chemistry – chapter 2 · 2019-11-11 · summary atoms want to have 8 electrons in their outer...
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CHEMISTRY
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Composition of Matter
Matter - Everything in universe is composed of matter
Matter is anything that occupies space or has mass
Mass – quantity of matter an object has
Weight – pull of gravity on an object + mass
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Elements
Pure substances that cannot be broken down chemically into simpler kinds of matter
More than 100 elements (92 naturally occurring)
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90% of the mass of an organism is composed of 4 elements (oxygen, carbon, hydrogen and nitrogen)
Each element unique chemical symbol
Consists of 1-2 letters
First letter is always capitalized
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Atoms The simplest particle of
an element that retains all the properties of that element
Properties of atoms determine the structure and properties of the matter they compose
Our understanding of the structure of atoms based on scientific models, not observation
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The Nucleus
Central core
Consists of positive charged protonsand neutral neutrons
Positively charged
Contains most of the mass of the atom
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The Protons
All atoms of a given element have the same number of protons
Number of protons called the atomic number
Number of protons balanced by an equal number of negatively charged electrons
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The Neutrons
The number varies slightly among atoms of the same element
Different number of neutrons produces isotopes of the same element
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Atomic Mass Protons & neutrons are found in the
nucleus of an atom Protons and neutrons each have a mass of
1 amu (atomic mass unit) The atomic mass of an atom is found by
adding the number of protons & neutronsin an atom
# of protons + # of neutrons =atomic mass
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The Electrons
Negatively charged high energy particles with little or no mass
Travel at very high speeds at various distances (energy levels) from the nucleus
Are located around the nucleus
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Periodic Table Elements are arranged by their
atomic number on the Periodic Table The horizontal rows are called
Periods & tell the number of energy levels
Vertical groups are called Groups & tell the outermost number of electrons
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Compounds Most
elements do not exist by themselves
Readily combine with other elements in a predictable fashion
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A compound is a puresubstance made up of atoms of two or more elements The proportion of
atoms are always fixed
Chemical formulas shows the kind and proportion of atoms of each element that occurs in a particular compound
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Molecules are the simplest part of a substance that retains all of the properties of the substance and exists in a freestate
Some molecules are large and complex
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Chemical Formulas Subscript after a symbol tell the
number of atoms of each element
H20 has 2 atoms of hydrogen & 1 atom of oxygen
Coefficients before a formula tell the number of molecules
3O2 represents 3 molecules of oxygen or (3x2) or 6 atoms of oxygen
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The tendency of elements to combine and form compoundsdepends on the numberand arrangement of electrons in their outermost energy level
Atoms are most stablewhen their outer most energy level is filled (when it has 8electrons fulfilling the octet rule)
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Most atoms are not stable in their natural state
Tend to react (combine) with other atoms in order to become more stable (undergo chemical reactions)
In chemical reactions bonds are broken; atoms rearranged and new chemical bonds are formed that store energy
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Covalent Bonds
Formed when two atoms share one or more pairs of electrons
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Ionic Bonds
Some atoms become stable by losing orgaining electrons
Atoms that lose electrons are called positive ions (cations)
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Atoms that gain electrons are called negative ions (anions)
Because positive and negative electrical charges attract each other ionic bonds form
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Summary
Atoms want to have 8 electrons in their outer
shell, so they will react to other atoms to get 8
electrons
Covalent bonds= two or more atoms SHARING
electrons
Ionic Bonds= When one atom STEALS electrons
from another, then there will be a positive and a
negative ion which become attracted to each other