basic chemistry mini lecture ap biology. atom electron – negative charge neutron; no charge
TRANSCRIPT
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Basic Chemistry
Mini LectureAP Biology
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AtomElectron – negative charge
Neutron; no charge
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Most atoms are neutral because the number of electrons equals the number of protons.
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Element—pure substance that contains only one kind of atom
• Living things are mostly composed of 6 elements:
• Carbon (C) Hydrogen (H) Nitrogen (N)
• Oxygen (O) Phosphorus (P) Sulfur (S)
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Basic Chemistry Concepts
• The number of protons identifies an element.• Number of protons = atomic number
• See periodic table handout!• For electrical neutrality, # protons = #
electrons.• Mass number—total number of protons and
neutrons
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Figure 2.1 Electron Shells
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• Atoms with unfilled outer shells tend to undergo chemical reactions to fill their outer shells.
• They can attain stability by sharing electrons with other atoms (covalent bond) or by losing or gaining electrons (ionic bond)
• The atoms are then bonded together into molecules.
• Octet rule—atoms with at least two electron shells form stable molecules so they have eight electrons in their outermost shells.
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Ionic bonds • Ions are charged particle that form when
an atom gains or loses one or more electrons.
• Cations—positively charged ions• Anions—negatively charged ions
• Ionic bonds result from the electrical attraction between ions with opposite charges.
• The resulting molecules are called salts.
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Covalent bonds• Covalent bonds form when two atoms
share pairs of electrons.• The atoms attain stability by having full
outer shells.• Each atom contributes one member of the
electron pair.
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• Carbon atoms have four electrons in the outer shell—they can form covalent bonds with four other atoms.
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Strength and stability—covalent bonds are very strong; it takes a lot of energy to break them.
• Multiple bonds• Single—sharing 1 pair of electrons
• Double—sharing 2 pairs of electrons
• Triple—sharing 3 pairs of electrons
C H
C C
N N
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Degree of sharing electrons is not always equal.
• Electronegativity—the attractive force that an atomic nucleus exerts on electrons
• It depends on the number of protons and the distance between the nucleus and electrons.
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• If two atoms have similar electronegativities, they share electrons equally, in what is called a nonpolar covalent bond.
• If atoms have different electronegativities, electrons tend to be near the most attractive atom, in what is called a polar covalent bond
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