Kimia Anorganik I
Atom dan Unsur
Atoms and Elements
• 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
• 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
• 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
• 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:
(a) Using chemical symbols and lines for “connections”
(b) A 3-D ball-and-stick model
• 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
• 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
• 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
• 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
• 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
• 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
• 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
• 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)
• 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.
Support for Dalton’s Atomic Theory: The Law of Multiple Proportions Whenever two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small whole numbers.
• 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
• 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
• 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)
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.
Abundance Mass
Isotope (%) (u) Contribution
• 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
• 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)
• 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
• 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
• 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
