Download Atomic Structure Notes file

Structure of the Atom
Atomic Number
All atoms are composed of the same basic particles, yet all atoms are not the same. Atoms of different elements have different
numbers of protons. Atoms of the same element all have the same number of protons. The
atomic number (Z) of an element is the number of protons in the nucleus of each atom of that
element.
Turn to the periodic table on the back of your resource guide. In the periodic table, an element’s
atomic number is usually indicated above its symbol. The atomic number identifies the element.
Notice the two images to along the right side of this page. It is worth noting that periodic tables
often differ slightly from one another. Depending on the preference of the publisher, the
arrangement in the squares can vary. The atomic number is always a whole number. The atomic
mass usually has several decimal places indicated, again, dependent upon the preference of the
publisher.
If you want to know which element has atomic number 47, for example, you would look at the
periodic table and see that atomic number 47 corresponds with silver, Ag. You would also then
know that all silver atoms contain 47 protons in their nuclei. And because atoms are neutral, we
also know from the atomic number that silver atoms must also contain 47 electrons.
Ions
When an atom gains or loses one or more electrons, it acquires a net electrical charge and is called an ion. If an atom has
more protons than electrons, it will have a positive charge and is called a cation. If the atom has more electrons than protons
it will have a negative charge and is called an anion. To write an ion with chemical symbols, place the charge of the ion to the
upper right of the chemical symbol of the element. The charge symbol may be placed before or after the numerical value of the
charge, as below:
Mg+2
or
Mg2+
Isotopes and Mass Number
The atoms of an element can differ in mass from each other because they have differing numbers of neutrons. Those with
more neutrons will weigh more and be more massive. The atomic mass (often referred to as atomic weight) of an element is
calculated by adding together the number of protons and the number of neutrons. Isotopes are atoms of the same element
having different masses. IMPORTANT: Remember Dalton’s postulates? This violates one of them! Can you explain how/why?
The simplest atoms are those of hydrogen. All hydrogen atoms contain only one proton. However, like many naturally
occurring elements, hydrogen atoms can contain different number of neutrons.
The diagram shows three isotopes of hydrogen. These three atoms are all atoms of the same
element because they each have one proton. Therefore they all have the same atomic number.
They are isotopes of hydrogen because they have different masses, caused by differing number
of neutrons. The mass of the hydrogen atom is 1, of the deuterium atom is 2 and the mass of
tritium is 3.
Carbon has three isotopes in nature. The most common isotope of carbon has 6 protons and 6
neutrons. Its atomic mass is 12. A much more rare isotope of carbon (<1% of all carbon atoms)
has an atomic mass of 13. A still more rare isotope of carbon has an atomic mass of 14. This
atom is unstable and radiates particles and energy from its nucleus. This spontaneous decay of the nucleus makes such an
atom a radioactive isotope of the element. Radioactive isotopes have many practical uses, from dating fossils to treating
diseases.
Identifying an isotope requires knowing both the name of atomic number of the element and the mass of the isotope. The
mass number is the total number of protons and neutrons in the nucleus of an isotope.
Protium/Hydrogen
Deuterium
Tritium
Mass Number of Hydrogen Isotopes
Atomic #/ # of protons
# of neutrons
1
0
1
1
1
2
Mass Number
1+0=1
1+1=2
1+2=3
Structure of the Atom
Ways to Express Isotopes
There are two acceptable ways to express isotopes. The first method is called hyphen-notation. In hyphen-notation, the
name or symbol of the element is written and then followed by a hyphen (imagine that!) and then the mass number, which will
reflect the change in the neutrons.
Example: Chlorine-37 or Cl-37
The second method of expressing isotopes is called nuclear symbol. When using nuclear symbol, the symbol of the element is
written, and then the atomic number is written to the lower left of the element, and the mass number is written to the upper
left of the symbol.
37
Example: 17 Cl
Skills Practice Problems:
1. For each of the following, indicate the missing information:
Symbol
Nitrogen
Atomic#
# protons
# neutrons
# electrons
7
7
10
21
24
18
Mass #
F1-
56
26
Fe2
Oxygen-18
2. For each of the following, indicate the missing information: Note the change in the column headings!!
Symbol
31
15
28
13
# of neutrons
# of protons
# of electrons
Atomic #
Mass #
38
38
80
P
Al 3
119
50
Sn
8
84
84
7
10
210
While the math of determining protons, neutrons, and electrons is very simple (it is just addition and subtraction of whole
numbers!), many students find it challenging. It takes practice and a willingness to learn what each number/particle tells us
about the structure of an atom. Without question, you will be called upon to tackle problems such as the ones above on tests
and quizzes. If you need additional practice, please utilize the class web page!
Structure of the Atom
Information: Structure of the Atom
Note the following symbols: (they are not to scale)
= proton (positive charge)
= electron (negative charge)
= neutron (no charge)
The following three diagrams represent hydrogen atoms:
1
1
2
1
H
3
1
H
The following three diagrams represent carbon atoms:
12
6
13
6
C
14
6
C
C
Notice the type of notation used for atoms:
A
Z
X
X = chemical symbol of the element
Z = “atomic number”
A = “mass number”
12
6
1
1
C,
13
6
C and
H , 12 H
and
14
6
3
1
C
are notations that represent isotopes of carbon.
H are notations that represent isotopes of hydrogen.
The part of the atom where the protons and neutrons are is called the nucleus.
Critical thinking Questions
1. How many protons are found in each of the following:
1
1
H , 12 H
and
3
1
H
2. How many neutrons are found in each of the following:
1
1
H , 12 H
and
3
1
H
3. How many electrons are found in each of the following:
1
1
H , 12 H
and
3
1
H
H
Structure of the Atom
4. What structural characteristics do all hydrogen atoms have in common?
5. What structural characteristics do all carbon atoms have in common?
6. What does the mass number tell you? Can you find the mass number of an element on the periodic table?
7. What does the atomic number tell you? Can you find the atomic number of the periodic table?
8. Define the term isotope.
9. How does one isotope of carbon differ from another isotope of carbon?
Information: Atoms, Ions, Masses of Subatomic Particles
The atomic mass unit (amu) is a special unit for measuring the mass of very small particles such as atoms. The
relationship between amu and grams is the following: 1.00 amu = 1.66 x 10 -24 g. Note the following diagrams
comparing atoms and ions.
Atom
Ion
9 protons
10 neutrons
9 protons
10 neutrons
19
9
F
mass = 18.9980 amu
Atom
19
9
F 1
mass = 18.9985 amu
Ion
12 protons
12 neutrons
12 protons
12 neutrons
24
12
mass = 23.9978 amu
Mg
24
2
12
mass = 23.9968 amu
Mg
Structure of the Atom
Critical Thinking Questions:
10. What is structurally different between an atom and an ion? Note: This is the ONLY
structural difference between an atom and an ion.
11. In atomic mass units (amu), what is the mass of an electron?
12. Is most of the mass of an atom located in the nucleus or outside the nucleus? How do you know?
13. If protons and neutrons have the same mass, what is the approximate mass of a proton and neutron in
atomic mass units?
14. The mass of
14
6
C
is about 14 amu. Does this agree with what you determined in questions 11 and 13?
15. The charge (in the upper right hand corner of the element symbol) is –1 for a fluorine ion. Why isn’t it +1 or
some other number?
16. What is the charge on every atom? Why is this the charge?
17. How do you determine the charge on an ion?
18. An oxygen ion has a –2 charge. (Use your periodic table if needed.)
a. How many protons does the oxygen ion have?
b. How many electrons does the oxygen ion have??
Use the back of this page to take notes – we are now connecting all this information to the organization of the
periodic table!