Unit C: Atoms, elements, and Compounds
6.7: Explaining the Periodic Table
pg. 234
Key Concepts:
3. Elements are organized according to their atomic number and electron
arrangement on the periodic table.
5. Atoms contain protons and neutrons in a central core surrounded by
electrons.
6. Elements can be both beneficial and harmful to humans and to the
environment.
Atomic Number
Atomic Number: the number of protons in an atom’s nucleus.
- Dalton’s Atomic Theory states that the atoms of each element are different
from atoms of other elements.
- Atoms are unique because of the number of protons they have in their
nucleus.
- The number of protons in the nucleus is known as the Atomic Number.
- Elements are arranged on the Periodic Table according to increasing
Atomic Number.
- You can not combine different atoms to make new elements.
Copper (Cu 29P+) + Tin (Sn 50P+)
→ Gold (Au 79P+)
- Protons are tightly contained within the Nucleus of an atom. They can not
combine with other atom’s nucleus to form new atoms.
Figure 2: the atomic number is given in
the top left-hand corner of each element
on the periodic table.
Mass Number and Atomic Mass
Mass Number: the number of protons and neutrons in an atom’s nucleus.
Isotope: an atom with the same number of protons but a different number of
neutrons.
Atomic Mass: the mass of an atom in atomic mass units (u).
- An atom is made up of mostly empty space. Electrons are found in this
empty space and make up very little mass.
_ the mass of an atom is found within its nucleus, yet the nucleus takes up
very little space.
- Example, Lithium: Lithium has the atomic number of 3. It has 3 protons in
the nucleus. - It has an Atomic Mass of 6.94 u.
- Atomic Mass relates to the mass of the sub atomic particles (Protons and
Neutrons) within the nucleus.
- The most common form of Lithium has an Atomic mass is 6.0 u and there
are 3 Protons in the nucleus the rest of the mass must come from neutrons
which are also found in the nucleus. 6u – 3 = 3 neutrons are found in the
nucleus.
- The mass of a proton is equal to the mass of a neutron. 1:1 ratio.
- An isotope is an atom with the same number of protons (3) for lithium, but
a different Atomic Mass, 7u for Lithium.
- Mass Number of 7 u – 3 (Protons) = 4 neutrons in the nucleus.
- The True Atomic Mass of Lithium is not a whole number but a
combination of Lithium 6u and Lithium 7u, mass number of 6.94 u. This is
the average weight of the two isotopes.
- When determining the number of neutrons; round the Atomic Mass to the
nearest whole number and then subtract the number of protons
Figure 4: A Lithium atom contains 3
protons and 4 neutrons, giving it a mass
number of 7.
Figure 5: One Lithium isotope contains
3 protons and 4 neutrons, giving it a mass
number of 7. The other lithium isotope contains
3 protons and 3 neutrons, giving it a mass
number of 6.
Figure 6: The element lithium has an
Atomic number of 3 and an atomic mass of 6.94 u.
Sample Problem 1: Finding the Number of Neutrons
Find the number of neutrons in the most common isotope of Aluminum ….
Bohr-Rutherford Diagrams of an Atom
Bohr-Rutherford diagram: a simple drawing that shows the numbers and
location of protons, neutrons, and electrons in an atom.
- Bohr-Rutherford diagram shows the number of protons and neutrons found
within the nucleus, and the number of electrons orbiting around the nucleus.
- The number of protons equals the Atomic Number.
- The number of protons always equals the number of electrons
for neutrally charged atoms.
- The number of neutrons is equal to the Atomic Mass subtract
the Atomic Number.
Sample Problem 2: Drawing a Bohr-Rutherford Diagram
Step 1: Determine the amount of protons and neutrons
Determine the number of Electrons.
Step 2: Draw circles to represent the nucleus and electron shells surrounding
the nucleus.
Step 3: Draw the electrons in their electron shells. There are a maximum of
2 electrons in the first shell, 8 electrons in the second shell and 8 electrons in
the third shell.
Identify the number of protons and neutrons found in the nucleus.
The Periodic Table Meets Bohr-Rutherford
- The Bohr-Rutherford model is used to explain the pattern of families of
elements in the periodic table, first 20 elements.
Patterns in the Periodic Table
- As you go down each family, the number of electron shells increase by one.
Each row increases by one orbit.
- Elements in the same family have the same number of electrons in their
outer shell.
- Elements found in the same family undergo similar chemical reactions,
because they have the same number of electrons in their outer electron shell
or orbit.
Figure 11: These elements have similar properties because they each
have one outer electron.
Table 1: Theory Meets Evidence
Evidence
Elements have unique properties
Same objects can attract each other
In Rutherford’s gold foil experiment,
positively charged particles were fired at a
sample of gold atoms.
* Most particles passed through the foil.
* Some particles were deflected at large
angles.
Elements of the same chemical family have
similar properties.
Each Element gives off unique pattern of
coloured lines when it is excited by energy.
Theory
- each element is made of different atoms
- All atoms contain three subatomic
particles: protons, neutrons, and electrons.
- Atoms are electrically neutral because
they have an equal number of protons and
electrons.
- Matter is positively charged if it contains
more protons than electrons.
- Matter is negatively charged if it contains
more electrons than protons.
- Objects that attract each other are
oppositely charged.
- Moat of the volume of the atom is empty
space.
- The centre of the atom is very small,
dense, positively charged core called the
nucleus.
- Electrons exist in specific orbits around
the nucleus.
- The number of electrons in the outermost
orbit determines many elemental
properties.
- Elements of the same chemical family
have the same number of electrons in their
outermost orbits.
- Each electron orbit has a definite amount
of energy.
- Electrons that absorb energy jump to
higher orbits.
- Electrons emit energy when they return to
lower orbits.
- The colour of light observed corresponds
to the energy difference between the two
electron orbits.
Evidence of Learning: Students can …
- describe and relate atomic number, atomic mass, and mass number.
- find the number of neutrons in an atom from its atomic number and atomic
mass.
- draw a Bohr-Rutherford diagram for a small atom.
- relate Bohr-Rutherford diagrams to the placement of atoms on the periodic
table.
Check Your Learning
Questions 1 – 10, page 240
Summary:
- Atomic number represents the number of protons in an atom. This number
is unique to each element.
- In a neutral atom, the number of electrons equals the number of protons.
- Atomic mass is the mass of an atom measured in atomic mass units, (U).
- Mass number represents the total number of protons and neutrons in an
atom.
- The maximum number of electrons in the first, second, and third orbits is 2,
8, and 8, respectively. Electrons fill lower orbits before filling higher orbits.
- Bohr-Rutherford diagrams of atoms show the number and location of
protons, neutrons, and electrons in the atom.
- All atoms within each family of elements have the same number of
electrons in their outermost orbits.