Transcript Atoms and the Periodic Table

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:
Mass
Number
Atomic
Number
X
Charge
57
26
Or just:
Fe
2
Fe
2
• For isotopes chemists use the following
notation:
• Name of element – mass number
• Examples:
carbon-12
carbon-14
uranium-236
Determine the Numbers of p+, n0,
and e- for the following symbols
given:
17
8
O
204
2
37
Hg
Cl

calcium - 46
nickel - 60
Radiation
Unstable Nuclei & Radioactive Decay
Early Pioneers in Radioactivity
Rutherford:
Roentgen:
Discoverer
Alpha and Beta
rays 1897
Discoverer of
X-rays 1895
The Curies:
Discoverers of
Radium and
Polonium 19001908
Becquerel:
Discoverer of
Radioactivity
1896
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.
“Normal” Chemical Reactions
• Nuclear reactions involve the nucleus
• The nucleus opens, and protons and
neutrons are rearranged
• The opening of the nucleus releases a
tremendous amount of energy that holds the
nucleus together – called binding energy
• “Normal” Chemical Reactions involve
electrons, not protons and neutrons
Types of Radiation
• Alpha (a) – 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
4
2
He
0
1
0
0


Other Nuclear Particles
• Neutron
1
0
• Positron – a positive
electron
0
1
•Proton – usually referred to
as hydrogen-1
•Any other elemental isotope
1
1
n
e
H
Three Common Types of Radioactive
Emissions
Alpha
Beta
Gamma