Chapter 2

Elements & Atoms

Dalton’s Atomic Theory• An element is composed of tiny particles called atoms. All

atoms of a given element show the same chemical properties.

• Atoms of different elements have different properties.• Compounds are formed when atoms of two or more

elements combine. In a given compound, the relative number of atoms of each kind are definite and constant.

• In an ordinary chemical reaction, no atom of any element disappears or is changed into an atom of another element. Chemical reactions involve changing the way in which the atoms are joined together.

• The same elements can be combined to form different compounds by combining the elements in different proportion.

Dalton predicted Law of Multiple Proportions

• 1g Cu + 0.252 g O 1.252 g CuO

• 1g Cu + 0.126 g O 1.126 g Cu2O

Rutherford’s Experiment

Rutherford’s Model of the Atom

• atom is composed mainly of vacant space

• all the positive charge and most of the mass is in a small area called the nucleus

• electrons are in the electron cloud surrounding the nucleus

Structure of the Atom

ChargeMass

• Electron e- -1 0.0005

• Proton p+ +1 ~1

• Neutron n 0 ~1

Relative size of atom

and atomic nucleus

Nucleus to atom is like a pencil dot to this lecture hall

Scanning Tunneling Microscope

Atomic number, Z

• the number of protons in the nucleus

• the number of electrons in a neutral atom

• the integer on the periodic table for each element

Mass Number, A

• the sum of the number of protons and neutrons in the nucleus

• Approximately the mass of an atom

Nuclear Notation

X-A e.g. C-12

AX e.g. 12C

XAZ C12

6e.g.

An isotope has 35 electrons and 46 neutrons. The identity of this isotope is best represented as

1 2 3 4 5

20% 20% 20%20%20%1. 2.

3.

4. 5.

Masses of Atoms

Carbon-12 Scale

Masses of the atoms are compared to the mass of C-12 isotope having a mass of 12.0000

Mass Spectrometer

Atomic Masses andIsotopic Abundances

natural atomic masses =

sum[(atomic mass of isotope)

(fractional isotopic abundance)]

The average mass of the naturally occurring isotopes

Cl 34.969x0.7577+36.966x0.2423=35.453amu

1 2 3 4

25% 25%25%25%1. 2.

3. 4.

The Mole

• a unit of measurement, quantity of matter present

• Avogadro’s Number6.022 1023 particles per mole

• Latin for “pile”

• 1 C-12 atom has mass 12 amu

• 1 mole C-12 has mass 12 g

• Avogadro’s number converts betweenmoles and number of particles

• Molar mass converts betweenmoles and mass

• You are given a 1 carat diamond, how many carbon atoms does it contain (1 carat=0.2 g)

• 1 carat C x 0.2 g/1carat = 0.2 g C

• 0.2 g C x 1 mol C/12.01 g C = 0.01665 mol C

• 0.01665 mol C x 6.022x1023 atoms C/1 mol C =1.003x1022 C atoms

Development of Periodic Table

Mendeleev

• 1869 - Periodic Law

• allowed him to predict properties of unknown elements

• the elements are arranged according to increasing atomic weights

• Missing elements: 44, 68, 72, & 100 amu

Predicted Properties of Ekasilicon

Property Ekasilicon Germanium

Atomic Weight 72 72.6

Color gray gray

Density, g/mL 5.5 5.36

Oxide EsO2 GeO2

Chloride EsCl4 GeCl4

Modern Periodic TableMoseley, Henry Gwyn Jeffreys

1887–1915, English physicist.

• studied the relations among spectra of different elements.

• concluded that the atomic number is equal to the charge on the nucleus based on the x-ray spectra emitted by the element.

• explained discrepancies in Mendeleev’s Periodic Law.

I A II A III B IV B V B VI B VII B VIII B I B II B III A IV A V A VI A VII A VIII A1 1 2

1 H H He1.008 1.008 4.0026

3 4 5 6 7 8 9 10

2 Li Be B C N O F Ne6.939 9.0122 10.811 12.011 14.007 15.999 18.998 20.183

11 12 13 14 15 16 17 18

3 Na Mg Al Si P S Cl Ar22.99 24.312 26.982 28.086 30.974 32.064 35.453 39.948

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

4 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr39.102 40.08 44.956 47.89 50.942 51.996 54.938 55.847 58.932 58.71 63.54 65.37 69.72 72.59 74.922 78.96 79.909 83.8

37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

5 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe85.468 87.62 88.906 91.224 92.906 95.94 * 98 101.07 102.91 106.42 107.9 112.41 114.82 118.71 121.75 127.61 126.9 131.29

55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86

6 Cs Ba **La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn132.91 137.33 138.91 178.49 180.95 183.85 186.21 190.2 192.22 195.08 196.97 200.29 204.38 207.2 208.98 * 209 * 210 * 222

87 88 89 104 105 106 107 108 109 110 111 112 114 116 118

7 Fr Ra ***Ac Rf Ha Sg Ns Hs Mt Uun Uuu Unb Uuq Uuh Uuo* 223 226.03 227.03 * 261 * 262 * 263 * 262 * 265 * 268 * 269 * 272 * 277 *285 *289 *293

Based on symbols used by ACS S.M.Condren 2001

58 59 60 61 62 63 64 65 66 67 68 69 70 71

* Designates that **Lanthanum Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Luall isotopes are Series 140.12 140.91 144.24 * 145 150.36 151.96 157.25 158.93 162.51 164.93 167.26 168.93 173.04 174.97

radioactive 90 91 92 93 94 95 96 97 98 99 100 101 102 103

*** Actinium Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Series 232.04 231.04 238.03 237.05 * 244 * 243 * 247 * 247 * 251 * 252 * 257 * 258 * 259 * 260

Periodic Table of the ElementsPeriodic Table of the Elements

Family Names

Group IA alkali metals

Group IIA alkaline earth metals

Group VIIA halogens

Group VIIIAnoble gases

transition metals

inner transition metals

• lanthanum series rare earths

• actinium series trans-uranium series

Diatomic Elements

• H2 N2 O2 F2 Cl2 Br2 I2 Memorize

• The “gens”

• Hydrogen, nitrogen, oxygen, halogens

• S8 and P4 No need to remember