Chapter 4 Section 1 Introduction to Atoms

Development of Atomic Models Why would it be difficult to figure out what atoms are made up of? Atoms are very, very small. In fact, it takes a special electron microscope to see an entire atom, let alone look inside! Around 5 hundred quadrillion atoms are in a speck of dust.

Ancient Greeks: Democritus The first people to think about atoms were the ancient Greeks. 430BC the Greek philosopher named Democritus proposed that matter is formed of small pieces These small pieces were “uncuttable” or could not be cut into smaller parts. He named these parts atomos which means uncuttable. Today, we call them ATOMS.

Dalton’s Atomic Theory 1803 English Chemist : “All atoms have certain characteristics” ALL ELEMENTS COMPOSED OF ATOMS THAT CAN’T BE DIVIDED ALL ATOMS OF THE SAME ELEMENTS ARE ALIKE & HAVE SAME MASS. ATOMS CAN’T BE CREATED OR DESTROYED, ONLY REARRANGED EVERY COMPOUND IS COMPOSED OF ATOMS OF DIFFERENT ELEMENTS IN SPECIFIC RATIO Theory still accepted today with only a few changes!

Thomson & Smaller Parts of the Atom 1897 British scientist J.J. Thomson found negative particles in atoms and called them ELECTRONS. Since he knew atoms have NO ELECTRICAL CHARGE, he reasoned that they must also have POSITIVE PARTICLES. He proposed that atoms had negative electrons embedded in a positive sphere. (berries in a muffin)

1911 Thomas Rutherford & Nucleus Found evidence that disproved Thomson’s model that electrons are embedded in positive sphere GOLD FOIL EXPERIMENT: Fired positive charged particles through gold foil. Most particles passed straight through; however, some were deflected Like charges repel like charges He found that positive charges in atoms are located in a central region of atom which he named the NUCLEUS

Rutherford continued… He also knew that electrons have almost no mass, so he reasoned that the mass of an atom must be located in the neucleus where the protons existed. Rutherford suggested that most of the atom was empty space With the electrons floating around. https://www.youtube.com/watch?v=5pZj0u_XMbc

1913 Neils Bohr, Danish Student of both Thomson and Rutherford Revised Atomic Model again Found electrons have specific amounts of energy This causes them to move in certain orbits like planets orbiting the sun.

1920’s Bohr’s Theory Disproved Scientists determined that electrons do not orbit nucleus like planets. Instead, electrons can be anywhere in a cloudlike region around nucleus. We call this the ELECTRON CLOUD An electron’s movement is related to its ENERGY LEVEL OR the specific amount of energy it has Electrons of different energy levels are found in different places

1930’s Chadwick: Modern Atomic Model James Chadwick discovers the NEUTRON He found that it has neutral charge He found that it is located in nucleus He found that its mass is almost the same as that of the PROTON The Atomic Model has not changed much since 1930’s

Particle Charges Protons + Electrons – Neutrons have NO charge Why does every atom have a neutral charge? # of protons = # of electrons (they cancel out) NEUTRONS have NO effect on the electrical charge of the atom.

Scale and Size of Atoms Most of an atoms volume is located in the electron cloud. Atoms are so small that one speck of dust contains 10 million billion atoms!!! Must use very high power microscopes to study.

Atomic Number – Atom Identity Every atom of the SAME element has the SAME number of PROTONS The nucleus of every Carbon atom has 6 protons, every Oxygen atom has 8 protons, every Iron atom has 26 protons. The number of protons in an atoms nucleus is it’s ATOMIC NUMBER Atomic

Isotopes All atoms of same element ALWAYS have the same number of protons; HOWEVER the number of NEUTRONS can differ.

Why are Isotopes Important? Used in medical imaging to diagnose & treat cancer Used in Smoke Detectors Batteries in Nasa Satellites To calibrate detectors in shipping ports to keep them safe from nuclear terrorism. To enable new sources of energy – nuclear fusion Used to date ancient remains – radioactive decay

Isotopes There are 90 naturally occurring elements with roughly 250 stable isotopes – stable = reliable reactions There are 3200 UNSTABLE isotopes – unreliable reactions Different isotopes often have different properties than the original element Production of isotopes is very expensive and time consuming