Atoms and Their Structure

Chapter 4

Atoms

• Elements are made of particles called atoms

• Atoms are the smallest pieces of matter that contain all the properties of a specific element

• Each element contains only one type of atom

Essential Question

• How do we know what the atom looks like when no one has ever seen it?

Inside the Atom

• Atoms are made up of smaller particles• These particles are found in different

regions of the atom

Protons

• Positively charged particles found in nucleus of atom

• Have an electrical charge of +1• Mass of 1 a.m.u.• Composed of quarks

Neutrons

• Neutral particles found in nucleus of atom

• Have no electrical charge• Mass of 1 a.m.u.• Composed of quarks

Electrons

• Negatively charged particles found in electron cloud

• Have an electrical charge of -1• Constantly moving around outside

nucleus• Have essentially no mass

Nucleus

• The nucleus is the positively charged dense core in the center of the atom

• Contains 99.9% of mass of atom

• Houses protons and neutrons

Electron Cloud

• Region around nucleus• Houses electrons

Atomic Models

• Have been revised many times to explain new discoveries

• Democritus (4th century B.C.) first theory - thought the atom could not be split

• Theory was modified when subatomic particles were discovered

Newer Models

• Bohr’s Model (1913) revised earlier thoughts– Showed electrons moving in paths like

planets

• Revised in 1925– Now electrons in cloud

Quantum Mechanical Model

Valence Electrons

• Electrons in the outermost energy level of an electron are called valence electrons

• These are the electrons furthest from the nucleus

Atomic Number & Mass Number

• Atomic number = # p+ – For atoms atomic number also = number of

e-

• Mass number = # p+ + # n0

– Mass number is a whole number– Number on the periodic table is average

atomic mass (not a whole number)

• How can you solve for numbers of neutrons?

How Atoms Differ: Ions and Isotopes

• Usually by way of differing numbers of p+, n0 and e-

– Different elements have different numbers of p+

– Ions have charges• the number of e- ≠ number of p+• charge = # p+ - # e-

– Atoms with the same number of p+ but different number of n0 are isotopes.

• Mass number of isotopes of the same element changes

Symbols of Ions and Isotopes

• For ions chemists use the following notation:

Or just:

MassChargeNumber

AtomicNumber

X57 226Fe

2Fe

• For isotopes chemists use the following notation:

• Name of element – mass number• Examples:

carbon-12carbon-14uranium-236

Determine the Numbers of p+, n0, and e- for the following

symbols given:17 28O

204Hg

37Cl

calcium - 46

nickel - 60

Radiation

Unstable Nuclei & Radioactive Decay

Early Pioneers in Radioactivity

Roentgen:

Discoverer of X-rays 1895

Becquerel:

Discoverer of Radioactivity

1896

The Curies:

Discoverers of Radium and

Polonium 1900-1908

Rutherford:

Discoverer Alpha and Beta

rays 1897

What do we mean by Radioactivity?

Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves.

There are numerous types of radioactive decay. The general idea:

An unstable nucleus releases energy to become more stable

Nuclear Reactions vs. Nuclear Reactions vs. “Normal” Chemical “Normal” Chemical

ReactionsReactions• Nuclear reactions involve the nucleusNuclear reactions involve the nucleus

• The nucleus opens, and protons and The nucleus opens, and protons and neutrons are rearrangedneutrons are rearranged

• The opening of the nucleus releases a The opening of the nucleus releases a tremendous amount of energy that holds tremendous amount of energy that holds the nucleus together – called the nucleus together – called binding binding energyenergy

• ““Normal” Chemical Reactions involve Normal” Chemical Reactions involve electronselectrons, not protons and neutrons, not protons and neutrons

Types of Radiation

• Alpha () – a positively charged helium isotope– we usually ignore the

charge because it involves electrons, not protons and neutrons

• Beta (β) – a fast moving (high energy) electron

• Gamma (γ) – pure high energy; called a ray rather than a particle

He42

01

00

Other Nuclear ParticlesOther Nuclear Particles

e01

n10• NeutronNeutron

• Positron – a positive Positron – a positive electronelectron

•Proton – usually referred to Proton – usually referred to as hydrogen-1as hydrogen-1

•Any other elemental isotopeAny other elemental isotope

H11

Three Common Types of Radioactive Emissions

Alpha

Beta

Gamma