Atoms and Elements
Jan 22, 2016
Atoms and Elements
Structure of an Atom
Charges•Electron – negative•Proton – positive•Neutron - neutral
Parts•Nucleus – neutrons & protons•Orbitals - electrons
Elements
• Pure chemical substances made up of one type of atom
• Building blocks of matter
• Over 115 elements known - and more being discovered
Elements – Symbol / Name
Symbol: one or two letter abbreviation
Examples: Latin
C Carbon Au Gold-aurum
O Oxygen Lb Lead-plumbum
Mg Magnesium K Potassium-kalium
Br Bromine Na Sodium-natrium
Elements – Atomic Number
DEFINES the element
• ALL Carbon atoms have six protons, and …ALL atoms with six protons are Carbon
• BUT… they can have different numbers of neutrons These are called isotopes of carbon
Atoms are neutral… so have same # electrons (-) as protons (+)
Elements – Atomic Mass
• MASS NUMBER = mass of all particles in one atom (protons, neutrons and electrons)
• Measured in “atomic mass units” (amu)
Calculated from Mass Number
Elements – Mass Number• MASS NUMBER = mass of all particles in one
atom (protons, neutrons and electrons)• Mass of 1 proton = 1 amu
Mass of 1 neutron = 1 amuMass of 1 electron = really, really tiny!
• SO… mass number = # protons + # neutrons
Carbon•6 protons x 1 amu/proton = 6 amu•6 neutrons x 1 amu/neutron = 6 amu•6 electrons x really tiny mass = 0 amu
TOTAL = 6 amu + 6 amu + 0 amu = 12 amu
Elements – Mass Number
Helium
Atomic Number =
Mass Number =
Nitrogen
Atomic Number =
Mass Number =
2 (protons)
4 amu (p + n)
7 (protons)
14 amu (p + n)
Elements – Atomic Mass
• BUT… Why is the atomic mass for Lithium (atomic number 3; with 3 protons) = 6.94?!?!
• …. IsotopesLithium-6 - 3 protons, 3 neutrons - mass number = 6 - 7.5% of all lithium
Lithium-7 - 3 protons, 4 neutrons - mass number = 7 - 92.5 % of all lithium
NOTE: Mass number is used to name isotope
Elements – Atomic Mass
• BUT… Why is the atomic mass for Lithium (atomic number 3; with 3 protons) = 6.94?!?!
• Calculation: Weighted Average (info. only):– 7.5 % Lithium-6 at 6 amu– 92.5 % Lithium-7 at 7 amu
• (0.075 x 6 amu) + (0.925 x 7 amu) = (0.45 amu) + (6.48 amu) = 6.93 amu
Elements – Atomic Mass
• The atomic mass of an element is calculated by doing a weighted average of the mass numbers of all naturally existing isotopes.
• If you round the atomic mass of an element to the nearest whole number, you get the mass number of the most common isotope.
7.0
Elements – Symbol / Name
Atomic Mass• “Average” for all carbon• Calculated from mass
numbers of isotopes• On the periodic table
Mass Number• Calculated for each
individual isotope• Used to name the
isotope• Averaged together to
represent all carbonAll have same # protons
Same # protons, different # neutrons
Elements - SUMMARY
Structure of an Atom
• Outside the nucleuselectron– Electron
• Atomic mass 0• Charge (-)
nucleus(containing protonsand neutrons)
Structure of an Atom
• Electrons are arranged in orbits– Orbit 1 holds
2 electrons– Orbit 2 holds
8 electrons– Orbit 3 holds
8 electrons…
Structure of an Atom
• Nucleus – the center of an atom contains…– Protons
• Atomic mass 1• Charge (+)
– Neutrons• Atomic mass 1• Charge none
Symbols
• Either 1 or 2 letters symbols– First letter Capital, second lower case– Some based on Latin nameExample LatinC Carbon Au Gold-aurumO Oxygen Lb Lead-plumbumMg Magnesium K Potassium-kaliumBr Bromine Na Sodium-
natrium
The Periodic Table
• Elements are arranged by size and chemical properties.
• Rows – Period
• Size
• Columns – Group
• Properties
Properties
• Properties are related to atomics structure.• The number of electrons in the outer most
orbit (valence electrons) determine how an atom will behave in chemical reactions.