atoms and molecules: the chemical basis of life. elements – substances that cannot be broken down...
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Atoms and Molecules:The Chemical Basis of Life
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Elements
– Substances that cannot be broken down into simpler substances by ordinary chemical reactions
– Each has a chemical symbol
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Proton Neutron Electron
Hydrogen1 proton1 electron
Oxygen8 protons8 neutrons8 electrons
+ 0 –
Everything is made of matter Matter is made of atoms
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The World of Elements
CC
Different kinds of atoms = elementsDifferent kinds of atoms = elementsDifferent kinds of atoms = elementsDifferent kinds of atoms = elements
HHOONN
PP SSNaNaKKMgMgCaCa
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C,H,N,O,P,S
• The most abundant elements in organisms
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Life requires ~25 chemical elements
• About 25 elements are essential for life– Four elements make up 96% of living matter:
• carbon (C) • hydrogen (H)
• oxygen (O) • nitrogen (N)
– Four elements make up most of remaining 4%: • phosphorus (P) • calcium (Ca)
• sulfur (S) • potassium (K)
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AtomThe smallest portion of an
element that retains its chemical properties
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Structure of the Atom
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The periodic table is arranged by atomic number
Every element hasA fixed number of protons in the atomic
nucleus, known as the atomic number
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Atomic mass – add up the number of protons and neutrons
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The average atomic mass (atomic weight) of an atom:
– Takes into account the mass of each isotope and their relative abundance in nature.
– Expressed by in atomic mass units (amu)– Usually not a whole number since it is an average!
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Contain the same number of protons and electrons, but the number of neutrons varies
Isotopes Two or more forms of atoms of the same element
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• The chemical behavior of an atom is determined by the number and arrangement of its valence electrons
• When the valence shell is not full, the atom tends to lose, gain, or share electrons
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Chemical Bonds
–Forces of attraction that hold atoms of a compound together–The two principal types are• Covalent bonds• Ionic bonds
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Covalent Bonds
– Share electrons between atoms– Each atom has a filled valence shell– Bond can be single, double, or triple
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Covalent bonds
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Carbon can form up to 4 covalent bonds!
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Covalent bonds can be Polar or Nonpolar
Polar Covalent Bonds – Electrons are not shared equally
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Nonpolar Covalent Bonds
Electrons shared equallyNo areas of partial charge
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Ions– Particle with one or more units of electrical charge– Results when an atom gains or loses electrons• Cations—positively charged ions• Anions—negatively charged ions
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Ionic Bonds
– Formed due to attraction between a cation and an anion
– An ionic compound is a substance consisting of cations and anions bonded together
– An example of ionic bond is the attraction between sodium ions and chloride ions
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Ionic bonding
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Hydrogen bonds
– Tend to form between an atom with partial negative charge and a hydrogen atom covalently bonded to oxygen or nitrogen
– Readily formed and broken– While individually weak, hydrogen bonds are
strong when present in large numbers
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Hydrogen bonding
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