chapter 4: atomic structure - king's science...
TRANSCRIPT
4.1 Defining the Atom
• An Atom is the smallest particle of an
element that retains its identity in a
chemical reaction.
• The Greek Philosopher
Democritus, was among the
First to suggest the existence
Of atoms.
•Democritus believed that
atoms were indivisible and
indestructible.
•Although Democritus’s ideas
agreed with later scientific
theory, they did not explain
chemical behavior.
•They also lacked experimental
support.
Dalton’s Atomic Theory
• By experimental methods, Dalton
transformed Democritus’s ideas on atoms
into scientific theory.
• Dalton studied the ratios in which
elements combine in chemical reactions.
Based on the results of his experiments,
Dalton formulated hypotheses and
theories to explain his observations.
Dalton’s Atomic Theory
1. All elements are composed of tiny indivisible particles called atoms.
2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element.
3. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.
4. Chemical reactions occur when atoms are separated, joined, or rearranged.
Sizing up the Atom….
• Copper atoms are very small.
• A copper coin the size of a penny contains
about 2.4 x 10^22 atoms. By comparison,
Earth’s population is only about 6 x 10^9
people.
• If you could line up100,000,000
Copper atoms side by side, they would
produce a line only 1 cm long.
4.2 Structure of the Nuclear Atom
• Much of Dalton’s atomic theory is
accepted today. One important change
however, is that atoms are now known to be divisible.
• Three kinds of subatomic particles are
electrons, protons, and neutrons.
Electrons• In 1897, English
physicist J.J. Thomson discovered the Electron.
• Electrons are negatively charged subatomic particles.
• Thomson called these particles corpuscles; later they were named electrons.
Robert A. Millikan (1868-1953)
• Carried out
experiments to find
the quantity of charge
carried by an
electron.
• An electron carries
exactly 1 negative
charge and its mass
is 1/1840 the mass of
a hydrogen atom.
And Finally…….
• When a negative particle combines
with a positive particle an electrically
neutral particle is formed.
+1 + -1 = 0
Eugene Goldstein
• Observed a cathode-ray tube and found
rays traveling in the direction opposite to
that of the cathode rays.
• He called these rays canal rays and
concluded that they were composed of
positive particles.
• These positive particles are called
Protons.
James Chadwick
• In 1932, an English Physicist confirmed
the existence of yet another subatomic
particle: The Neutron.
• A neutron is a subatomic particle with no
charge but with a mass nearly equal to
that of a proton.
Plum Pudding Model
• In Thomson’s atomic
model, known as the
plum pudding model,
electrons were stuck
into a lump of positive
charge, similar to
raisins stuck in
dough.
• This model of the atom turned out to be short
lived, however, due to the groundbreaking
work of Ernest Rutherford (1871-1937), a
former student of Thomson.
• Rutherford’s Gold Foil
Experiment
»Here is a video of what
happened!!!
Rutherford Atomic Model
• He proposed that the atom is mostly empty space, thus explaining the lack of deflection of most of the alpha particles.
• He concluded that all the positive charge and almost all the mass are concentrated in a small region.
• He called this region the Nucleus.• The nucleus is the tiny central core of an atom and is
composed of protons and neutrons.
Key Concept
• In the nuclear atom, the protons and
neutrons are located in the nucleus. The
electrons are distributed around the
nucleus and occupy almost all the volume
of the atom.
• Although it was an improvement over Thomson’s model of the atom, Rutherford’s model turned out to be incomplete.
• In Chapter 5, we will learn how it was revised.
4.3 Distinguishing Among Atoms
•Elements are different because they contain different numbers of protons.
Atomic Number
• The atomic number of an element is the
number of protons in the nucleus of an
atom of that element.
Mass Number
• The total number of protons and neutrons
in an atom is called the mass number.
• Number of neutrons = mass number – atomic number
• The number of neutrons in an atom is the difference
between the mass number and atomic number.
• If you know the
atomic number
and mass
number of an
atom of any
element, you can
determine the
atom’s
composition.
• How do you calculate
mass number?
Isotopes
Isotopes are atoms that have the same number of protons but different numbers of neutrons.
Because isotopes of an element have different numbers of neutrons, they have different mass numbers.
Isotopes Cont…
• Atomic mass unit is defined as one twelfth of the mass of a carbon-12 atom.
• In nature most elements occur as a mixture of two or more isotopes.
• Isotopes of an element has a fixed mass and a natural percent abundance.
Relative Abundance
• The Atomic Mass of an element is a weighted average mass of the atoms in a naturally occurring sample of the element.
• A weighted average mass reflects both the mass and the relative abundance of the isotopes as they occur in nature.
Determine Atomic Mass
1. # stable isotopes
2. Mass of each isotope
3. Natural % abundance of each isotope
Atomic Mass = Mass of Isotope x natural
abundance = add them together
Look on page 117 for practice problems!!
Periodic Table Cont….
• Each vertical column of the periodic table is called a group.
• Elements within a group have similar chemical and physical properties.
Lets Review!!!
• Atoms are the smallest particle of an element that retains its identity in a chemical reaction.
• Democritus believed that atoms were indivisible and indestructible.
• Dalton’s atomic theory states that
1. All elements are composed of atoms
2. Atoms of different elements combine to form compounds
3. Atoms of the same element are identical
4. Chemical reactions occur when atoms are separated, joined, or rearranged.
Review Cont….
• There are 3 subatomic particles
1. Proton
– Identifies the atom
– Positive charge
2. Electron
– Determines chemical reactions
– Negative Charge
3. Neutron
– Determines Isotope
– Neutral Charge
Review Cont…
• Thomson atomic model, known as the plum pudding model, electrons were stuck into a lump of positive charge.
• Rutherford’s Gold Foil Experiment: He proposed that atoms are mostly empty space.
• The nucleus is the tiny central core of an atom made of protons and neutrons. Electrons surround the nucleus.
• Atomic number is the number of protons.
• Mass number is the # of protons and neutrons.
• Isotopes are atoms that have the same number of protons but a different number of neutrons.
Review Cont…
• Atomic mass unit is defined as one twelfth of the mass of a carbon-12 atom.
• The atomic mass of an element is the weighted average mass of the atoms in a naturally occurring sample of the element.
• To calculate the atomic mass of an element, multiply the mass by its natural abundance and then add the products.