Structure of Atoms
Physical Science
Chapter 4 – Section 2
State Standards
• CLE.3202.1.2 – Describe the structure and
arrangement of atomic particles
What is an Atom?
• The three main components of an atom are
distinquished by mass, charge, and location
• The chemistry of each element depends
directly on these three components
• Nucleus:
– Protons ( positively charged )
– Neutrons ( neutral – NO CHARGE )
• Electrons ( negatively charged )
Electric Forces in Atoms
• Protons and electrons attract each other
• Electric force holds components together; the
same forces hold solids and liquids together
• Liquid water is held together by electric forces
Atomic Components
• Protons ( +1 charge / 1.67 x 10-27 kg )
• Neutrons ( 0 charge / 1.67 x 10-27 kg )
• Electrons ( -1 charge / 9.11 x 10-31 kg )
• All ( most ) elements contain all three
• Atoms of the same element contain the same
number of protons, but neutrons may vary
• Each element has a unique number of protons
Distinguishing Elements and Atoms
• Atomic number – number of protons in an atom
• Mass number – number of protons & neutrons
• Isotopes – exist for an element if different atoms
( with the same atomic number ) have varying
numbers of protons ( different mass numbers )
Hydrogen & Helium
• Hydrogen ( H ) contains 1 electron, 1 proton, and
0 neutrons [ Protium isotope ]
• Helium ( He ) contains 2 electrons, 2 protons, and
2 neutrons
• What is atomic number, mass number of H, He?
Isotopes of Hydrogen
• Protium – Most common form on Earth and Sun
– 1 electron, 1 proton, 0 neutrons
• Deuterium – small fraction of hydrogen; 1 out of
every 6,000 hydrogen atoms in Earth’s crust
– 1 electron, 1 proton, 1 neutron
• Trillium – very unstable and so very rare
– 1 electron, 1 proton, 2 neutrons
Isotopes of Chlorine
• The common form of chlorine ( Cl ) has 17 protons,
17 electrons, and 18 neutrons
• Another form has 20 neutrons
– Neutrons can be calculated by using these
• How do we know which form is most common?
Atomic Mass Units
• Used to express the mass of such tiny particles
• Unified Atomic Mass Unit is 1/12 the mass of a
Carbon-12
– Carbon-12 isotope has a fairly even mass number
– 6 protons and 6 neutrons
– Remember electrons contribute little mass to atom
– Each proton/neutron has mass of 1.0 u
– Units given as a u
Atomic Mass Units
• The value of AVERAGE ATOMIC MASS found in the
periodic table tells us which isotope of any
element is most common ( HYDROGEN )
– weighted average so most common form counts most
• Zinc with average atomic mass 65.4 u
– 65 ( closest mass # ) – 30 ( atomic # ) = 35 neutrons
– Most common Zn isotope has 35 neutrons
• Cl has average of 35.453 u , with 35 mass number
– Most common Cl form contains 18 neutrons
The Mole
• In order to count large numbers of small particles, a
COUNTING UNIT was devised – THE MOLE ( mol )
• Examples ( Avogadro’s number ):
– 1 mol = 602,213,670,000,000,000,000,000 particles
– Equals 6.022 x 1023 when put into scientific notation
– 1 mol of marbles = 6.022 x 1023 marbles
– 1 mol of atoms = 6.022 x 1023 atoms ( any atom )
– 1 mol of He atoms = 6.022 x 1023 He atoms
The Mole
• How many stars in the universe?
– 2003, roughly 7 x 1022 stars counted ( within range
– Value could increase in future
• 6.022 x 1023 popcorn kernels would cover the USA
in a pile 500 km ( or 310 mi ) tall!!
• Avogadro’s number is used only for small particles
Relating Moles and Grams
• Mass of 1 mol random atoms difficult to find
– Each element has a unique MOLAR MASS
– The mass of 1 mol of this type of atom ( element )
– Usually accounts for several isotopes
• Mass of 1 mol of the same atom easy to find
• For Carbon-12, molar mass is 12.00 g
– Molar mass ( in g ) equals average atomic mass ( in u )
– Carbon-12 has 12.01 u as well as 12.01 g/mol
• Figure 7 in text
Relating Moles and Grams
• Mass of 1 mol Carbon-12 atoms is 12.01 g
– USE PERIODIC TABLE
• 24 g Carbon-12 = 2 mol Carbon-12
• 3 mol Carbon-12 = 36 g Carbon-12
Molar Mass of Compounds, Molecules
• As elements ( atoms with same atomic number ) have a
MOLAR MASS, so do coumpounds
• H2O : 2 Hydrogen with 1.00 g/mol & 1 Oxygen with 16 g/mol
– Gives ( 2.00 + 16 ) g/mol = 18 g/mol
H
O
• CO2 : 1 Carbon with 12.01 g/mol & 2 Oxygens with 16 g/mol
– Gives ( 12.01 + 32 ) g/mol = 44 g/mol
• O2 : 2 Oxygen atoms at 16 g each
– Gives: 32 g/mol for O2 molecules