Download CHAPTER 2: ATOMS, IONS, AND COMPOUNDS

CHAPTER 2: ATOMS, IONS, AND COMPOUNDS
(Topics to Review)
Early Models
Democritus (462-370 B.C.): proposed that all matter was made up of tiny, indivisible particles
called atomos (meaning “not to cut) or atoms.
Empedocles (490-430 B.C.): suggested all matter was composed of four basic elements: air,
water, fire, and earth.
Aristotle (384-321 B.C.): accepted Empedocles idea and added a fifth element, heavenly
ether, which is perfect, eternal, and incorruptible.
Aristotle’s idea of five basic elements was accepted for 2000 years.
John Dalton (1766-1844), an English chemist and physicist, established the foundation for
modern chemistry with the following:
The Modern Model of the Atom
1. All matter is composed of tiny indivisible* particles called atoms.
2. An element is composed of only one type of atom
– All atoms of one type of an element are identical*.
– Atoms of different elements behave in different ways.
Note: Parts of these statements (indicated with *) were later proven to be wrong.
3. Two or more elements combine to form compounds.
law of definite proportions (also the law of constant composition):
– A compound always has same elements in the same proportion by mass
– i.e., a compound always has the same formula
→ Water is always H2O.
law of multiple proportions:
– Two or more elements can combine to form different compounds
– e.g. C and O can combine to form CO or CO2
– For every compound the element’s masses are always in a fixed ratio.
– because always have whole #’s of atoms of each element in the compound
4. A chemical reaction involves only the rearrangement of atoms—NEVER their creation
or destruction. This is called the law of conservation of mass.
Chemical Reaction:
REACTANTS
(starting materials)
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PRODUCTS
(substances after reaction)
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The mass of the product(s) in a reaction must equal the mass of the reactant(s).
For example,
11.2 g hydrogen + 88.8 g oxygen = 100.0 g water
Ex. 1: Methane burns by reacting with oxygen present in air to produce steam and
carbon dioxide gas. Calculate the mass of methane that reacts if 199.4 g of
oxygen react to produce 112.3 g of steam and 137.1 g of carbon dioxide.
2.5 AVERAGE ATOMIC MASSES
Atoms are too small to weigh directly
– e.g. one carbon atom has a mass of 1.99 x 10-23 g–too inconvenient!
→ need more convenient unit for mass
→ atomic mass unit (amu)
Carbon-12 was chosen and given a mass value of 12 amu
→ 1 amu = 1/12 the mass of carbon-12
→ Mass of all other atoms measured relative to mass of carbon-12
Average Atomic Mass of an Element
– Why is carbon’s mass on Periodic Table 12.01 amu, NOT 12.00 amu?!
– Atomic masses reported on the Periodic Table are weighted averages of all the naturally
occurring isotopes for each element.
Ex. 1 If 98.892% of carbon exists as carbon-12, with a mass of 12.00000, while 1.108% exists
as carbon-13, with a mass of 13.00335, calculate the average atomic mass for carbon.
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Ex. 2 The atomic masses of the three naturally occurring isotopes or argon, Ar-36 (0.3365%),
Ar-38 (0.0632%) and Ar-40 (99.6003%), are 35.96754552 amu, 37.9627325 amu, and
39.9623837 amu, respectively. Calculate the average atomic mass for argon.
2.4 THE PERIODIC TABLE OF THE ELEMENTS
– A vertical column is called a group or family.
– Elements belonging to the same group exhibit similar chemical properties
– A horizontal row is called a period or series.
Main-Group (Representative or A Group) Elements
Those elements in groups 1, 2, 13, 14, 15, 16, 17, 18 (or IA to VIIIA)
– Group 1 or IA: alkali metals
– Group 2 or IIA: alkaline earth metals
– Group 17 or VIIA: halogens
– Group 18 or VIIIA: noble gases (because they are all gases that do not react)
Transition Metals (or B Group Elements)
– Elements in Groups 3 to 12 (middle of the Periodic Table)
Inner Transition Elements (beneath the main body of Periodic Table)
– Lanthanide series: Ce-Lu, also called rare earth metals, make up <0.005% of Earth's crust
– Actinide series: Th-Lr, also called transuranium elements, generally all man-made and exist
for only very short periods of time before decaying to other elements
Periodic Law:
Elements can be arranged to display recurring properties.
→ We can use the Periodic Table to predict the properties of elements.
Dimitri Mendeleev (1834-1907), a Russian chemist, published his periodic table in 1869,
before many elements were even discovered.
– He organized elements to display recurring properties according to increasing atomic mass.
Henry G. J. Moseley (1887-1915), an English physicist, carried out high-energy X-ray radiation
experiments on atomic nuclei.
→ Repeating properties of elements are more clearly reflected if elements are arranged
according to increasing atomic number (not increasing atomic mass).
→ Periodic Table’s arrangement today
– Trends for increasing atomic mass are identical with those for increasing atomic number,
except for Ni & Co, Ar & K, Te & I.
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