Saturday, July 21, 2012

Chapter Three chemistry note


·         Air’s atomic mass is 29
·         Pure substance is a type of matter that has definite composition (elements are pure because they are only composed of only one kind of atom)
·         Compounds are pure substances that consist of two or more elements always in the same proportion
·         Difference between elements and compounds is that chemical processes can break down compounds into simpler substances such as elements
·         Mixtures are mixed, not combined
o   Homogeneous: called a solution, the composition is uniform throughout (salt water)
o   Heterogeneous: not a uniform composition throughout (oil and water)
·         Elements cannot be broken down into simpler substances
·         All elements on the periodic table are made up of atoms. Atoms are the smallest particles of elements that retain the characteristics of that element.

·         Dalton’s Atomic Theory:
o    All matter is made of atoms
o   All atoms within an element are similar to each other, but different from atoms of other elements
o   Atoms of two or more can combine to form compounds, but a particular compound is always made of the same kinds of atoms and always has the same number of those atoms
o   A chemical reaction involves the rearrangement, separation, or combination of atoms. Atoms are never created or destroyed during a chemical reaction.
·         Groups in the periodic table are vertical. Groups with the letter A after them refer to the amount of valence electrons they have (1A has one, 2A has two, all the way up to 8A)
·         Periods in the periodic table are horizontal.
·         Atoms have a neutral charge (protons = electrons)
·         Electrons are in the outer shells of atoms, are negatively charged, and weigh nothing
·         Protons are in the nucleus (the positively charged center of an atom), are positively charged, and are heavier than electrons
·         The amount of protons and electrons in an atom are equal, but the electrons don’t contribute to the weight.
·         Neutrons are in the nucleus, are neutral in charge, and also contribute to the weight of the atom
·         Atomic mass is the protons + neutrons
·         Atomic number is the number of protons (can also be the number of electrons since those numbers are equal)
·         Finding the number of protons, electrons, or neurons is easy is you have the atomic mass and the atomic number
o   To find the number of electrons, look at the atomic number
o   Same thing to find the number of protons
o   To find the number of neutrons, subtract the atomic number from the atomic mass
·         Isotopes are atoms of the same element that have different numbers of neutrons; all elements of Mg have 12 protons, but some have 12 neutrons, 13 neutrons, or 14 neutrons. This causes their mass numbers to be different but not their chemical behavior. When writing a symbol for an isotope: ­mass number on top of the atomic number (bigger on top) then the elemental symbol
·         The average atom has an atomic mass, which is an average of the mass of all of the naturally occurring isotopes of that element. Multiply the % X mass and average among the three
·         Electrons of similar energy are grouped into energy levels. The max number of electrons allowed in each shell (energy level) is given by the formula: 2n2  where n is the number of the energy level; level 6 can hold 144 (2 X 6 = 12 and 12 X 12 is 144). Can go up to 7 energy levels.
·         If the shell has its maximum amount of electrons than it can’t react
·         An orbital is a region in space around the nucleus in which an electron is most likely to be found.
·         The electron dot symbol (Lewis dot structure) is a way to represent the number of valence electrons.
·         The volume of atomic size of an atom is determined by its atomic radius (the distance from the nucleus to the valence electrons)
·         Ionization energy is the energy needed to remove the least tightly bound electron from an atom in the gaseous state.
·         When an electron is removed from a neutral atom, the particle is called a cation, w/ a 1+ charge. It removes the least tightly bound electron from the outer shell.
·         The ionization energy decreases going down a group on the periodic table.

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