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

Energy Levels

Electrons

Ordinary Hydrogen

DeuteriumTritium

Isotopes

Isotopes are:Different forms of the same element

with the SAME # of Protons but with DIFFERENT #’s of Neutrons

Mass Number

Mass Number = Total Protons + Neutrons in an isotope of an element

3HMass Number

P + N

1 P + 2 N’s

Also called “Hydrogen – 3”

Mass Number

3HCalled Nuclear Notation

Practice Problem To find P’s and N’s from Nuclear

Notation

Calcium’s atomicNumber = 20

20 Protons

To find # ofNeutrons, putAtomic NumberHere:

Mass # = P + N

Subtract to get #of Neutrons

21

So it has 21 Neutrons

41Ca20

Find the # of Protons and # of Neutrons inEach of the Following Isotopes:

Find the # of Protons and # of Neutrons inEach 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:# 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, 58neutrons and 42 electrons.Write the nuclear notation:

Answer: 104Pd 4+

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 = 1.007276 uThe mass of a neutron = 1.00866 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