Atomic Theory Chapter 2 Pg: 41-57

2.1.1 State the position of protons, electrons and neutrons in the atom2.1.2 State the relative masses and relative charges of protons, neutrons and

electrons2.1.3 Define the terms mass number (A), atomic number (Z) and isotopes of an

element2.1.4 Deduce the symbol for an isotope given its mass number and atomic number2.1.5 Calculate the number of protons, neutrons and electrons in atoms and ions

from the mass number, atomic number and charge.2.1.6 Compare the properties of the isotopes of an element2.1.7 Discuss the uses of radioisotopes

History of the atom•Democritus (400 BC) suggested that the material world was made up of tiny, indivisible particles

• atomos, Greek for “uncuttable”

• Aristotle believed that all matter was made up of 4 elements, combined in different proportions

• Fire - Hot

• Earth - Cool, heavy

• Water - Wet

• Air - Light

• The “atomic” view of matter faded for centuries, until early scientists attempted to explain the properties of gases

Re-emergence of Atomic Theory

John Dalton postulated that:1. All matter is composed

of extremely small, indivisible particles called atoms

2. All atoms of a given element are identical (same properties); the atoms of different elements are different

3. Atoms are neither created nor destroyed in chemical reactions, only rearranged

4. Compounds are formed when atoms of more than one element combine• A given compound always has the same relative number and kind of atoms

Atoms are divisible!

• By the 1850s, scientists began to realize that the atom was made up of subatomic particles

• Thought to be positive and negative

• How would we know this if we can’t see it or touch it?

Cathode Rays and Electrons• Mid-1800’s scientists began to study electrical discharge through cathode-ray tubes. Ex: neon signs

• Partially evacuated tube in which a current passes through• Forms a beam of electrons which move from cathode to anode• Electrons themselves can’t be seen, but certain materials fluoresce (give off light) when energised

Oh there you are!

• JJ Thompson observed that when a magnetic or electric field are placed near the electron beam, they influence the direction of flow• opposite charges attract each other,

and like charges repel.• The beam is negatively charged so it

was repelled by the negative end of the magnet

• http://www.chem.uiuc.edu/clcwebsite/video/Cath.mov

• Magnetic field forces the beam to bend depending on orientation

• Thompson concluded that:• Cathode rays consist of beams of particles• The particles have a negative charge

• Thompson understood that all matter was inherently neutral, so there must be a counter

• A positively charged particle, but where to put it

• It was suggested that the negative charges were balanced by a positive umbrella-charge

• “Plum pudding model” “chocolate chip cookie model”

Rutherford and the Nucleus

• This theory was replaced with another, more modern one

• Ernest Rutherford (1910) studied angles at which particles (nucleus of helium) were scattered as they passed through a thin gold foil

• http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

Rutherford expected …• Rutherford believed that the mass and positive charge was evenly distributed throughout the atom, allowing the particles to pass through unhindered

particles

Rutherford explained …

+

• Atom is mostly empty space• Small, dense, and positive at the center• Alpha particles were deflected if they got close enough

particles

• Nucleus: Containing protons and neutrons, it is the bulk of the atom and has a positive charge associated with it

• Electron cloud: Responsible for the majority of the volume of the atom, it is here that the electrons can be found orbiting the nucleus (extranuclear)

The modern atom is composed of two regions:

Major Subatomic Particles

• Atoms are measured in picometers, 10-12 meters• Hydrogen atom, 32 pm radius

• Nucleus tiny compared to atom• If the atom were a stadium, the nucleus would be a marble

• Radius of the nucleus is on the order of 10-15 m• Density within the atom is near 1014 g/cm3

Name Symbol Charge Relative Mass (amu)

Actual Mass (g)

Electron e- -1 1/1840 9.11x10-28

Proton p+ +1 1 1.67x10-24

Neutron no 0 1 1.67x10-24

Elemental Classification• Atomic Number (Z) = number of protons (p+) in the nucleus

• Determines the type of atom• Li atoms always have 3 protons in the nucleus, Hg always 80

• Mass Number (A) = number of protons + neutrons [Sum of p+ and nº]

• Electrons have a negligible contribution to overall mass

• In a neutral atom there is the same number of electrons (e-) and protons (atomic number)

Nuclear Symbols• Every element is given a corresponding symbol which is composed of 1 or 2 letters (first letter upper case, second lower), as well as the mass number and atomic number

E A

Z

elemental symbolelemental symbol

mass numbermass number

atomic numberatomic number

• Find the Find the • number of protonsnumber of protons• number of neutronsnumber of neutrons• number of electronsnumber of electrons• atomic numberatomic number• mass numbermass number

W184 74F

199 Br

80 35

Ions

• Cation is a positively charged particle. Electrons have been removed from the element to form the + charge. ex: Na has 11 e-, Na+ has 10 e-

• Anion is a negatively charged particle. Electrons have been added to the atom to form the – charge.

ex: F has 9 e-, F- has 10 e-

Isotopes• Atoms of the same element can have different numbers of neutrons and therefore have different mass numbers

• The atoms of the same element that differ in the number of neutrons are called isotopes of that element

• When naming, write the mass number after the name of the element

H11

Hydrogen-1

H21Hydrogen-2

H31

Hydrogen-3

How heavy is an atom of oxygen?•There are different kinds of oxygen atoms (different isotopes)

• 16O, 17O, 18O

• We are more concerned with average atomic masses, rather than exact ones

• Based on abundance of each isotope found in nature

• We can’t use grams as the unit of measure because the numbers would be too small

• Instead we use Atomic Mass Units (amu)• Standard amu is 1/12 the mass of a carbon-12 atom

• Each isotope has its own atomic mass

Calculating AveragesAverage = (% as decimal) x (mass1) +

(% as decimal) x (mass2) + (% as decimal) x

(mass3) + …

Problem:Silver has two naturally occurring isotopes, 107Ag with a mass of 106.90509 amu and abundance of 51.84 % ,and 109Ag with a mass of 108.90476 amu and abundance of 48.16 % What is the average atomic mass?

Average = (0.5184)(106.90509) + (0.4816)(108.90476)

= 107.87 amu

• If not told otherwise, the mass of the isotope is the mass number in amu

• The average atomic masses are not whole numbers because they are an average mass value

• Remember, the atomic masses are the decimal numbers on the periodic table

Average Atomic Masses

Properties of Isotopes

• Chemical properties are primarily determined by the number of electrons

• All isotopes has the same number of electrons, so they have nearly identical chemical properties even though they have different masses.

• Physical properties often depend on the mass of the particle, so among isotopes they will have slightly different physical properties such as density, rate of diffusion, boiling point…

• Calculate the atomic mass of copper if copper has two isotopes

• 69.1% has a mass of 62.93 amu• The rest (30.9%) has a mass of 64.93 amu

• Magnesium has three isotopes• 78.99% magnesium 24 with a mass of 23.9850 amu• 10.00% magnesium 25 with a mass of 24.9858 amu• The rest magnesium 26 with a mass of 25.9826 amu• What is the atomic mass of magnesium?

More Practice Calculating Averages

Radioisotopes

• Isotopes of atoms that have had an extra neutron attached to their nucleus.

• Carbon-14 radioactive decay is used to measures the date of objects.– After 5700 years the amount of 14C will be half

its original value.

• Iodine-125 or 131 is used to monitor the activity of the thyroid gland (b/c the thyroid tends to absorb iodine)

• Cobalt-60 produces gamma rays (intense radioactivity) and is used in radiation treatment of cancer.

• Note: gamma rays are the shortest wavelength on the electromagnetic spectrum. They are the most dangerous and difficult to shield from.