An ATOM is the smallest unit of an element that maintains the properties of that element.
• Postulated existence of atoms and void (makes change possible) • Elements differ in shape, position, and arrangement • Believed that atoms are: – – – –
Microscopic Indestructible Entirely solid (no holes or gaps) Homogeneous with no internal structure
• Aristotle and other well-known Greek philosophers didn’t believe Democritus
• The late 1700’s –definitions and basic laws had been discovered and accepted by chemists. – Element – substance that cannot be broken down by ordinary chemical means. – Chemical Reaction – transformation of substance or substances into one or more new substances.
Mass cannot be created or destroyed just changed from one form to another. (Antoine Lavosier)
A chemical compound contains exactly the same elements in the same proportion regardless of sample size. (Joseph Proust from work of Gay-Lussac & Amadeo Avogadro – 1802/1804)
If two or more different compounds are composed of the same two elements, then the ratio of the masses of those elements will always exist as a ratio of small whole numbers. (John Dalton – 1808)
1. All elements are composed of tiny indivisible particles called atoms. 2. Atoms of the same element are identical. 3. The atoms of one element are different from the atoms of another element. 4. Atoms combine in simple whole-number ratios. 5. Atoms are separated, joined or rearranged in chemical reactions. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction.
• Rutherford, Geiger & Marsden (1912) -showed that most of the atom was empty space, but that atoms had a solid, positive core.
• Most particles passed through the foil undisturbed (black arrows). • A few were deflected (red arrows). • Rutherford reasoned that each atom in the foil contained a small, dense, positively charged nucleus surrounded by electrons.
• Rutherford, Geiger & Marsden (1912) -showed that most of the atom was empty space, but that atoms had a solid, positive core.
Rutherford’s Nuclear Atom
Thomson’s “Plum Pudding” Model of the Atom Electrons Uniform, positively charged sphere
Nucleus
Electron Cloud
• Atomic Number – Represents the number of protons – Defines the identity of the element
• Mass Number – Represents the total number of protons and neutrons
• Atomic Mass – average mass of the isotopes (based on relative frequency in nature)
• Location: Electron cloud • Relative Mass: 1/1840 amu
An ISOTOPE is an atom with a different number of Neutrons and therefore a different atomic mass. Example: C-12 vs. C-14
An ION is an atom that has lost or gained electrons, resulting in a positively or negatively charged particle.
Atomic Number = # of protons 8
O Atomic Symbol
Mass Number = # protons & neutrons (round to 16)
15.999 Mass Number (unrounded) = average mass of the isotopes
o A scale designed for atoms gives their small atomic masses in atomic mass units (amu) o An atom of 12C was assigned an exact mass of 12.00 amu o Relative masses of all other atoms was determined by comparing each to the mass of 12C
o An atom twice as heavy has a mass of 24.00 amu. An atom half as heavy is 6.00 amu.
o Listed on the periodic table o Gives the mass of “average” atom of each element compared to 12C o Average atom based on all the isotopes and their abundance %. o Atomic mass is not a whole number due to isotopes.
Percent(%) abundance of isotopes Mass of each isotope of that element Weighted average = mass isotope1(%) + mass isotope2(%) + … 100
100
Isotopes
Mass of Isotope
Abundance
24Mg
=
24.0 amu
78.70%
25Mg
=
25.0 amu
10.13%
26Mg
=
26.0 amu
11.17%
Atomic mass (average mass) Mg = 24.3 amu
Mg 24.3
The element copper has naturally occurring isotopes with mass numbers of 63 and 65. Isotope
Atomic Mass
Relative Abundance
Cu-63
63 amu
69.2%
Cu-65
65 amu
30.8%
Calculate the average atomic mass of copper. Cu-63:
(63 amu)(0.692) = 43.596 amu
Cu-65:
(65 amu)(0.308) = 20.02 amu
Total:
63.616 amu
Naturally occurring boron is 80.20% B-11 and 19.80% of a different isotope. What must the mass of this isotope be if the average atomic mass of boron is 10.81 amu? Isotope
