The Structure of the Atom

Chapter 4

The Roots of Atomic Theory

DemocritusAristotleJohn Dalton

Ideas versus ScienceWhy was it hard for Democritus to defend

his ideas?

Dalton’s Atomic Theory All elements are composed of tiny indivisible

particles called atoms. Atoms of the same element are identical. The

atoms of any one element are different from those of any other element.

Atoms of different elements can chemically combine with one another in simple whole-number ratios to form compounds.

Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element are never changed into another element as a result of a chemical reaction.

Defining the Atom

An atom is the smallest particle of an element that retains the properties of that element.

100,000,000 atoms = 1 centimeter

The Electron

Using a vacuum pump, scientists passed electricity through glass tubes with an anode at one end and a cathode at the other. Cathode rays led to the discovery of negatively charged particles called electrons.

Electrons

JJ Thomson did experiments using cathode rays, discovered that opposite charges attract and like charges repel

Robert Millikan determined electrons negative charge and very small mass

found in cloud (sublevels) outside the nucleus; valence electrons are the outermost electrons

Plum Pudding Model

Atomic Nucleus

Ernest Rutherford proposed that the atom is mostly empty space (gold foil experiment)

all the positive charge and almost all the mass is concentrated in a small region

Rutherford called this region the nucleusnucleus is the center core of an atom

composed of protons and neutrons

Rutherford’s Nuclear Model

The atom is composed of a dense, positively charged nucleus surrounded by negative electrons.

What force causes the deflection of alpha particles?

Protons and Neutrons atoms are electrically neutral protons are positively charged

almost 2000 times the mass of an electron neutrons have no charge, but equal mass to a

proton protons and neutrons are collectively called

nucleons

Summary of Atom Anatomy

The nucleus of the atom is made of protons and neutrons.

Electrons orbit the nucleus in a cloud.

Protons + charge

Neutrons no charge

Electrons - charge

Atomic Number

elements are different due to different numbers of protons

number of protons is the atomic number number of protons equals number of electrons in

a neutral atom

Mass Number

total number of protons and neutrons in an atom is the mass number

number of neutrons = mass # - atomic #oxygen has an atomic # 8, mass # 16

atomic number can be written as the subscript and mass number as the superscript

can also designate element by mass numbergold-197

Isotopes

atoms with same number of protons but different number of neutrons are called isotopes

different mass numbers; chemically alike due to same # of protons and electrons

hydrogen-1 (mass number 1, no neutrons)hydrogen-2 (mass number 2, 1 neutron)

deuterium

Atomic Mass Unit defined as one twelfth the

mass of a carbon-12 atom (amu)

mass of a single proton (or neutron) is about 1 amu

Why isn’t atomic mass always a whole number?

Atomic mass is the weighted average mass of the atoms in a naturally occurring sample of the element.

Isotope Abundance

Analyzing an element’s mass can indicate the most abundant isotope for that element.

Using the atomic mass, you can usually assume that the most common isotope is the atomic mass rounded to the nearest whole number.

Example Problems

Find the weighted average of 3 assignments…Term paper 20% A = 4.0Midterm 30% B = 3.0Final 50% C = 2.0

Student receives a “B” on term paper, “C” on midterm, and “B” on final. What is the overall grade for this student?

Example Problems

Calculate the average atomic mass of nitrogen…Nitrogen-14 99.63% 14.003Nitrogen-15 0.37% 15.000

What is the average atomic mass?

Example Problems

Three isotopes of Element X have atomic masses and relative abundances of 23.985 amu (78.99%), 24.986 amu (10.00%), and 25.982 amu (11.01%). Calculate the atomic mass.

Radioactivity

Nuclear reactions involve an atom of one element changing into an atom of another element.

radioactive atoms undergo changes that alter their identities

unstable nuclei lose energy by emitting radiation through radioactive decay

Radioactivity

Alpha (helium-4 nucleus)Beta (fast moving electron)Gamma (high energy, no mass)