Pearson Prentice Hall Physical Science: Concepts in Action Chapter 4 Atomic Structure

Atom V Element

ATOMIC THEORY Democritus (4th century B.C) said the universe was made of invisible units called atoms In 1808, an Englishman named John Dalton proposed a theory: 1. every element is made of tiny unique particles called atoms that cannot be subdivided 2. atoms of the same element have the same mass & atoms of different elements have different masses 3. compounds contain atoms of more than one element 4. in any particular compound, atoms of different elements always combine in the same way

Thomson & Rutherford Q: What did Dalton notice that all compounds have in common? A: Compounds contain atoms of more than one element JJ Thomson (1856-1940) used a sealed tube of gas to show that particles are charged

In Thomson’s experiment, the particles repelled the negative plate and were attracted to the positive plate Q: What was the charge on the particles in the beam of the gas? Thomson’s experiments provided the first evidence that atoms are made of even smaller particles

Q: How do objects of the same charge behave when they come close to one another? Thomson developed the “plum pudding” model of the atom His model suggested that negative charges were evenly scattered throughout an atom filled with a positively charged mass of matter

Ernest Rutherford (1871-1937) discovered fast moving particles in uranium that he called alpha particles Uranium is a radioactive substance He used a zinc sulfide screen, gold foil and alpha particles to produce a very famous experiment in which he showed that the positive charges are not evenly dispersed throughout the atom

Rutherford concluded that atoms must have a concentrated central core Definition: a nucleus is a dense, positively charged mass located in the center of the atom According to Rutherford’s model, all of an atom’s positive charge is concentrated in its nucleus Today, much of an atom consists of empty space outside the positively charged nucleus

Atomic Theory Timeline This Photo by Unknown Author is licensed under CC BY-SA

4.2 The Structure of an Atom Objectives: 1. Describe the three subatomic particles 2. Compare protons, electrons and neutrons in terms of physical properties 3. Explain how atoms of one element are different from atoms of other elements 4. Describe the difference between 2 isotopes of the same element

Three Subatomic Particles Protons, neutrons & electrons are subatomic particles Proton is a positively charged subatomic particle that is found in the nucleus of an atom and has an atomic mass of 1 amu (atomic mass unit)

Three Subatomic Particles Protons, neutrons & electrons are subatomic particles Electron is a negatively charged subatomic particle found in the space OUTSIDE THE NUCLEUS (ORBITALS), electrons are so small they are said to have no mass

Three Subatomic Particles Protons, neutrons & electrons are subatomic particles Neutron is a neutral (no charge) subatomic particle found in the nucleus of an atom and have an atomic mass of 1 amu

Comparing Protons, Neutrons & Electrons Protons, neutrons and electrons can be distinguished by mass, charge & location in an atom The mass of a neutron is almost exactly equal to the mass of a proton Electrons are nearly 2,000 times less massive than either a proton or a neutron

The mass of an electron is insignificant Thomson’s experiment with the gas tube detected electrons Rutherford discovered the positive central core The mass of each proton or neutron equals one atomic mass unit (amu)

Atoms of Different Elements Atoms of any given element ALWAYS have the same number of PROTONS Atomic number of an element equals the number of protons The number of PROTONS determines which element you have ATOMIC NUMBER = THE NUMBER OF PROTONS

ATOMIC MASS: is the sum of the protons plus the neutrons Neutrons + Protons = Mass # # neutrons = mass # - atomic # ATOMIC MASS = THE NUMBER OF PROTONS + THE NUMBER OF NEUTRONS

WHY AREN’T ELETRONS PART OF THE ATOMIC MASS MASS IS TOO SMALL SO WE SAY ELECTRONS HAVE NO MASS

WHY DON’T ATOMIC MASSES HAVE FULL NUMBERS? Some atoms of the same element may have a different number of neutrons (not protons or it would be a different element; change the number of protons you change the element) Elements with different number of neutrons are called isotopes When calculating atomic mass we take the average of all isotopes, thus creating a mass with a decimal point.