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 to 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 x g.

1 amu = 1/12 of the mass of a C-12 atom = x g The mass of a proton is x g = amu The mass of a neutron is x g = amu The mass of a an electron 1/1837 of the proton mass = amu atomic mass H = amu (mass of proton) amu (mass of electron) = amu. atomic mass He = 2 x x x = amu

calculated atomic mass of He atom (amu) : actual atomic mass of He atom (amu): mass difference (amu):

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 = amu (proton) amu (electron) amu (mass defect) = amu The actual value listed in the periodic table is amu

natural atomic Calculation abundance mass (amu) H atom % amu (proton) amu (electron) amu (mass defect) = D atom % amu (proton) amu (neutron) amu (electron) amu (mass defect) = naturally occurring amu x Hydrogen amu x = amu

Atomic mass of Mg Mg-24, % Mg-25, % Mg-26, % 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 : x (12 x amu + 12 x amu + 12 x ) x (12 x amu + 13 x amu + 12 x ) x (12 x amu + 14 x amu + 12 x ) amu ( mass defect) = amu Approximate calculation: (Mp, Mn =1 amu, neglect Me, mass defect) Atomic weight: Mg= x x x 26 = 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 X- rays. Gamma radiation is usually associated with alpha or beta decay. Ra-226 decays to Rn He (alpha radiation) + gamma radiation

Radioactive decay equation N t = N o e - γ t N o = number atoms initially present ( at time to) N t = 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 N o 1/2N o = N o e - γ t [half life] e + γ t[half life] = 2 γ t [half life] = ln2 or t [half life] = 0.693/γ N t = N o e - γ t N t = N o 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 years. Uranium-235 has a half life time of 7.04 x 10 6 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 addingadding 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+ + e- E ionization = 2 x 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 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