The Structure of the Atom Chapter 4 The Structure of the Atom

Chapter 4 Democritus: Greek philosopher coined the word “atom”. Greek word comes from “atomos” meaning indivisible. Antoine Lavoisier: Law of Conservation of Matter Joseph Proust: Law of Constant Composition (aka Law of Definite Proportion)

John Dalton’s Atomic Theory All matter is made up of atoms, which are indivisible. All atoms of the same element are identical, but differ from those of other elements. Atoms are neither created nor destroyed in any chemical reaction. A given compound always has the same relative numbers and kinds of atoms.

Dalton Model of the Atom Atoms are indivisible No subatomic particles have been discovered! Atoms of each element are identical Isotopes unknown at this time! 1803!

Electricity Ben Franklin: studied static electricity; objects could have negative or positive charge. The word electricity comes from elektron (Greek for amber) Static electricity is charge in motion.

Discovering Atomic Structure Opposite charges attract Like charges repel Crookes: studied behavior of gases in a vacuum tube. Cathode Ray Tube

Thomson: Discovered Electrons He used a magnet in the CRT to deflect the rays away from the negative pole of the magnet (rays were pulled toward the positive pole). No deflection without the magnet, so the rays must be negative. Using other gases in the tube gave the same effect, so it is characteristic to all elements. Thomson discovered electrons. He thought the positive charges were like pudding and that electrons were like raisins!

Plum Pudding Model

Millikan’s Oil Drop Experiment He is known for measuring the charge on an electron (from which he calculated the mass of an electron). He concluded the charge on an electron was 1.60 x 10-19 coulombs He used a charge/mass ratio to calculate the mass of an electron as 9.109 x 10-28 g.

Millikan’s Experiment

Rutherford’s Gold-Foil Experiment He discovered the positively charged nucleus by shooting positive alpha particles through gold foil. Most went through, but some were deflected. Since some of the particles deflected backwards, it had to get close to something positive; the nucleus is positive; protons were discovered.

Rutherford He concluded the most dense part of the atom was in the center, which he called the nucleus. Most of the atom was empty space (shells).

Rutherford’s Lighter Side

Rutherford Model

The Neutron Discovered in 1932 by James Chadwick (Coworker of Rutherford) Same mass as proton No electrical charge Also found in the nucleus

Subatomic Particles Particle Location Mass Charge Proton Nucleus 1 amu +1 Neutron Electron Outside 1/1836 amu -1

Radioactivity Henri Becqerel accidently discovered uranium exhibits radioactivity. Marie and Pierre Curie isolated radium (Ra) and polonium (Po) which are both radioactive.

Types of Radiation

Modern Atomic Theory Atoms consist of subatomic particles: protons, neutrons, and electrons. Atoms are not permanently changed during chemical reactions, merely rearranged. Atoms can be changed to other elements, but not by ordinary chemical reactions (nuclear).

Cont. Atoms of the same element are not all alike (masses may differ-ISOTOPES).

Atomic Number (Z) An atom’s identity comes from the number of protons in its nucleus. Atomic number is defined as the number of protons (which is also the number of electrons) in an atom’s nucleus. Atoms are electrically neutral. Atomic # = # protons = # electrons (abbrev. Z)

Mass Number (A) The total number of protons and neutrons of an isotope. # neutrons = mass # - atomic # or A - Z

Isotopes Atoms of the same element with the same number of protons, but different number of neutrons. Ex. 3 isotopes of hydrogen Hydrogen 1 (protium) 1 p+, 0 no, 1 e- Hydrogen 2 (deuterium) 1 p+, 1 no, 1 e- Hydrogen 3 (tritium) 1 p+, 2 n0, 1 e- radioactive! It gets heavier!

Designating Isotopes Hyphen notation- the mass # is added with a hyphen to the name of the element. Ex. Carbon – 12 **LOOK UP THE ATOMIC # on the periodic chart to find protons, (which is = to the electrons). Carbon -12 has 6 protons, 6 electrons, and 6 neutrons (12-6 = 6).

Isotopes, cont. Nuclear symbol- the atomic # is added to the symbol for the element as a subscript to the LEFT and the mass # is added as a superscript on the left. Ex. 1737Cl How many protons, neutrons and electrons does this isotope of chlorine have?

Ex. How many protons, neutrons, and electrons does uranium -238 have?

Ions An atom that has gained or lost one or more electrons. An ion may have a positive or negative charge. Cations are positively charged because they lose electrons. Anions are negatively charged because they gain electrons.

Example Calculate the charges on the following ions and indicate whether the ions are cations or anions. ION Protons Electrons X 35 36 Y 3 2 Z 20 18