The Discovery of Isotopes Francis William Aston and Antoine Becquerel

An atom consists of a nucleusnucleus –(of protons and neutrons) electrons in space about the nucleus.electrons in space about the nucleus. The Atom Nucleus Electron cloud

Copper atoms on silica surface. An _____ is the smallest particle of an element that has the chemical properties of the element.An _____ is the smallest particle of an element that has the chemical properties of the element. Distance across = 1.8 nanometer (1.8 x m)

ATOM COMPOSITION protons and neutrons in the nucleus.protons and neutrons in the nucleus. the number of electrons is equal to the number of protons.the number of electrons is equal to the number of protons. electrons in space around the nucleus.electrons in space around the nucleus. extremely small. One teaspoon of water has 3 times as many atoms as the Atlantic Ocean has teaspoons of water.extremely small. One teaspoon of water has 3 times as many atoms as the Atlantic Ocean has teaspoons of water. The atom is mostly empty space

ATOMIC COMPOSITION Protons (p + )Protons (p + ) –+ electrical charge –mass = x g –relative mass = atomic mass units (amu) but we can round to 1 Electrons (e - )Electrons (e - ) – negative electrical charge –relative mass = amu but we can round to 0 Neutrons (n o )Neutrons (n o ) – no electrical charge –mass = amu but we can round to 1

Atomic Number, Z All atoms of the same element have the same number of protons in the nucleus, Z 13 Al Atomic number Atom symbol AVERAGE Atomic Mass

Mass Number, A C atom with 6 protons and 6 neutrons is the mass standardC atom with 6 protons and 6 neutrons is the mass standard = 12 atomic mass units= 12 atomic mass units Mass Number (A) = # protons + # neutronsMass Number (A) = # protons + # neutrons NOT on the periodic table…(it is the AVERAGE atomic mass on the table)NOT on the periodic table…(it is the AVERAGE atomic mass on the table) A boron atom can have A = 5 p + 5 n = 10 amuA boron atom can have A = 5 p + 5 n = 10 amu

Isotopes Atoms of the same element (same Z) but different mass number (A).Atoms of the same element (same Z) but different mass number (A). Boron-10 ( 10 B) has 5 p and 5 nBoron-10 ( 10 B) has 5 p and 5 n Boron-11 ( 11 B) has 5 p and 6 nBoron-11 ( 11 B) has 5 p and 6 n 10 B 11 B

Isotopes The existence of isotopes explains the apparent contradiction between the mass number and atomic mass of the elements. The various isotopes of the elements in nature occur in different proportions. Therefore, the atomic mass on the periodic table is the weighted average of all the natural isotopes of that element,

Two isotopes of sodium

How we classify Isotopes Stable isotopes are chemical isotopes that are not radioactive.By this definition, there are 256 known stable isotopes of the 80 elements which have one or more stable isotopes.isotopesradioactive Commonly analyzed stable isotopes include oxygen, carbon, nitrogen, hydrogen and sulfur oxygencarbonnitrogenhydrogen sulfur

Unstable or Radioactive Isotopes Naturally occurring fall into three categories: primordial secondary cosmogenic Artificially produced radioisotopes can be produced by particle accelerators or by nuclear reactions.

What are Radio Isotopes since they have different numbers of neutrons, isotopes of the same element may have different radioactivity. An isotope that is radioactive is called a radioisotope.

What is Half-life of the Isotopes The half-life he amount of time it takes for half of the atoms in a sample to decay. The half- life for a given isotope is always the samealways the same Suppose there's an alien species with a half- life of, say, 70 years. You randomly pick out 16 baby aliens and track them to see how long they live. After 70 years, of course, 8 of them will still be alive.

Examples The most stable isotope of uranium, U-238, has an atomic number of 92 (protons) and an atomic weight of 238 (92 protons plus 146 neutrons). The isotope of uranium of greatest importance in atomic bombs, U- 235, though, has three fewer neutrons. Thus, it also has an atomic number of 92 (since the number of protons has not changed) but an atomic weight of 235 (92 protons plus only 143 neutrons). The chemical behavior of U-235 is identical to all other forms of uranium, but its nucleus is less stable, giving it higher radioactivity and greater susceptibility to the chain reactions that power both atomic bombs and nuclear fission reactors.

Examples Iodine is one of the earliest elements whose radioisotopes were used in what is now called nuclear medicine. The most common, stable form of iodine has an atomic number of 53 (protons) and an atomic weight of 127 (53 protons plus 74 neutrons). Because its nucleus has the "correct" number of neutrons, it is stable and is not radioactive. A less stable form of iodine also has 53 protons (this is what makes it behave chemically as iodine) but four extra neutrons, for a total atomic weight of 131 (53 protons and 78 neutrons). With "too many" neutrons in its nucleus, it is unstable and radioactive, with a half-life of eight days. Because it behaves chemically as iodine, it travels throughout the body and localizes in the thyroid gland just like the stable form of iodine. But, because it is radioactive, its presence can be detected. Iodine 131 thus became one of the earliest radioactive tracers.

Isotopes & Their Uses Bone scans with radioactive technetium-99.

Separation of Isotopes Difference in physical properties enables separation of the isotope to be carried out. Evaporation Distillation Centrifuging Laser methods Diffusion

Calculating the% abundance Lithium has 2 isotopes. Li-6 and Li-7. If the atomic mass of Lithium is 6.94, calculate the proportion ( % of abundance) of the isotopes.

Atomic Symbols Show the name of the element, a hyphen, and the mass number in hyphen notation sodium-23 Show the mass number and atomic number in nuclear symbol form mass number 23 Na atomic number 11

Counting Protons, Neutrons, and Electrons Protons: Atomic Number (from periodic table) Neutrons: Mass Number minus the number of protons (mass number is protons and neutrons because the mass of electrons is negligible) Electrons: –If it’s an atom, the protons and electrons must be the SAME so that it is has a net charge of zero (equal numbers of + and -) –If it does NOT have an equal number of electrons, it is not an atom, it is an ION. For each negative charge, add an extra electron. For each positive charge, subtract an electron (Don’t add a proton!!! That changes the element!)

Learning Check – Counting Naturally occurring carbon consists of three isotopes, 12 C, 13 C, and 14 C. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12 C 13 C 14 C #p + _______ _______ _______ #n o _______ _______ _______ #e - _______ _______ _______

Answers 12 C 13 C 14 C #p #n o #e

Learning Check An atom has 14 protons and 20 neutrons. A.Its atomic number is 1) 142) 163) 34 B. Its mass number is 1) 142) 163) 34 C. The element is 1) Si2) Ca3) Se D.Another isotope of this element is 1) 34 X 2) 34 X 3) 36 X

Learning Check – Counting State the number of protons, neutrons, and electrons in each of these ions. 39 K + 16 O -241 Ca #p + ___________________ #n o ___________________ #e - ___________________

AVERAGE ATOMIC MASS Because of the existence of isotopes, the mass of a collection of atoms has an average value.Because of the existence of isotopes, the mass of a collection of atoms has an average value. Boron is 20% 10 B and 80% 11 B. That is, 11 B is 80 percent abundant on earth.Boron is 20% 10 B and 80% 11 B. That is, 11 B is 80 percent abundant on earth. For boron atomic weightFor boron atomic weight = 0.20 (10 amu) (11 amu) = 10.8 amu = 0.20 (10 amu) (11 amu) = 10.8 amu 10 B 11 B

Isotopes & Average Atomic Mass Because of the existence of isotopes, the mass of a collection of atoms has an average value.Because of the existence of isotopes, the mass of a collection of atoms has an average value. 6 Li = 7.5% abundant and 7 Li = 92.5% 6 Li = 7.5% abundant and 7 Li = 92.5% –Avg. Atomic mass of Li = ______________ 28 Si = 92.23%, 29 Si = 4.67%, 30 Si = 3.10% 28 Si = 92.23%, 29 Si = 4.67%, 30 Si = 3.10% –Avg. Atomic mass of Si = ______________