Learning Outline
Introductory ChemistryPre-A&P
Chemistry is the basis for biology
Basic structural concepts
- Atom: smallest part of a pure substance, or element, that still has the properties of that substance
- Molecule: a particle made up of one or more atoms bound together
- Element: a pure substance, made entirely of one kind of atom (all known elements are listed in the Periodic Table)
- Compound: substance made of molecules that contain more than one kind of atom
Atomic structure and function
- Many different models exist; none are perfect
- We’ll look at the Bohr model and electron cloud model first —then later structural and space-filling models
- Bohr model (named for Niels Bohr) shows electrons orbiting the nucleus like planets orbit the sun
- Electron cloud model (ECM) shows electrons as a cloud surrounding the nucleus
- Nucleus
- Protons have positive (+) charge; number of protons is “atomic number” and signifies kind of atom (which element)
- Neutrons have no charge; optional and variable in number (isotopes)
- Electrons
- Electrons have a negative (-) charge and do not affect the type of element
- Electrons may be present in regions called energy levels
- The further from the nucleus, the more energy is needed by the electron to stay there –so the higher energy levels are farther from the nucleus
- The lowest (first) energy level can contain up to two electrons
- The next few can hold up to eight electrons each (the “octet rule”)
- Energy levels fill from the inside out
Behavior of atoms: chemical bonds
- Atoms want to be “happy”—meaning having a full outer energy level
- One path to happiness: ionic bonds
- Occurs when an atom gives one or two electrons to another atom, giving both “full” outer energy levels
- Ion: charged particle (atom or group of atoms)
- Ions “stick” together because opposite charges attract —forming a bond
- Electrolyte: molecule that dissolves in water to form ions
- Sometimes the individual, dissolved ions are also called electrolytes
- Another path to happiness: covalent bonds
pH: acids, bases, and buffers
- Water molecules may dissociate into H+ and OH–
- H+ is the hydrogen ion
- OH– is the hydroxide ion
- Solutions with equal proportion of H+ and OH– are “neutral”
- Acids: solutions with higher proportion of H+
- Bases: solutions with a lower proportion of H+ (alkaline solutions)
- Can be expressed as “power of Hydrogen” or pH
- pH scale is an inverse (base 10) logarithm of relative H+ concentration
- 7 = neutral
- acid = anything lower than 7
- base = anything higher than 7
- human blood plasma = pH 7.35-7.45
- Buffer: system of molecules that absorb or release H+ —maintaining a relatively stable pH
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Last updated: October 23, 2019 at 0:11 am