Atoms and Elements

Posted on February 25, 2009

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Chemistry is a science that studies the composition and properties of matter
Matter is anything that takes up space and has mass
Mass is a measure of the amount matter in a sample
Chemistry holds a unique place among the sciences because all things are composed of chemicals
A knowledge of chemistry will be valuable whatever branch of science you study
Chemistry is constantly changing as new discoveries are made by researchers
Researchers use a commonsense approach to the study of natural phenomena called the scientific method
A scientific study normally:
Begins with a question about nature
Involves a search of the work of others
Requires observing the results of experiments
Often results in a conclusion, or a statement based on what is thought about a series of observations
Experiments provide empirical facts
Facts are called data
A broad generalization based on the results of many experiments is called a (scientific) law
Laws are often expressed as mathematical equations
Laws summarize the results of experiments
Theoretical models attempt to explain why substances behave as they do
A hypothesis is a tentative explanation
A theory is an experimentally tested explanation of the behavior of nature
Chemical substances are comprised of atoms
Atoms combine to form molecules which can be represented in a number of ways, including:
Characteristics or properties of materials distinguish one type of substance from another
Properties can be classified as physical or chemical
Physical properties can be observed without changing the chemical makeup of the substance
Chemical properties involve a chemical change and result in different substances
Chemical changes are described by chemical reactions
Properties can also be described as intensive or extensive
Intensive properties are independent of sample size
Examples: sample color and melting point
Extensive properties depend on sample size
Examples: sample volume and mass
In general, intensive properties are more useful in identifying a substance
Matter is often classified by properties
The three common physical states of matter have different properties:
Solids have a fixed shape and volume
Particles are close together and have restricted motion
Liquids have indefinite shape but fixed volume
Particles are close together but are able to flow
Gases have indefinite shape and volume
Particles are separated by lots of empty space
Elements are substances that cannot be decomposed by chemical means into simpler substances
Each element is assigned a unique chemical symbol
Most are one or two letters
First letter is always capitalized
All remaining letters are lowercase
Names and chemical symbols of the elements are listed on the inside front cover of the book
Compounds are substances formed from two or more different elements combined in a fixed proportion by mass
The physical and chemical properties of a compound are, in general, different than the physical and chemical properties of the elements of which it is comprised
Elements and compounds are examples of pure substances whose composition is the same, regardless of source
A mixture consists of varying amounts of two or more elements or compounds
Homogeneous mixtures or solutions have the same properties throughout the sample
Heterogeneous mixtures consist of two or more phases
Matter can be classified:
We take for granted the existence of atoms and molecules
The concept of the atom had limited scientific usefulness until the discovery of two important laws: the Law of conservation of mass and the Law of Definite Proportions
These laws summarized the results of the experimental observations of many scientists
Law of Conservation of Mass:
No detectable gain or loss of mass occurs in chemical reactions. Mass is conserved.
Law of Definite Proportions:
In a given chemical compound, the elements are always combined in the same proportions by mass.
In the sciences mass is measured in units of grams (symbol, g)
One pound equals 453.6 g
The laws of conservation of mass and definite proportions provided the experimental foundation for the atomic theory
Dalton’s Atomic Theory:
Matter consists of tiny particles called atoms.
Atoms are indestructible. In chemical reactions, the atoms rearrange but they do not themselves break apart.
In any sample of a pure element, all the atoms are identical in mass and other properties.
The atoms of different elements differ in mass and other properties.
In a given compound the constituent atoms are always present in the same fixed numerical ratio.
It follows from Dalton’s Atomic Theory that atoms of an element have a constant, characteristic atomic mass or atomic weight
For example, for any sample of hydrogen fluoride:
F-to-H atom ratio: 1 to 1
F-to-H mass ratio: 19.0 to 1.00
This is only possible if each fluorine atom is 19.0 times heavier than each hydrogen atom
It turns out that most elements in nature are uniform mixtures of two or more kinds of atoms with slightly different masses
Atoms of the same element with different masses are called isotopes
For example: there are 3 isotopes of hydrogen and 4 isotopes of iron
Chemically, isotopes have virtually identical chemical properties
The relative proportions of the different isotopes are essentially constant
A uniform mass scale for atoms requires a standard
For atomic mass units (amu, given the symbol u) the standard is based on carbon:
1 atom of carbon-12 = 12 u (exactly)
1 u = 1/12 mass 1 atom of carbon-12 (exactly)
This definition results in the assignment of approximately 1 u for the mass of hydrogen (the lightest atom)
Example: Naturally occurring chlorine is a mixture of two isotopes. In every sample of this element, 75.77% of the atoms are chlorine-35 and 24.23% are chlorine-37. The measured mass of chlorine-35 is 34.9689 u and that of chlorine-37 is 36.9659 u. Calculate the average atomic mass of chlorine.
Experiments have been performed that show atoms are comprised of subatomic particles
There are three principal kinds of subatomic particles:
Proton – carries a positive charge, found in the nucleus
Electron – carries a negative charge, found outside the nucleus, about 1/1800 the mass of a proton
Neutron – carries no charge, found in the nucleus, a bit heavier than a proton, about 1800 times heavier than an electron
An element can be defined as a substance whose atoms all contain the identical number of protons, called the atomic number (Z)
Isotopes are distinguished by mass number (A):
Atomic number, Z = number of protons
Mass number, A = (number of protons) + (number of neutrons)
For charge neutrality, the number of electrons and protons must be equal
This information can be summarized
Example: For uranium-235
Number of protons = 92 ( = number of electrons)
Number of neutrons = 143
Atomic number (Z) = 92
Mass number (A) = 92 + 143 = 235
Chemical symbol = U
Summary for uranium-235:
The Periodic Table summarizes chemical and physical properties of the elements
The first Periodic Tables were arrange by increasing atomic mass
The Modern Periodic table is arranged by increasing atomic number:
Elements are arranged in numbered rows called periods
The vertical columns are called groups or families (group labels vary)
Modern Periodic Table with group labels and chemical families identified
Some important classifications:
A groups = representative elements or main group elements
I A = alkali metals
II A = alkaline earth metals
VII A = halogens
VIII = noble gases
B groups = transition elements
Inner transition elements = elements 58 – 71 and 90 – 103
58 – 71 = lanthanide elements
90 – 103 = actinide elements
Classification as metals, nonmetals, and metalloids
Metals
Tend to shine (have metallic luster)
Can be hammered or rolled into thin sheets (malleable) and can be drawn into wire (ductile)
Are solids at room temperature and conduct electricity
Nonmetals
Lack the properties of metals
React with metals to form (ionic) compounds
Metalloids
Have properties between metals and nonmetals

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