Download Intro to Atoms

Chapters 11 and 12
Physical Science
 Atom
– the smallest particle into which
an element can be divided and still be
the same substance.
 Atoms can join with other atoms to
make new substances.
 By the 1700s, scientists knew elements
could combine in specific combinations to
form compounds (Hydrogen + Oxygen =
Water). John Dalton found that
elements combine due to their
individual atoms.
 All
substances are made up of atoms.
Atoms are small particles that cannot be
created, divided or destroyed.
 Atoms of the same element are exactly
alike, while atoms of different elements
are different.
 For the most part, Dalton was correct.
Later there would be some revisions.
 100
years later, British scientist J.J. Thomson
discovered there were particles inside of
atoms (so atoms could be divided)
 He discovered that there were positivelycharged particles in the atoms. He called
those particles protons. He found they
were attracted to negative charges.
 Thomson
also discovered there were
negative charges that were attracted to
positive charges. He called those
electrons.
 Since an atom by itself doesn’t have a
charge, then Thomson realized that
each atom must have the same number
of protons and neutron. He created his
atomic model based on his experiments
(the plum-pudding model)
 Ernest
Rutherford, a student of Thomson,
realized that, when he sent atomic charges
(postive and negative) to metal, some of the
particles went through the metal (which he
expected but then some charges bounced
back (which he didn’t expect). This made
him realize that the atomic structure of the
atom is a little different than Thomson
believed.
 Niels
Bohr, a Danish scientist who worked
with Rutherford, he suggested that electrons
travel around the nucleus in definite levels,
and that these electrons could jump from
level to level.
 We now know that electrons travel in
levels (shells) in a region called the
electron cloud.
 Scientists
now know how atoms are
structured:
 Nucleus – the center of the atom, it is
very small and very dense with a lot of
particles. These particles are the
protons (+ charge) and the neutrons (no
charge at all, but are “heavier” than a
proton).
 The atomic number is the number of
protons in the nucleus of an atom. A
proton and neutron each have a mass of
1 amu (atomic mass unit)
 So,
what is the purpose of neutrons if they
have no charge?
 Neutrons act as a “buffer” between the
protons. If the protons were around each
other, they would constantly repel (since
they are all positive charges). The larger
neutrons help keep the protons from
repelling.
 There
is a “cloud” outside of the nucleus
where the electrons (- charge) are found
(electron cloud). These electrons move
around the nucleus in circular layers called
shells, and there are different numbers of
electrons on each shell.
 The mass of an electron is so small, it is
considered to be 0.
 If
there are the same number of protons and
electrons in an atom, then the atom has no
electric charge (it is neutral). However,
sometimes an atom has more protons
(becomes + charged), or electrons (becomes
– charged).
 When an atom has a charge, it is called an
ion.
 There
are over 110 identified elements, so
how do each of their atoms differ?
 It depends on the number of protons and
electrons each atom of each element has.
Hydrogen (H) = 1 proton, 1 electron (no
need for a neutron – why?)
Carbon (C) = 6 protons, 6 neutrons and 6
electrons
Oxygen (O) = 8 protons, 9 neutrons, 8
electrons
Gold (Au) = 79 protons, 118 neutrons, 79
electrons
Notice that you don’t have to have the
same number of neutrons as protons in
 You
already know that the mass of an atom is
all found in the nucleus.
 However, atoms are identified by their
atomic number. The atomic number is the
number of PROTONS you find in a nucleus.
 Example: Hydrogen has an atomic number
1
 Sometimes
the same element will have
atoms that have more particles (neutrons) in
it’s nucleus. For example, you know that
Hydrogen has only 1 proton, so it has an
atomic number of 1. However, if you add
neutrons to it’s nucleus, it will still have an
atomic number of 1, but the atomic mass has
changed. This “heavier” atom is called an
isotope.
 Isotopes
are atoms that have the same
number of protons but a different
number of neutrons.
 Isotopes share the same chemical and
physical properties of the atoms in the
same element, but some isotopes are
unstable. This means that their
neutrons can change their composition
and become radioactive (ex: Carbon 14
dating).
 Since
atoms are identified by their protons
(atomic number), isotopes are special.
Their big difference is the number of
neutrons, which means the mass of an
isotope is larger than the mass of an atom
of the same element. Therefore, isotopes
are identified by their mass number

1)
2)
3)
To identify a specific isotope of an
element:
Write the name of the element.
Take the mass number, subtract the
number of protons (the atomic number).
The answer is the number of neutrons in
the atom.
 Carbon
–12
12 Mass Number (protons + neutrons)
- 6 Number of Protons (atomic number)
6 Number of neutrons
The mass number of Carbon-12 is 6.
 Atomic
mass is the weighted average of
the masses of all naturally occurring
isotopes of an element.
 Example: Isotopes of Copper (Cu) - the
Statue of Liberty is made of isotopes of
copper (69% is Cu-63, 31% is Cu-65). If
you average the masses of all isotopes
of copper, the atomic mass of copper is
63.6 amu.
 In
an atom, you have both attraction and
repelling forces:
 Gravity – because the particles in an
atom are so small, gravitational
attraction is very small.
 Electromagnetic
Force – particles with
the same charge will repel, opposite
charges will attract. Electrons circle the
nucleus because they “feel” the
attraction to the protons inside the
nucleus.
 Strong
Force – Because the protons in
the nucleus repel each other
(electromagnetic force), there is a
Strong Force within the nucleus that is
stronger than the repelling, so the
nucleus stays together.
 Weak
Force – In radioactive isotopes, a
neutron can change into a proton or an
electron. This makes the isotope
unstable.
.