Transcript Ch 4 Intro to Atoms

Introduction to Atoms
Chapter 4
Democritus
• Greek Philosopher
• “uncutable particle”
• Named the atom from
the Greek word Atomos
• Atoms are small, hard, different
shapes and sizes, always moving
and join together
Aristotle
• Greek philosopher
• Disagreed with Democritus
• Strong influence
Dalton
• British Chemist 1766
• Elements combine in specific
proportions because they are made of
individual atoms
• He observed that no matter how large
or small the sample, the ratio of the
masses of the elements in the
compound is always the same.
• Compounds have a fixed position
Dalton’s Theory
• Dalton proposed the theory
that all matter is made up of
individual particles called
atoms, which cannot be
divided.
Main Points of Dalton’s Theory
• All elements are composed of atoms
• All atoms of the same element have the
same mass, and atoms of different
elements have a different masses.
• Compounds contain atoms of more than
one element.
• In a particular compound, atoms of
different elements always combine in the
same way
What did Dalton notice that all
compounds have in common?
• Dalton noticed that the ratio
of masses of elements in a
compound is always the
same!
Objects with like charges repel,
or push apart.
• Objects with opposite charges
attract, or pull together.
Thomson’s Experiments
• He used a sealed tube of gas
in his experiments.
– When the current was on,
the disks became charged
and glowing beam
appeared in the tube.
– The beam bent toward a
positively charged plate
placed outside the tube.
• He concluded that the
particles in the beam had a
negative charge because
they were attracted to the
positive plate.
• Thomson’s
experiments
provided the first
evidence that
atoms are made
of even smaller
particles
Thomson’s Model
• Thomson found a negative charge, but
knew that atoms were neutral.
• Atoms must contain a positive charge
as well.
• The negative charges were evenly
scattered throughout an atom filled
with a positively charged mass of
matter.
• Plum Pudding Model
Rutherford’s Atomic Theory
(1871-1937)
• He hypothesized
that the mass and
charge at any
location in the gold
would be too small
to change the path
of an alpha particle
(positive charge).
• When the gold foil experiment
was done more particles were
deflected and bounced straight
back, as though they had struck
an object.
• The alpha particles must have
come close to another charged
particle
• Rutherford
concluded that the
positive charge of
an atom is not
evenly spread
through the atom.
• It is concentrated
in a very small,
central area called
the nucleus.
• According to
Rutherford’s
model, all of an
atom’s positive
charge is
concentrated in
its nucleus.
Bohr
• Danish scientist
• Discovered electrons travel
around nucleus in definite paths
• Located in levels around nucleus
• Electrons can jump from one level
to another
Schrodinger and Heisenberg
• Modern theory
• Electron clouds surround
nucleus
• Electrons do not travel in
definite paths
• Movement cannot be predicted
Chapter 4 Section 2
Atom Structure
Nucleus
• A dense, positively
charged mass
located at the
center of the atom
• New Model: All of
an atom’s positive
charge is
concentrated in its
nucleus.
Properties of subatomic particles
• 1. Protons
electron
• 2. Electrons
• 3. Neutrons
neutron
proton
Protons
• Positively charged subatomic
particle that is found in the
nucleus of an atom
• Charge of 1+
Electron
• A negatively charged subatomic
particle that is found in the space
outside the nucleus
• Charge of 1-
Neutrons
• Neutral subatomic particle that is
found in the nucleus of an atom
• Mass almost exactly equal to that
of a proton
Subatomic particles
• Distinguished by their mass,
charge and location in the atom
Atomic Number
• Atoms of any element have
the same number of protons
• Atomic number = number of
protons in an atom of that
element
• Ex: Hydrogen has 1 proton,
atomic number is 1
Mass Number
• Sum of protons and
neutrons in the nucleus of
an atom
• Number of neutrons = Mass
number – Atomic Number
Isotopes
• Every atom of a given element has the
same number of protons and electrons,
but every element does not have the same
number of neutrons.
• Isotopes – have same atomic number, but
different mass numbers because they
have different number of neutrons
Isotopes
• Atoms that have the same number of
protons but have a different number of
neutrons
• Atoms that are isotopes of each other are
always the same element because the
number of protons in each atom is the
same
Bohr’s Model of the Atom
• Focused on Electrons that move
with constant speed in fixed orbits
around the nucleus
• Electrons can change energy
levels when the atom gains or
loses energy
Electron Cloud Model
• Visual model used to find the
locations for electrons
Energy levels, orbitals and
Electrons
Energy Level
# of Orbitals
1
1
Maximum # of
Electrons
2
2
4
8
3
9
18
4
16
32
Forces that Work in Atoms
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•*
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Gravity
Electromagnetic Force
Strong Force
Weak Force