Transcript Slides

The smallest particles of matter are atoms. Atoms have a
nucleus, with protons and neutrons as major components
and electrons which orbit the nucleus.
A compound, composed of the same kind of atoms is called
an element.
Each element has a characteristic number of positively charged
protons in its nucleus. The number of protons present in a nucleus
is called atomic number.
The total number of protons and neutrons in an nucleus
are called the mass number.
The mass of atoms is very small in the order of
10-23 to 10-22 g.
Because the use of such small numbers is inconvenient, a
relative mass scale is used, known as
atomic mass units (abbreviated amu) or sometimes called
dalton.
The scale is based on the carbon-12 atom which has 6
protons, 6 neutrons and 6 electrons and and has a mass of
1.9926 x 10-23 g.
1 amu = 1/12 of the mass of a C-12 atom = 1.66054 x 10-24 g
The mass of a proton is 1.6726 x 10-24 g = 1.00728 amu
The mass of a neutron is 1.6749 x 10-24 g = 1.00867 amu
The mass of a an electron 1/1837 of the proton mass
= 0.000549 amu
atomic mass H = 1.00728 amu (mass of proton) + 0.00055
amu (mass of electron) = 1.00783 amu.
atomic mass He = 2 x 1.00728 + 2 x 1.00867 + 2 x 0.00055 =
4.03300 amu
calculated atomic mass of He atom (amu) : 4.03300
actual atomic mass of He atom (amu): 4.00260
mass difference (amu): 0.03040
Isotopes are atoms of the same element, differing by
the number of neutrons.
Naturally occurring chlorine has two stable
isotopes Cl-35 and Cl-37.
The nucleus of Cl-35 has 17 protons and 18 neutrons,
the nucleus of Cl-37 has 17 protons and 20 neutrons.
The atomic mass or atomic weight - as it is more
frequently called - of an atom is the
average mass (amu) of the mixture of isotopes that reflects
the masses and relative abundance of the elements as
they occur in nature.
For hydrogen we would expect :
mass H-atom = 1.00728 amu (proton) + 0.00055 amu
(electron) - 0.00004 amu (mass defect) = 1.00779 amu
The actual value listed in the periodic table is 1.00794 amu
natural
abundance
atomic
mass
(amu)
Calculation
H atom
99.985 %
1.00779
1.00728 amu (proton)
+ 0.00055 amu (electron)
- 0.00004 amu (mass defect)
= 1.00779
D atom
0.015 %
1.01355
1.00728 amu (proton)
+ 1.00867 amu (neutron)
+ 0.00055 amu (electron)
- 0.00295 amu (mass defect)
= 2.01355
1.00794
1.00779 amu x 0.99985
+ 2.01355 amu x 0.00015
= 1.00794 amu
naturally
occurring
Hydrogen
Atomic mass of Mg
• Mg-24, 78.99 %
Mg-25, 10.00 %
Mg-26, 11.01 %
• Since Mg has an atomic number of 12, we know its nucleus has 12
protons. Thus, the atomic mass (atomic weight) of naturally occurring
Mg is :
0.7899 x (12 x 1.00728 amu + 12 x 1.00867 amu + 12 x 0.00055)
+ 0.1000 x (12 x 1.00728 amu + 13 x 1.00867 amu + 12 x 0.00055)
+ 0.1101 x (12 x 1.00728 amu + 14 x 1.00867 amu + 12 x 0.00055)
- 0.215976 amu ( mass defect) = 24.30500 amu
• Approximate calculation: (Mp, Mn =1 amu, neglect Me, mass defect)
Atomic weight: Mg= 0.7899 x 24 + 0.1 x 25 + 0.1101 x 26 = 24.32 amu
Radioactivity
Most elements exist in several isotopic forms. Some isotopes are instable
and their nuclei break apart. During this break up, energy is emitted in form
of radiation and the element is said to be radioactive.
Radiation
Alpha Radiation
The radioactive nucleus becomes more stable by giving off alpha radiation.
Alpha radiation consists of helium nuclei ( no electrons).
U-238 emits alpha radiation to form Th-234.
Beta Radiation
Beta radiation consists of electrons. During beta decay a neutron in the
nucleus is converted into a proton and an electron. The electron is emitted.
The carbon-14 isotope is a beta emitter.
Gamma Radiation
Gamma radiation is a high energy electromagnetic radiation similar to Xrays. Gamma radiation is usually associated with alpha or beta decay.
Ra-226 decays to Rn 222 + He (alpha radiation) + gamma radiation
Radioactive decay equation
• Nt = No e - γ t
No = number atoms initially present ( at time to)
Nt = number of atom left after time t .
γ = decay constant
• The half life of a radioactive nuclide is defined as the time it takes
half of the sample to decay.
• N t[half life]) = 1/2 No
• 1/2No = No e - γ t[half life]
e + γ t[half life] = 2
γ t[half life] = ln2 or t[half life] = 0.693/γ
Nt = No e - γ t
Nt = No e - (0.693 / t[half life]) t
The rate of radioactive emission or the rate of nuclear decay is
measured in half life. The half life time is the time it takes for half
of a given amount of an radioactive element to degrade.
Radium-226 has a half life time of 1600 years.
( 1g of Ra-226 will have degraded to 0.5 g after 1600 years).
C-14 is often used to date organic matter. With a half life 5730
years. C-14 dating is used to date objects up to 40000 years.
Uranium-235 has a half life time of 7.04 x 106 years. It decays to a
number of intermediary isotopes, with Pb-207 being the final stable isotope.
Using ratios of the intermediate isotopes and Pb-207, rocks can be dated to the
beginning of the formation of the earth (4.8 billion years ago).
P-32 a beta emitter, its half life is 14 days . It is mostly used in
molecular biology research..
I-131 (half life = 8 days, beta and gamma) and I-123 (half life 13
hrs) can be used to test the thyroid function.
The position of the electrons around the atom has to be
described as a space in which there is a high probability
to find the electron. This region is called an orbital.
S-orbital
P-orbital
adding electrons to the shells
Cations (positively charged ions) are formed if electrons are
removed from an atom. The energy necessary to remove one
electron from an atom is called ionization energy. The energy
required to remove one electron from a sodium atom is:
Na → Na+ + eEionization = 2 x 10-19 cal or 5.1 eV .
Anions are formed if a neutral atom accepts an electron.
The tendency of an atom to attract electrons is called
electronegativity . If the electronegativity is strong enough, the
electron can be transferred completely to the atom, forming a
negatively charged ion or anion.
Atomic and Molecular Interactions
The outer electrons of atoms can interact to form :
Covalent bonds
Ionic bonds
In addition molecules can form non covalent associations:
Non polar interactions
Polar interactions
Hydrogen bonds.
Ionic bonds
If an electron is transferred form one atom to another atom two
oppositely charged atoms or ions are formed. The force of
attraction between the ions is called an ionic bond. All metals
are capable of forming ionic bonds.
Bond Energies
Covalent bonds are strongest atomic interactions.
Bond energies are between 60 - 110 kcal/mol for single
bonds, approximately 150 kcal/mol for double bonds and
about 200 kcal/mol for triple bonds
Ionic bonds :
4 – 8 kcal/mol
H-bonds : 4 -5 kcal/mol
Polar interactions : 2 – 3 kcal/mol
Hydrophobic interactions : 1 kcal/mol