Proton

A nuclear particle having a positive charge equal to that of the electron and a mass more than 1800 times that of the electron

The number of protons in an atom is called the atomic number, Z.

An element is a substance whose atoms have the same number of protons and thus the same atomic number, Z.

Neutron

A nuclear particle having a mass almost equal to that of the proton but no electrical charge

The mass number, A, is the total number of protons and neutrons in the nucleus.

Isotopes are atoms whose nuclei have the same atomic number (number of protons) but different numbers of neutrons (mass number).

Fig. 2.13

•Nuclide

•An atom characterized by a certain atomic number, Z, and mass number, A.

•Nuclide symbol

•Examples:

Li73 C13

6

symbolChemicalAZ

Na2311

Atomic Definitions I: Symbols, Isotopes,Numbers

XA

Z

X = Atomic symbol of the element, or element symbol

A = The Mass number; A = Z + N

Z = The Atomic Number, the Number of Protons in the Nucleus

N = The Number of Neutrons in the Nucleus

Isotopes = atoms of an element with the same number of protons, but different numbers of Neutrons in the Nucleus

The Nuclear Symbol of the Atom, or Isotope

• Write the nuclide symbol for the atom that has 19 protons and 20 neutrons.

K3919

Atomic number: Z = 19The element is potassium, K.

Mass number: A = 19 + 20 = 39

The nuclide symbol is

Depicting the Atom

Fig. 2.14

Neutral ATOMS

• 51 Cr = P+ (24), e- (24), • N (27)

• 239 Pu = P+(94), e- (94), • N (145)

• 15 N = P+(7), e-(7), N(8)

• 56 Fe = P+(26), e-(26),• N (50)

• 235 U =P+(92), e-(92), • N (143)

Atomic Mass

The average atomic mass for the naturally occurring element expressed in atomic mass units (amu) (Note that this is a weighted average of the relative abundances and masses of the isotopes.)

Atomic Mass Unit

Equal to exactly one-twelfth the mass of a carbon-12 atom

Isotopes of Hydrogen

11H 1 Proton 0 Neutrons 99.985 % 1.00782503 amu

21H (D) 1 Proton 1 Neutron 0.015 % 2.01410178 amu

31H (T) 1 Proton 2 Neutrons -------- ----------

The average mass of Hydrogen is 1.008 amu

• 3H is Radioactive with a half life of 12 years.

• H2O Normal water “light water “

• mass = 18.0 amu , BP = 100.000000C

• D2O Heavy water

• mass = 20.0 amu , BP = 101.42 0C

Figure 2.9: A representation of two isotopes of carbon.

Figure 2.11: Diagram of a simple mass spectrometer.

Figure 2.12: The mass spectrum of neon.

Element #8 : Oxygen, Isotopes

• 168O 8 Protons 8 Neutrons

• 99.759% 15.99491462 amu

• 178O 8 Protons 9 Neutrons

• 0.037% 16.9997341 amu

• 188O 8 Protons 10 Neutrons

• 0.204 % 17.999160 amu

Calculating the “Average” Atomic Mass of an Element

24Mg (78.7%) 23.98504 amu x 0.787 = 18.876226 amu 25Mg (10.2%) 24.98584 amu x 0.102 = 2.548556 amu26Mg (11.1%) 25.98636 amu x 0.111 = 2.884486 amu

24.309268 amu

With Significant Digits = 24.3 amu

Problem: Calculate the average atomic mass of Magnesium! Magnesium Has three stable isotopes, 24Mg ( 78.7%); 25Mg (10.2%); 26Mg (11.1%).

Problem: Calculate the abundance of the two Bromine isotopes: 79Br = 78.918336 g/mol and 81Br = 80.91629 g/mol , given that the average mass of Bromine is 79.904 g/mol.

Plan: Let the abundance of 79Br = X and of 81Br = Y and X + Y = 1.0

Solution: X(78.918336) + Y(80.91629) = 79.904

X + Y = 1.00 therefore X = 1.00 - Y (1.00 - Y)(78.918336) + Y(80.91629) = 79.904

78.918336 - 78.918336 Y + 80.91629 Y = 79.904

1.997954 Y = 0.985664 or Y = 0.4933

X = 1.00 - Y = 1.00 - 0.4933 = 0.5067

%X = % 79Br = 0.5067 x 100% = 50.67% = 79Br %Y = % 81Br = 0.4933 x 100% = 49.33% = 81Br

• An element has four naturally occurring isotopes. The mass and percentage of each isotope are as follows:

Percentage Abundance Mass (amu)

1.48 203.973

23.6 205.9745

22.6 206.9759

52.3 207.9766

What is the atomic weight and name of the element?

To find the portion of the atomic weight due to each isotope, multiply the fraction by the mass of

the isotope. The atomic weight is the sum of these products.

Fractional Abundance

Mass (amu) Mass From Isotope

0.0148 203.973 3.01880040

0.236 205.9745 48.6099820

0.226 206.9759 46.7765534

0.523 207.9766 108.771762

207.177098

The atomic weight is 207 amu; the element is lead.

2. (8 points) Atoms X, Y, Z, and R have the following nuclear compositions:

186410 X 183

410Y 186412Z 185

412R

Which two are isotopes of each other? Circle them and explain.

• 6. (11 points) Fill in the blank spaces and write out all the symbols in the left hand column in full, in the form (i.e. include the appropriate values of Z and A as well as the correct symbol X and the charge, if any).

Symbol # protons #electrons #neutrons

1840Ar

Au 118

11 10 12

37 37 48

17 18 17

• (15 points) Lithium forms compounds which are used in dry cells and storage batteries and in high-temperature lubricants. It has two naturally occurring isotopes, 6Li (isotopic mass = 6.015121 amu) and 7Li (isotopic mass = 7.016003 amu). Lithium has an atomic mass of 6.9409 amu. What is the percent abundance of lithium-6?