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Basic Chemistry Mini Lecture AP Biology Atom Electron – negative charge Neutron; no charge Most atoms are neutral because the number of electrons equals the number of protons. 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) 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 Figure 2.1 Electron Shells • 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. 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. 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. • Carbon atoms have four electrons in the outer shell—they can form covalent bonds with four other atoms. Strength and stability—covalent bonds are very strong; it takes a lot of energy to break them. • Multiple bonds • Single—sharing 1 pair of electronsC H • C C Double—sharing 2 pairs of electrons • N N Triple—sharing 3 pairs of electrons 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. • 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