Download Unit 2.4 Understanding the Elements Listed on the Periodic Table

Unit 2.4 Understanding the Elements Listed on the Periodic Table
How many gallons of gas can fill a car's tank?
The current system of measurement in the Unites States is
a hodge-podge of different units, many of which are hard to
interconvert, unlike the metric system. We at least have
standardized units these days, unlike centuries past. At one
time, measurements of length often were defined as the
distance from the end of one appendage to another. For
example, the yard would be defined as the distance from
the king’s nose to the tip of his thumb when his arm was
stretched out.
Standardized measurements make it possible for people everywhere to get the same amount of
something. Note the red and white labels on the gas pump above. These labels certify that the gas pump
has been checked and is pumping an accurate gallon of gas. Standard measurements in science are very
important so that we can compare experimental data from one lab to another and make sure we all are
talking about the same thing.
Atomic Mass Units
Masses of individual atoms are very, very small. Using a modern device called a mass spectrometer, it is
possible to measure such minuscule masses. An atom of oxygen-16, for example, has a mass of 2.66 × 1023
g. While comparisons of masses measured in grams would have some usefulness, it is far more practical
to have a system that will allow us to more easily compare relative atomic masses. Scientists decided on
using the carbon-12 nuclide as the reference standard by which all other masses would be compared. By
definition, one atom of carbon-12 is assigned a mass of 12 atomic mass units (amu). An atomic mass unit
is defined as a mass equal to one twelfth the mass of an atom of carbon-12. The mass of any isotope of
any element is expressed in relation to the carbon-12 standard. For example, one atom of helium-4 has a
mass of 4.0026 amu. An atom of sulfur-32 has a mass of 31.972 amu.
What Is a Proton?
A proton is one of three main particles that make up the atom. The other two particles are the neutron
and electron. Protons are found in the nucleus of the atom. This is a tiny, dense region at the center of
the atom. Protons have a positive electrical charge of one (+1) and a mass of 1 atomic mass unit
(amu), which is about 1.67 × 10-27 kilograms.
Watch out! Lightning is extremely dangerous. A single bolt of
lightning can carry a billion volts of electricity. That’s enough energy
to light a 100-watt light bulb—for three months! As impressive as it
is, lightning is nothing more than a sudden flow of extremely tiny
particles. What are the particles that flow in a lightning bolt? The
answer is electrons.
What Is an Electron?
Electrons are extremely small. The mass of an electron is only about 1/1837 the mass of a proton or
neutron, so electrons contribute virtually nothing to the total mass of an atom. Electrons have a negative
electric charge of -1, which is equal but opposite to the charge of proton, which is +1. All atoms have
the same number of electrons as protons, so the positive and negative charges “cancel out,” making atoms
electrically neutral. Electrons are found outside of the nucleus in the “electron cloud” or “energy levels”.
What Is a Neutron?
Atoms of all elements—except for most atoms of hydrogen—have neutrons in their nucleus. Atoms generally
have about the same number of neutrons as protons, or slightly more. Neutron have no electrical charge, or
zero charge, however, they have a mass of 1 amu or about 1.67 x 10-27 kilograms. Together with protons,
they make up virtually all of the mass of an atom.
Atomic Number
The atomic number of an atom is the number of protons found in the nucleus. Atoms of the same element
have the same number of protons, and atoms of different elements have a different number of protons.
The modern periodic table uses atomic number to arrange the atoms. Each successive atom has one more
proton, thus the atomic number increases by one with each element.
History of Atomic Mass Determinations
As a part of his research on atoms, John Dalton determined a number of atomic weights of elements in
the early 1800s. Atomic masses were the basis for the periodic table that Mendeleev developed.
Originally all atomic masses were based on a comparison to hydrogen, which has an atomic mass of one.
After the discovery of the proton, scientists assumed that the weight of an atom was essentially that of
the protons – electrons were known to contribute almost nothing to the atomic weight of the element.
This approach worked until we learned how to determine the number of protons in an element. We then
saw that the atomic mass for an element was often twice the number of protons (or more). The
discovery of the neutron provided the missing part of the picture. The atomic mass is now known to be
the sum of the protons and neutrons in the nucleus..
For example, all helium atoms have
two protons, and no other elements
have atoms with two protons. In the
case of helium, the atomic number is
2. The atomic number of an element is
usually written in front of and slightly
Nuclear Notation
Shows the information required to determine the
element, as well as the number of protons, electrons,
and neutrons.
below the element’s symbol. The
atomic mass, the number of protons
plus neutrons is written above the
atomic number, before the element
symbol.
Atoms of the First Six Elements
Name
Symbol
Atomic Number
Protons
Neutrons
Electrons
Atomic Mass
Hydrogen
H
1
1
0
1
1
Helium
He
2
2
2
2
4
Lithium
Li
3
3
4
3
7
Beryllium
Be
4
4
5
4
9
Boron
B
5
5
6
5
11
Carbon
C
6
6
6
6
12
Consider the element helium. Its atomic number is 2, so it has two protons in its nucleus. Its nucleus also
contains two neutrons. Since 2 + 2 = 4, we know that the mass number of the helium atom is 4. Finally,
the helium atom also contains two electrons since the number of electrons must equal the number of
protons. This example may lead you to believe that atoms have the same number of protons and neutrons,
but further examination of the table above will show that this is not the case. Lithium, for example has
three protons and four neutrons, leaving it with a mass number of 7.
Knowing the mass number and the atomic number of an atom allows you to determine the number of
neutrons present in that atom by subtraction.
Number of neutrons = atomic mass - atomic number
Atoms of the element chromium (Cr) have an atomic number of 24 and an atomic mass of 52.
How many neutrons are in the nucleus of a chromium atom?
52 - 24 = 28 neutrons in a chromium atom
The number of electrons would be equal to the number of protons in neutral atoms.
How many electrons would be in the chromium atom?
Chromium has 24 protons and 24 electrons.
The composition of any atom can be illustrated with a shorthand notation using the atomic number and
the mass number. Both are written before the chemical symbol, with the mass number written as a
superscript and the atomic number written as a subscript. The chromium atom discussed above would be
written as:
Another way to refer to a specific atom is to write the mass number of the atom after the name,
separated by a hyphen. The above atom would be written as chromium-52.
What Are Isotopes?
All atoms of the same element have the same number of protons, but some may have different numbers
of neutrons. For example, all carbon atoms have six protons, and most have six neutrons as well. But some
carbon atoms have seven or eight neutrons instead of the usual six. Atoms of the same element that
differ in their numbers of neutrons are called isotopes. All elements have isotopes. Many isotopes occur
naturally. Usually one or two isotopes of an element are the most stable and common. Different isotopes
of an element generally have the same physical and chemical properties. That’s because they have the
same numbers of protons and electrons.
An Example: Hydrogen Isotopes
Hydrogen is an example of an element that has isotopes. Three isotopes of hydrogen are modeled in
the figure below. Most hydrogen atoms have just one proton and one electron and lack a neutron. These
atoms are just called hydrogen. Some hydrogen atoms have one neutron as well. These atoms are the
isotope named deuterium. Other hydrogen atoms have two neutrons. These atoms are the isotope named
tritium
e-
Q: The mass number of an atom is the sum of its protons and neutrons. What is the mass number of each
isotope of hydrogen shown in the Figure above?
A: The mass numbers are: hydrogen = 1, deuterium = 2, and tritium = 3. (See the top number of the
nuclear notation.)
For most elements other than hydrogen, isotopes are named for their mass number. For example, carbon
atoms with the usual 6 neutrons have a mass number of 12 (6 protons + 6 neutrons = 12), so they are
called carbon-12. Carbon atoms with 7 neutrons have an atomic mass of 13 (6 protons + 7 neutrons = 13).
These atoms are the isotope called carbon-13.
Q: Some carbon atoms have 8 neutrons. What is the name of this isotope of carbon?
A: Carbon atoms with 8 neutrons have an atomic mass of 14 (6 protons + 8 neutrons = 14), so this isotope
of carbon is named carbon-14.
Q: The element oxygen has 8 protons. How many protons and neutrons are there in oxygen-17?
A: Oxygen-17—like all atoms of oxygen—has 8 protons. Its name provides the clue that it has a total of
17 protons and neutrons. Therefore, it must have 9 neutrons (8 + 9 = 17).
Elements Listed on the Periodic Table
Each square representing an element on the periodic table shows information about the number of
particles found in that element.
Atomic mass is the average number
of protons plus neutrons (p+ + n)
Atomic number is the number of
protons (p+) found in the nucleus
Carbon
12.0111
C
6
Chemical Symbol: 1 or 2 letters used
to represent the element. The 1st
letter is always capital. If there is a 2nd
letter it is lower case.
When ( ) is used around the atomic mass, it means that very little of that elements exists at any given
time. Scientists have not been able to attain a good average atomic mass.
Remember that all elements have isotopes. The atomic mass listed is the average mass.
Summary

An atom’s mass number is its mass in atomic mass units (amu), which is about equal to the total
number of protons and neutrons in the atom.

A proton one of three main particles that make up the atom. It is found in the nucleus. It has an
electrical charge of one +1 and a mass of 1 atomic mass unit (amu).

Atoms of any given element have a unique number of protons that is different from the numbers
of protons of all other elements.

Electrons are one of three main types of particles that make up the atom. They are extremely
small and have an electric charge of -1, and virtually no mass. All neutral atoms have the same
number of electrons as protons.


Electrons are found outside of the nucleus in “electron clouds” or “energy levels”.
A neutron is one of three main particles that make up the atom. It is found in the nucleus and is
neutral in electric charge. It has about the same mass as a proton. Neutrons are found in all
atoms except for most atoms of hydrogen.

The nucleus is a small, dense region at the center of the atom. It consists of positive protons and
neutral neutrons, so it has an overall positive charge.


The nucleus is just a tiny part of the atom, but it contains virtually all of the atom’s mass.
The number of protons in an atom is called its atomic number. This is also unique for each
element.



The atomic mass is defined as the total number of protons and neutrons in an atom.
The number of neutrons = mass number - atomic number.
All the atoms of a given element have the same number of protons and electrons, but they may
vary in their numbers of neutrons. Atoms of the same element that differ in their numbers of
neutrons are called isotopes.

Different isotopes of an element generally have the same physical and chemical properties
because they have the same numbers of protons and electrons.

Carbon-12 is the reference for all atomic mass calculations.
Review
1.
What is an atomic mass unit? What is it equal to?
2.
Why is the nucleus positive in charge?
3.
Explain why the nucleus is very dense.
4.
Describe protons.
5.
What is the relationship between protons and elements?
6.
What are electrons?
7.
What is the relationship between the number of electrons and protons in a neutral atom?
8.
What is a neutron?
9.
What is the atomic number of an atom?
10. What is the atomic mass of an atom?
11. A tin atom has an atomic number of 50 and an atomic mass of 118, how many protons, electrons
and neutrons would be found in the tin atom?
12. What is the mass of a cobalt atom that has 27 protons and 30 neutrons?
13. The symbol below represents an isotope of helium. How many protons and neutrons does it have?
14. What are isotopes?
15. All carbon atoms have six protons. Most also have six neutrons, but some have seven or eight
neutrons. What is the mass number of a carbon isotope that has seven neutrons?
16. Why do isotopes of an element generally have the same physical and chemical properties?
Answers
1.
An atomic mass unit is a unit of measure for subatomic particles. It is equal to 1.7 x 10
-24
kilograms.
2.
The nucleus is positive because it is made of protons which carry a +1 charge, and neutrons which
are neutral.
3.
The nucleus is a tightly packed area in the center of the atoms containing both protons and
neutrons, which constitute most of the mass of an atom.
4.
Protons are a particle found in the nucleus of an atom, with a +1 charge and a mass of 1 amu.
5.
The number of protons is unique to each element.
6.
Electrons are a particle found around the nucleus of an atom, with a -1 charge and virtually no
mass.
7.
In a neutral atom, the number of electrons is equal to the number of protons.
8.
Neutrons are particles found in the nucleus of an atom, with no charge and a mass of 1 amu.
9.
Atomic number is the number of protons found in the atom.
10. Atomic mass is the number of protons plus the number of neutrons found in the atom.
11. 50 protons, 50 electrons (number of protons is equal to the number of electrons), 68 neutrons
(atomic mass – atomic number, 118 – 50)
12. Atomic mass is 57 (protons plus neutrons, 27 + 30)
13. 2 protons, 3 neutrons (and 2 electrons)
14. Isotopes are atoms of the same element with a different atomic mass because they have a
different number of neutrons.
15. With 7 neutrons it is carbon – 13
16. Isotopes have the same properties because they have the same number of protons and electrons.