25.1 Bonding &Nuclear Radiation

Chemical Reactions: Chemical Bond The force of attraction between two or more elements. 3 types: Covalent Ionic Metallic

Ionic: Transfer of electrons (from metal to nonmetal). Metals lose electrons to form cations (+ ions) Nonmetals gain electrons to form anions (- ions) Both achieve stable octet. Electrostatic forces: Opposite charges hold ions together

Covalent: Valence Electrons are shared by both atoms Occurs between 2 nonmetals Electron pairs form a bond Both atoms achieve octet (8) in outermost orbital

Polar vs. Nonpolar Bonds Covalent Nonpolar Bond: Electrons shared equally 2 like atoms Polar Bond: Electrons shared unequally Water is polar!

Metallic Bond Occurs when large d and f sublevels of transition metals overlap and allow the electrons to travel freely between them. “Sea of Electrons”

Chemical vs. Nuclear Reactions 1) Bonds are broken or formed. 2) Atoms remain unchanged (may be rearranged) 3) Involve valence electrons outside the nucleus. 1) Nuclei emit particles and/or rays. 2) Atoms often converted into atoms of another element. 3) Involve protons & neutrons inside the nucleus.

(Chemical vs. Nuclear) 4) Small energy changes. 5) Mass & energy are conserved. 6) Reaction rate influenced by temperature, concentration, etc. Large energy changes (E = mc2) Small amount of mass converted into energy. Reaction rate not affected by these outside conditions.

Chemical vs. Nuclear 7) Used to fuel coal powered plants. 8) Photosynthesis 7) U-235 used to fuel nuclear power (fission). 8) Source of energy used in photosynthesis (fusion)

25.2 Radioisotopes – isotopes of atoms having unstable nuclei. Attain more stable nuclei during radioactive decay. Unstable atoms lose energy by emitting radiation. Nuclear Stability depends on the ratio of protons: neutrons.

25.3 Transmutation – conversion of 1 element to another element. Involves nearly all nuclear reactions. Types: Natural: by unstable nuclei Induced: by striking nuclei with high-velocity charged particles. Transuranium elements (at.#>92) formed: 238U + 1n  239U  239Np + 0 92 0 92 93 -1

Neutron Capture: Atomic nucleus collides with neutron forming heavier nucleus.

Half Life: Time needed for half of radioisotope’s nuclei to decay into products. Half life of Sr– 90 is 29 yrs. Suppose you have 100 g sample: At 0 years: Have 100 g Sr-90 At 29 yrs. Have 50 g At 58 yrs. Have 25 g At 87 yrs. Have 12.5g

Half-Life

Half-Life The half-life of tritium, H-3 is 12 years. If you start with 200 g of tritium, then how many half-lives have passed if you only have 25 g remaining? 200 g  100 g (1 half-life) 100 g  50 g (2 half-lives) 50 g  25 g (3 half-lives) Ans. 3 half-lives. (12x3 = 36 years)

100-g of sample “X” decays to 25 g in 60 sec. What is its half-life? 1) Find how many half-lives occurred: Start with 100 g: 1 half-life – Now have 50 g 2 half-lives- Have 25 g 2) It took 60 sec. for 2 half-lives. Thus, it takes 60/2 = 30 sec. for 1 half-life.

Radioactive Dating – uses radioisotopes to determine the age of an object. Carbon dating – measures the amount of C-14 to determine age.

The End Geiger Counter – used to detect radiation: