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University of Rhode Island
CHM 103

Chapter 2: Atoms and Molecules Symbols and Formulas ● a unique symbol is used to represent each element ● formulas are used to represent compounds Elemental Symbols ● a symbol is assigned to each element. ● the symbol is based on the name of the element and consists of one capital letter or a capital letter followed by a lowercase letter ● some symbols are based on the Latin or German name of it (know the Latin ones) Halogens ● 7a ● gas, liquid, solid as you go down the group ● human body has a high tolerance for halogens Compound Formulas ● a compound formula consists of the symbols of the elements found in the compound. Each elemental symbol represents one atom of the element. If more than one atom is represented, a subscript following the elemental symbol is used. ○ molecular formula ○ structural formula ○ ball-and-stick model ○ space-filling model ● examples ○ CO ○ H20 ○ NH3 - ammonia ○ H202 - hydrogen peroxide ○ CH4 - methane The structure of atoms ● atoms are made up of three subatomic particles, protons, neutrons, and electrons ● the protons and neutrons are tightly bound together to form the central portion of an atom called the nucleus ● the electrons are located outside of the nucleus and thought to move very rapidly throughout a relatively large volume of space surrounding the small but very heavy nucleus Neutrons ● the weight is the sum of the protons and electrons ● neutral ● 1 atomic mass unit ● n ● 1.67E-24 Electrons ● 1/1836 amu ● -1 charge ● e- ● 9.07E-28 Protons ● +1 charge ● 1 amu ● 1.67E-24 ● p, p+, H+ ● protons and neutrons are nucleons Atomic Number of an atom ● the atomic number of an atom is equal to the number of protons in the nucleus of the atom ● atomic numbers are represented by the symbol Z (number). Mass Number of an Atom ● the mass number of an atom is equal to the sum of the number of protons and neutrons in the nucleus of the atom ● mass numbers are represented by the symbol A (number). ● Fluorine - 19 contains: 9 protons, 10 neutrons, and 9 electrons Isotopes ● isotopes are atoms that have the same number of protons in the nucleus but different numbers of neutrons. that is, they have the same atomic number but different mass numbers. ● because they have the same number of protons in the nucleus, all isotopes of the same element have the same number of electrons outside the nucleus ● 35/17Cl - 17 p, 18 n, e- 17 ● 37/17Cl - 17 p, 20 n, e- 17 Symbols of isotopes ● isotopes are represented by the symbol A/ZE where Z is the atomic number, A is the mass number and E is the elemental symbol ○ Ex. 60/28Ni - contains 28 protons and 32 neutrons in the nucleus ● Isotopes are also represented by the notation Name-A where Name if the name of the element and A is the mass number of the isotope ○ Ex. Magnesium-26 Relative Mass ● the extremely small size of atoms and molecules makes it inconvenient to use their actual masses for measurements or calculations. Relative masses are used instead ● relative masses are comparisons of actual masses to ea
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