Unit 1 – Atomic Structure Bravo – 15,000 kilotons

Modern Atomic Theory  Atoms of an element have a characteristic average mass which is unique to that element.  Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!  All matter is composed of atoms  Atoms of any one element differ in properties from atoms of another element

Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle. Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Conclusions from the Study of the Electron  Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.  Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons  Electrons have so little mass that atoms must contain other particles that account for most of the mass

Thomson’s Atomic Model Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

Rutherford’s Gold Foil Experiment  Alpha particles are helium nuclei  Particles were fired at a thin sheet of gold foil  Particle hits on the detecting screen (film) are recorded

Try it Yourself! In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

The Answers Target #1Target #2

Rutherford’s Findings  The nucleus is small  The nucleus is dense  The nucleus is positively charged  Most of the particles passed right through  A few particles were deflected  VERY FEW were greatly deflected “Like howitzer shells bouncing off of tissue paper!” Conclusions:

–Atoms are extremely small particles, which cannot be seen, even with microscopes. –All matter is made up of atoms. –Scientists have created models to describe atoms. Models are similar to theories, but often include a picture or physical representation.

Atomic Structure –An atom is mostly empty space. –The rest consists of a nucleus, which is located in the very center of the atom, and electrons, which are located around the nucleus. –The nucleus is very small (it would be nothing more than a tiny speck in our drawings, if we were to draw it to scale).

–The nucleus is also very dense and consists of two types of particles— neutrons and protons. –A neutron is a neutral particle with no charge on it. –A proton is a positively charged particle. –Tightly bound together, neutrons and protons make a positively charged nucleus.

Atomic Particles ParticleCharge Mass # Location Electron0 Electron cloud Proton+11Nucleus Neutron01Nucleus

So,What Forces Keep an Atom Together ?  There are four natural forces that are responsible for the behavior of an atom’s particles... The Fantastic Four Forces within the Atom

ELECTROMAGNETIC FORCES THE STRONG FORCE—the greatest of the four forces! WEAK FORCES GRAVITY—mentioned last, is it the least?

Atomic Number Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element. Element# of protonsAtomic # (Z) Carbon66 Phosphorus15 Gold79

Mass Number Mass number is the number of protons and neutrons in the nucleus of an isotope. Mass # = p + + n 0 Nuclidep+p+ n0n0 e-e- Mass # Oxygen Arsenic Phosphorus153116

–Atomic mass is the “weight” or mass of a single atom. –Atomic weight is the decimal number on the periodic table.

–Each successive element has one more proton than the element preceding it. –The atomic number is equal to the number of protons. –Atomic Number =P

–The number of electrons is equal to the number of protons (as long as the atom is neutral). –E = P

Isotopes Isotopes are atoms of the same element having different masses due to varying numbers of neutrons. IsotopeProtonsElectronsNeutronsNucleus Hydrogen–1 (protium) 110 Hydrogen-2 (deuterium) 111 Hydrogen-3 (tritium) 112

Atomic Masses IsotopeSymbolComposition of the nucleus % in nature Carbon C6 protons 6 neutrons 98.89% Carbon C6 protons 7 neutrons 1.11% Carbon C6 protons 8 neutrons <0.01% Atomic mass is the average of all the naturally occuring isotopes of that element. Carbon =