# Chapter 2 ATOMS AND ELEMENTS - Yale University

ATOMS AND ELEMENTS Radioactivity One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of Marie Curie (1876-1934). She discovered radioactivity, the spontaneous disintegration of some elements into smaller pieces.

ATOMIC COMPOSITION Protons + electrical charge mass = 1.672623 x 10-24 g relative mass = 1.007 atomic mass units (amu) Electrons

negative electrical charge relative mass = 0.0005 amu Neutrons no electrical charge mass = 1.009 amu 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.

The modern view of the atom was developed by Ernest Rutherford of New Zealand (1871-1937). Ernest Rutherford

Canterbury University in Christchurch, NZ Rutherford laboratory The modern view of the atom was developed by Ernest Rutherford (1871-1937). Screen 2.9

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 26.981

Atomic weight Atomic Weight This tells us the mass of one atom of an element relative to one atom of another element. OR the mass of 1000 atoms of one relative to 1000 atoms of another. For example, an O atom is approximately 16 times heavier than an H atom.

Define one element as the standard against which all others are measured Standard = carbon Mass Number, A C atom with 6 protons and 6 neutrons is the mass standard = 12 atomic mass units Mass Number (A) = # protons + # neutrons

A boron atom can have A = 5 p + 5 n = 10 amu A A ZZ 10 10 B B

55 Isotopes Atoms of the same element (same Z) but different mass number (A). Boron-10 (10B) has 5 p and 5 n Boron-11 (11B) has 5 p and 6 n B 11

B 10 Isotopes & Their Uses Bone scans with radioactive technetium-99.

Isotopes & Their Uses 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. Masses of Isotopes determined with a mass spectrometer

Isotope s B 11 B 10

Because of the existence of isotopes, the mass of a collection of atoms has an average value. Average mass = ATOMIC WEIGHT Boron is 20% 10B and 80% 11B. That is, 11B is 80 percent abundant on earth. For boron atomic weight = 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu Isotopes & Atomic

Weight Because of the existence of isotopes, the mass of a collection of atoms has an average value. 6Li = 7.5% abundant and 7Li = 92.5% Atomic weight of Li = ______________ Si = 92.23%, 29Si = 4.67%, 30Si = 3.10% 28

Atomic weight of Si = ______________ Counting Atoms Mg burns in air (O2) to produce white magnesium oxide, MgO. How can we figure out how much oxide is produced

from a given mass of Mg? Counting Atoms Chemistry is a quantitative sciencewe need a counting unit. MOLE 11 mole mole is is the

the amount amount of of substance substance that that contains contains as as many many particles particles (atoms,

(atoms, molecules) molecules) as as there there are are in in 12C. 12.0 12.0 g g of

of 12 C. Particles in a Mole Avogadros Number Amedeo Avogadro 1776-1856 6.02214199 x 1023 There

There is is Avogadros Avogadros number number of of particles particles in in aa mole mole of of any any substance.

substance. Molar Mass 1 mol of 12C = 12.00 g of C = 6.022 x 1023 atoms of C 12.00 g of 12C is its MOLAR MASS Taking into account all of the isotopes of C,

the molar mass of C is 12.011 g/mol One-mole Amounts PROBLEM: PROBLEM: What What amount amount of of Mg Mg

is is represented represented by by 0.200 0.200 g? g? How How many many atoms? atoms? Mg has a molar mass of 24.3050 g/mol.

1 mol 0.200 g = 8.23 x 10 -3 mol 24.31 g How many atoms in this piece of Mg? 23 atoms 6.022 x 10 8.23 x 10-3 mol

1 mol = 4.95 x 1021 atoms Mg Periodic Table Dmitri Mendeleev developed the modern periodic table. Argued that element properties are periodic functions of their atomic weights. We now know that element

properties are periodic functions of their ATOMIC NUMBERS. See CD-ROM, Screen 2.16. Periods in the Periodic Table Groups in the Periodic Table Regions of the Periodic

Table Element Abundance C O Al Si

Fe http://www.webelements.com/webelements/elements/text/Si/geol.html Hydrogen Shuttle main engines use H2 and O2 The Hindenburg crash, May 1939.

Group Group 1A: 1A: Alkali Alkali Metals Metals Reaction of potassium + H2O Cutting sodium metal

Group 2A: Alkaline Earth Metals Magnesium Magnesium oxide Calcium Carbonate Limestone The Appian Way, Italy

Champagne cave carved into chalk in France Group 3A: B, Al, Ga, In, Tl Aluminum Boron halides BF3 & BI3 Gems & Minerals

Sapphire: Al2O3 with Fe3+ or Ti3+ impurity gives blue whereas V3+ gives violet. Ruby: Al2O3 with Cr3+ impurity Group 4A: C, Si, Ge, Sn, Pb Quartz, SiO2

Diamond Group 5A: N, P, As, Sb, Bi Ammonia, NH3 White and red phosphorus Phosphor

us Phosphorus first isolated by Brandt from urine, 1669 Group 6A: O, S, Se, Te, Po Sulfuric acid dripping from snot-tite in cave in Mexico

Sulfur from a volcano Group 7A: F, Cl, Br, I, At Group 8A: He, Ne, Ar, Kr, Xe, Rn Lighter than air balloons Neon signs

XeOF4 Transition Elements Lanthanides and actinides Iron in air gives iron(III) oxide Colors of Transition Metal Compounds

Iron Cobalt Nickel Copper Zinc

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