Atomic Structure: Basic Concepts
• What is an atom?
• What are atoms made of?
• Where are protons
located and what is their
electrical charge?
• Where are neutrons
located and what is their
electrical charge?
• Where are electrons
located and what is their
electrical charge?
• Atom is the basic unit of
matter
• Atoms are made of
protons, neutrons and
electrons.
• Protons are located in the
nucleus of atom having
positive electrical charge.
• Neutrons are located in
the nucleus of atom
having positive electrical
charge
• Electrons are located in
clouds outside nucleolus
and have negative
electrical charge.
What we think atoms look like…
Atomic Structure: Basic Concepts
Dalton’s Atomic Theory
• The following statements are the main points
of Dalton’s atomic theory.
1.All elements are composed of tiny
indivisible particles called atoms.
2.Atoms of the same element are
identical and different from atoms of
other elements.
Dalton’s Atomic Theory
3. Atoms of different elements can mix together
to form compounds.
4. Chemical reactions occur when atoms are
separated, joined, or rearranged. Atoms are
never changed into atoms of another element
as a result of chemical reaction.
Table of Elements
A chemical element is a pure substance
that is made of only one kind of atom.
Ex: Carbon, Hydrogen, Oxygen, Nitrogen.
Carbon – a chemical element.
The number
of electrons
in a neutral
atom is the
same as the
number of
protons.
6
C
Carbon
12.011
The number
above the letter
is the element’s
atomic number,
which is the
same as the
number of
protons in its
nucleus.
Atoms are neutral (no electrical charge)
because their number of protons is
equal to the number of electrons.
Atomic Structure: Basic Concepts
Masses
• The mass of a neutron is almost the same as
the mass of a proton. However an electron is
about 2,000 times lighter than a proton.
• The sum of the protons and neutrons in the
nucleus is the mass number of that particular
atom.
* Mass number is also called atomic
weight/atomic mass
Atomic Structure: Basic Concepts
Atomic Mass
• In order to have a simpler way of comparing
the masses of individual atoms, chemists
have devised a different unit of mass called
an atomic mass unit, which is given the
symbol u or amu.
• Chemists have defined the carbon-12 atom as
having a mass of 12 atomic mass units.
Isotopes of Carbon
Nonradioactive carbon-12
6 electrons
6 protons
6 neutrons
Nonradioactive carbon-13
6 electrons
6 protons
7 neutrons
Radioactive carbon-14
6 electrons
6 protons
8 neutrons
Isotopes are atoms with the same number of protons
and electrons but different number of neutrons.
Isotopes like carbon-14, C14, can be used by scientists
to determine the age of rocks and fossils.
Isotopes of Lithium
Atomic Structure: Basic Concepts
Electrons in Motion
• Niels Bohr (1885-1962), a Danish scientist
who worked with Rutherford, proposed that
electrons must have enough energy to keep
them in constant motion around the nucleus.
• Electrons have energy of motion that enables
them to overcome the attraction of the
positive nucleus.
Atomic Structure: Basic Concepts
Electrons in Motion
• This energy keeps
the electrons
moving around the
nucleus.
• Bohr’s view of
the atom, which
he proposed in
1913, was called
the planetary
model.
Atomic Structure: Basic Concepts
The Electron Cloud Model
• Electrons themselves take up little space but
travel rapidly through the space surrounding
the nucleus.
• These spherical regions where electrons
travel may be depicted as clouds around the
nucleus.
• The space around the nucleus of an atom
where the atom’s electrons are found is called
the electron cloud.
Atomic Structure: Basic Concepts
The Electron Cloud Model
Atomic Structure: Basic Concepts
Electrons in Energy Level
• How are electrons arranged in energy levels?
• Each energy level can hold a limited number
of electrons.
• The lowest energy level is the smallest and
the closest to the nucleus.
Atomic Structure: Basic Concepts
Electrons in Energy Level
• This first energy level holds a maximum of
two electrons.
• The second energy level is larger because it is
farther away from the nucleus. It holds a
maximum of eight electrons.
• The third energy level is larger still and holds
a maximum of 18 electrons.
Atomic Structure: Basic Concepts
Energy Levels
• A hydrogen atom has only one electron. It’s in
the first energy level.
Atomic Structure: Basic Concepts
Electrons in Energy Level
• The electrons in the outermost energy level
are called valence electrons.
• You can also use the periodic table as a tool
to predict the number of valence electrons in
any atom in Groups 1, 2, 13, 14, 15, 16, 17,
and 18.
• All atoms in Group 1, like hydrogen, have
one valence electron. Likewise, atoms in
Group 2 have two valence electrons.
Atomic Structure: Basic Concepts
Electrons in Energy Level
• An oxygen atom has eight electrons. Two of
these fill the first energy level, and the
remaining six are in the second energy level.