# Unit II: Atoms and The Periodic Table

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Unit II: Atoms and The Periodic Table
ATOMIC NUMBER, ATOMIC MASS, AND ISOTOPES

Subatomic Particles Particles which make up the atom are called subatomic particles.

Atom: Nucleus Protons Neutrons

Atom: Nitrogen Electrons Energy Levels

Ordinary Hydrogen Tritium Deuterium

Isotopes Isotopes are:
Different forms of the same element with the SAME # of Protons but with DIFFERENT #’s of Neutrons

Also called “Hydrogen – 3”
Mass Number Mass Number = Total Protons + Neutrons in an isotope of an element Mass Number P + N 3H 1 P + 2 N’s Also called “Hydrogen – 3”

Called Nuclear Notation
Mass Number 3H Called Nuclear Notation

41Ca Practice Problem 20 To find P’s and N’s from Nuclear Notation
Mass # = P + N To find # of Neutrons, put Atomic Number Here: 41Ca 20 21 Calcium’s atomic Number = 20 Subtract to get # of Neutrons 20 Protons So it has 21 Neutrons

Find the # of Protons and # of Neutrons in Each of the Following Isotopes:

Find the # of Protons and # of Neutrons in Each of the Following Isotopes:

Now Try the Other Way

Now Try the Other Way

Now Try the Other Way

Now Try the Other Way

Neutral Atom In a Neutral Atom of an Element:
The # of Electrons(-) = The # of Protons(+)

Ion An ion is an atom where: In a + ion (cation) In a - ion (anion)
# of e-’s ¹ ≠ # of p’s In a + ion (cation) # of e-’s < # of p’s In a - ion (anion) # of e-’s > # of p’s

Example #1 The ion Ca 2+ has 20 protons (atomic #) and it has 18 e-’s (2 less –’s than +’s, hence a 2+ charge)

Example 2 The ion Se 2-has 34 protons (atomic #) and it has 36 e-’s (2 more –’s than +’s, hence a 2- charge)

Example 3 An atom of Bi has 83 protons (atomic #) and it has 83 e-’s (e-’s are equal to protons, hence, no charge (neutral))

Isotopes Continued The isotope: 56Fe 3+ has 26 protons
30 neutrons (56-26) 23 electrons (26-3)

Isotopes Continued The isotope: 76As 3- has 33 protons
43 neutrons (76-33) 36 electrons (33 +3)

Isotopes Continued The isotope: 201Au+ has 79 protons
122 neutrons (201-79) 78 electrons (79 - 1)

An isotope has 46 protons, 58 neutrons and 42 electrons
An isotope has 46 protons, 58 neutrons and 42 electrons. Write the nuclear notation: Answer: 104Pd 4+

An isotope has 52 protons, 79 neutrons and 54 electrons
An isotope has 52 protons, 79 neutrons and 54 electrons. Write the nuclear notation: Answer: 131Te2-

Finding (average) atomic mass using isotope abundances
Assumption: Assume mass of a neutron = mass of proton = 1 u (atomic mass unit) this is not “exactly” correct The mass of a proton = u The mass of a neutron = u

Example #1 Chlorine in nature is 75.77% 35Cl and 24.23% 37Cl
Using these: Find the average atomic mass of Cl Change % to decimals