ChemCom Unit 1B
Phases of the moon
The phases of the moon
repeat at regular
intervals or periods.
- This is termed
“periodicity”
The history of the periodic table
 Johann Dobereiner – Law of Triads (1817)
The history of the periodic table
 1862- Alexandre-Emile Béguyer de Chancourtois was
the first person to organize the elements by atomic
mass
 John Newlands – Law of Octaves 1865
 Dmitri Mendeleev – 1872
Early Periodic tables
 The tables of the 1800’s were organized according to
two characteristics
 Atomic mass
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

Physical property
Hydrogen atoms have the lowest mass, oxygen atoms are
about 16 times more massive, sulfur was about 32 times more
massive
An average atomic mass was assigned to each element based
on these comparisons
 Combining capacity with other elements, such as
chlorine and oxygen


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Chemical property
One atom of potassium (K) or cesium (Cs) combines with
only one atom of chlorine to produce KCl or CsCl
One atom of magnesium (Mg) or strontium (Sr) combines
with two atoms of chlorine to produce MgCl2 or SrCl2
 Vertical columns
 Elements with similar chemical properties
 Horizontal arrangements
 Based on increasing atomic mass
 Creators of early periodic tables were unable to explain
the similarities in properties among neighboring
elements

50 years after Mendeleev, the reason for these patterns was
proposed
The Electrical Nature of Matter
 Like charges repel
+
+
-
 Unlike charges attract
+
-
-
 Every electrically neutral (uncharged) atom contains
equal numbers of positively and negatively charged
particles:
 Protons – positively charged particles found in the
nucleus (center) of the atom

The number of protons identifies what element the atom is
 Electrons – negatively charged particles found outside
the nucleus of the atom
 An electrically neutral atom of chlorine contains


17 protons
17 electrons
 Ions (atoms or groups of atoms that are NOT
electrically neutral) contain different numbers of
protons and electrons
 A potassium atom with 19 protons and 18 electrons will
have a charge of:


+1
Because there is one more proton than there are electrons.
 Neutrons
 Electrically neutral particles within the nucleus of most
atoms
 Only hydrogen contains no neutrons
 Does not affect the charge of the atom
The Modern Periodic table
 Elements are placed in sequence according to their
increasing atomic number (number of protons)
rather than their atomic mass
 Early periodic tables used atomic mass rather than
atomic number



Works well for relatively small atoms
Does not work well for larger/ more massive atoms
Note that Cobalt (Co) and Nickel (Ni) are not in mass order
 The nucleus of the atom
 A concentrated region of positive charge in the center of
the atom
 Contains both protons (positive charge) and neutrons
(neutral charge)
 The total number of protons and neutrons in the
nucleus is called the mass number
 Electrons are not included in the mass number because
each electron is only about 1/2000th the mass of a proton
or neutron
 Atoms of the same element
 All have the same number of protons
 May have differing numbers of neutrons
 Atoms with the same number of protons but different
numbers of neutrons are known as isotopes

Isotopes are atoms of the same element with different mass
numbers
 Atomic masses
 Are decimals because it represents the average mass of
all of the different isotopes of that particular element.
Counting Subatomic Particles
 Atomic number:
 Equals the number of protons
 Equals the number of electrons (in a neutral atom)
 Atomic mass:
 Is an average of the masses of all the possible isotopes of
a particular element
 Rounded to the nearest whole number, it equals the
mass number for the most common isotope
 Mass number:
 Equals the number of protons + number of neutrons
 In a neutral atom:
 # Protons = # electrons = atomic number
 # Neutrons = mass number – # protons
 In an ion (charged atom):
 # Protons = atomic number
 # Electrons = # protons – charge


Ex. 1
Na+
 11 protons
 11 – 1 = 10 electrons
Ex. 2 Cl 17 protons
 17 – (-1) = 18 electrons
 # Neutrons = mass number - # protons
 To find the charge of an ion when the number of
protons and electrons is known:
 Charge = # Protons - # electrons


Ex. 1
an ion of cobalt with 25 electrons will have what
charge?
 27 -25 = +2
Ex. 2 an ion of sulfur has 18 electrons. What is its charge?
 16 – 18 = -2
 You can now complete B.5 Extra Practice packet