Lecture 7 The Atom and Elements Subatomic Particles Isotopes Radioactivity

Atom: the fundamental unit of an element. Mostly empty space: if an atom were expanded to a diameter of 3 km, the nucleus would be the size of a basketball.

Element: a substance that cannot be broken down into simpler substances by chemical or physical means. –Composed entirely of one type of atom.

Periodic Table: arranges elements into rows so that those with the same properties are found in the same column.

Subatomic Particles Protons: Atomic number: number of protons in the nucleus of an atom. Elements are defined by their atomic number. Number of protons = Number of electrons in an electrically neutral atom.

Subatomic Particles Neutrons: Accompany protons in the nucleus. Atomic Mass Number: total number of protons and neutrons in the nucleus. Number of neutrons = atomic mass number – atomic number.

1 atomic mass unit = X grams

Subatomic Particles Electrons: Behave as both a wave and a particle. Travel at speeds of 2 million m/s within an atom. Electrical repulsion with electrons of neighboring atoms prevents one atom from passing through another.

Isotopes Isotopes: atoms of the same element that contain different numbers of neutrons in the nucleus. –Different isotopes = different mass numbers. –Isotopes of an element have similar chemical properties – same electron structure.

Isotopes

Radioactivity Radioactivity: results from radioactive decay - the process whereby unstable atomic nuclei break down and emit radiation. –An imbalance of neutrons to protons can result in radioactivity. A large amount of Earth’s internal heat is maintained by the presence of abundant radioactive isotopes.

Origins of radiation exposure for an average individual in the United States.

Note: lethal radiation doses begin at 500 rems.

Types of Radiation

α = Alpha Particle β = Beta Particle γ = Gamma Ray

Radiation and Energy Energy is released during radioactive decay. –Kinetic Energy of particles released from atom. –Kinetic Energy of atom when it recoils. –Gamma rays.

Forms of Electromagnetic Radiation Low Frequency (Low Energy) High Frequency (High Energy)

The Atomic Nucleus Strong Nuclear Force: attractive force between neutrons and protons within the nucleus. –Very strong over extremely short distances (within the diameter of a typical atomic nucleus). –Easily overcomes the repulsive electrical force of protons with same charge.

The strong nuclear force is a very distance-sensitive attraction.

The presence of neutrons helps hold the nucleus together.

In large nuclei, the nucleons are farther apart which weakens the strong nuclear force. Larger nuclei require more neutrons than protons in order to overcome the repulsive electrical forces. Exanple: Pb has 1.5 times as many neutrons as protons. All nuclei with more than 83 protons are radioactive.

Decay of a Neutron to a Proton by emission of an electron (Beta particle) Farther apart neutrons are unstable - require the presence of nearby protons to remain stable.

Unstable nucleus undergoing radioactive decay

Transmutation Transmutation: changing of one chemical element into another. –Occurs both naturally and artificially.

Alpha emission from a nucleus: mass number decreases by 4 atomic number decreases by 2 Transmutation

Transmutation Beta emission from a nucleus: One neutron transforms to a proton but no change in mass number. Atomic number increases by 1

Transmutation Gamma emission from a nucleus: No change in mass number No change in atomic number