Nucleus 1 Å = m = 100 pm Seahawks Stadium in Seattle, Washington. Imagine a small marble at the center of the field in the stadium. The marble represents the size of the atoms nucleus, in comparison to the entire atom, which is represented by the whole stadium. Electrons reside around the nucleus, filling the remaining volume of the atom. Electrons therefore occupy majority of the volume of an atom. 7

8 The Modern View of Atomic Structure The subatomic particles obey the Law of Electrostatics:. This law describes how the electrons remain in constant contact with the nucleus of the atom! The charge on the electron (e - ) and proton (p + ) is EQUAL in magnitude (1.602x10-19 C) but opposite in sign. The mass of the proton and the neutron is similar (1.01 amu), but the electron is a whole lot lighter ( amu). (1 amu = 1.661x10-24 g) 8

9 Isotopes, Atomic Numbers, and Mass Numbers What makes an atom of 1 element different from an atom of another element?. All atoms of the same element have the same number of protons. You cannot characterize an atom by its number of electrons, or its number of neutrons! BUT WHY? 9

10 Isotopes, Atomic Numbers, and Mass Numbers NEUTRONS: There can be a variable number of neutrons for the same number of protons. Isotopes have the same number of protons but different number of neutrons. (Isotopes therefore differ in mass.) ELECTRONS: Atoms can in reactions. Electrons are and are easily transferred between different atoms. 10

11 Isotopes, Atomic Numbers, and Mass Numbers A Z X X symbolizes the element. A symbolizes the mass number the total number of nucleons in the nucleus (A = protons + neutrons). Z symbolizes the atomic number. Atomic number = number of protons in the nucleus. This never changes: All atoms of the same element have the same atomic number! Isotopes have the same Z but different A. 11

12 Mass number 127 nucleons = 77 neutrons Chemical symbol for TIN 127 Sn 50 Atomic number 50 protons = 50 electrons 12

13 Often write just as: 127 Sn because we know from the periodic table that Z=50 for Sn. 13

14 Isotopes 79 Br 81 Br Notice how the mass numbers in the two bromine atoms differ! The number of Number of What is responsible for this difference? differ in isotopes. ALWAYS remain constant! 14

15 Isotopes, Atomic Numbers, and Mass Numbers Example 1 1.How many protons, neutrons, and electrons are in an atom of 197 Au? 2.Which of the following pairs are isotopes? a) 2 1H + and 3 1H b) 3 2He and 4 2He c) 12 6C and 14 7C - d) 3 1H and 4 2He - 15

16 Atomic weights Atoms have very small masses, example 1 H has a mass g 16 O has a mass of g. Instead of quoting such small masses using grams, scientists developed a unit specifically for the mass of atoms ATOMIC MASS UNITS (amu) The amu is defined by assigning a mass of exactly 12 amu to the 12 C isotope of carbon. 16

17 Atomic weights For conversion purposes: 1 amu = x g 1 g = x amu All elements on the periodic table are expressed in amu for convenience, and we can interconvert between and. What is hydrogen s mass in amu? Hydrogen = x g Atomic mass of 1 H on the periodic table is (which can also be expressed as g mol -1 will see this in Chapter 3). 17

18 Atomic weights Masses of all atoms are expressed relative to the mass of 12 C. Such masses are called relative atomic masses or atomic weights. Symbol : A r Atomic weight is the average mass of the atoms of an element in amu. It is numerically equal to the mass in grams per mole (g.mol -1 ) of the element. An atomic weight (relative atomic mass) of an element from a specified source is the ratio of the average mass per atom of the element to 1/12 of the mass of an atom of 12 C. 18

19 Measuring the mass of atoms Accurate values of atomic weights determined by mass spectrometry. 19

20 Results from mass spectrometry are isotopic peaks with relative intensities. 20

21 Atomic weights listed in the Periodic Table are weighted averages of the atomic weights of the isotopes of the elements. Example 2 Chlorine is a mixture of two isotopes % consists of 35 Cl which has an atomic weight of amu % is due to 37 Cl, amu. What is the atomic weight of chlorine? 21

22 Express percentages as fractional numbers (divide by 100): 100.0% = 1 50% = 0.5 therefore 75.77% = SO has mass amu and has mass amu We will visit this in more detail in chapter 3! 22

23 Periodic table Periodic table is laid out according to similarities between the elements. Lithium (Li), sodium (Na) and potassium (K) are all metals that are soft and very reactive. Helium (He), neon (Ne) and argon (Ar) are all gases that are nonreactive. For an interactive periodic table go to 23

24 The periodic table is arranged in increasing values of Z (Atomic #). Z for Mg = 12 U = Uranium Know the names and symbols of all elements up to Z = 88, excluding f block, except Z = 92.

25 Columns in the periodic table are called groups (numbered from 1A to 8A or 1 to 18). Alkali metals Alkaline earth metals Chalcogens Halogens Noble gases 25

26 Rows in the periodic table are called periods. Period 1 or Row 1 Period 2 or Row 2 26

27 Metals are located on the left hand side of the periodic table (most of the elements are metals). 27

28 Non-metals are located in the top right hand side of the periodic table. 28

29 Metalloids are located at the interface between the metals and non-metals. Metalloids have some metallic and some non-metallic properties. 29

30 Increasing Non-metallic Nature Increasing Metallic Nature 30

31 Identify: main group elements transition elements inner transition elements 31

32 Identify: s block p block d block f block 32

33 S, p, d and f? This is an Aufbau diagram. It shows the orbitals around the nucleus in an atom. The s orbital is usually the lowest in energy, and it can only carry 2 electrons. The p orbital has room for 6 electrons. The d orbital houses a maximum of 10 electrons. BUT NOW HOW DOES THIS CORRESPOND TO THE PERIODIC TABLE? 33

34 S, p, d and f? Atoms on the periodic table are found in differing blocks (s, p, d and f) because of where their VALENCE electrons reside. {Are they in the s, p, d, or f orbital?} Lets look at an example: oxygen The valence electrons are found in a p orbital and so oxygen should therefore be found in the p block on the periodic table! IS IT? 34

35 Identify: p block 35