Download Atomic Structure of hydrogen

Unit 5 - Atomic Structure, Isotopes & the Periodic Table
5.1
Atomic Structure & Isotopes
Elements
An element is a pure substance consisting of only one type of atom. Most elements on the earth
exist in compounds (pure substances containing two or more elements chemically bonded together).
A symbol is used (like an initial) to represent an atom of an element - refer to the Periodic Table.
A formula is used to represent a compound, e.g. sodium chloride, NaCl or water H2O.
Note: Some element symbols exist as formulas since they only exist as diatomic elements. For
example, Hydrogen (H2), Oxygen (O2), Nitrogen (N2), Fluorine (F2), Chlorine (Cl2), Bromine (Br2)
and Iodine (I2). You need to memorize these!!
Atomic Structure
Each element has its own special type of atom, made up of a nucleus containing the protons and
neutrons, surrounded by electrons. The number of protons is unique to each type of atom and thus
can be used to identify the element.
•
•
•
Protons, found in the nucleus, have a positive charge (+) and a relative mass of 1.
Neutrons, also found in most nuclei, have no charge and a relative mass of 1.
Electrons, found surrounding the nucleus, have a negative charge (-) and negligible mass.
Because atoms are neutral, they have the same number of electrons as protons.
symbol
location
mass (relative)
charge (relative)
•
•
protons
p+
nucleus
1
+1
neutrons
n
nucleus
1
0
Most of the mass of an atom is concentrated in its nucleus.
Most of the volume of an atom is occupied by the electrons.
TASIS 9th GradeScience
electrons
eorbit
near 0
-1
Atomic Number (Symbol Z)
The atomic number (or proton number) of an element is the number of protons in the nucleus.
In atoms (which are neutral by definition) the number of electrons equals the number of protons.
Z = Number of protons (p+)
Mass Number (Symbol A)
The mass number (or nucleon number) of any atom is the sum of the number of protons and neutrons
in the nucleus.
A = Number of protons + number of neutrons (p+ + n)
Symbols
Atoms are given symbols based on mass and atomic numbers
For element X with Z protons and A protons & neutrons then the symbol would be
Name
Mass Number (Number of p + n)
Atomic Number (number of p)
Full atomic symbol
Hydrogen – H
1 p+ + 0 n = 1
1
1
1 H
A
Z X
Helium - He
2 p+ + 2n = 4
2
4
2 He
Atomic Structure of hydrogen
All hydrogen atoms have 1 proton and most hydrogen atoms have no neutrons. Surrounding the one
proton is one electron, moving extremely quickly. The diagram of atoms is simplified to show an
electron in a fixed position.
The Nucleus
The nucleus is made up of protons and neutrons and these are not involved in chemical reactions.
Isotopes
Atoms of the same element have the same number of protons. However the number of neutrons for
atoms of the same element may be different.
Example 1
All atoms of carbon have 6 protons, but:
• One type of carbon atom has 6 neutrons (mass number 12).
• Another less common type of carbon atom has 7 neutrons (mass number 13).
• A third (the least common) type of carbon atom has 8 neutrons (mass number 14).
The full name of the atom of an element shows the mass number of the atom
- e.g. atoms of carbon-12 all have a mass number of 12.
TASIS 9th GradeScience
Symbol
Proton and atomic
number (Z)
Number of
neutrons
Mass number
(A)
Carbon - 12
12
6 C
6
6
6 + 6 = 12
Carbon - 13
13
6 C
6
7
6 + 7 = 13
Carbon - 14
14
6 C
6
8
6 + 8 = 14
Full Name
Isotopes are atoms of the same element (same atomic number and number of protons) with
different mass numbers (numbers of neutrons).
Radioactive Isotopes
Some isotopes have a nucleus that is unstable. The balance between the numbers of protons and
neutrons is not stable. These atoms spontaneously throw particles out of their nucleus in order to
become more stable, in a process is known as decay. Unstable isotopes are said to be radioactive and
their decay releases radiation. Many elements have naturally occuring radioactive isotopes, or
radioisotopes. Others are made in laboratories or nuclear reactors.
Radioisotopes are used in a wide variety of tasks:
Tracers – The radioisotope is injected into the body. It decays and the radiation can be detected
and viewed on a screen where it shows up as bright spots. Dark patches indicate faulty cells or poorly
functioning organs. Tracers can also be placed into a liquid to detect if a pipe is leaking.
Cancer treatment – A weak beam of radiation will kill cancer cells more readily than healthy cells.
Carbon dating – All living things contain a known proportion of radioactive carbon-14 atoms. When an
organism dies it stops taking in new carbon atoms so the proportion of carbon-14 atoms slowly drops
as the atoms decay. By measuring the radiation from the remaining carbon-14 atoms the age of an
item can be determined.
Nuclear Power
Nuclear power is generated in reactors where energy from the fission of uranium-235 is harnessed.
When the unstable nucleus of a radioactive isotope splits to form smaller atoms, a large amount of
energy is produced when some of the mass of the isotope is converted into energy. This energy is
used to convert water to steam and then drive turbines to generate electricity.
Electron Arrangements
In atoms, the negatively charged electrons are:
•
repelled by other electrons.
•
attracted to the positive nucleus (protons are positive).
found in orbits (or shells) around the nucleus.(each shell or orbit has a maximum capacity of
electrons).
•
The maximum capacity of the electron shells around an atom are
•
1st shell
•
2nd shell
•
3rd shell etc
Maximum 2eMaximum 8e-
For the elements we deal with 8 e- (but larger shells can hold more)
No atom has more than 8 electrons in its outside (valence) shell.
TASIS 9th GradeScience
Symbols and simple electronic structures
Important Points
• Elements in the same column, called a group or family, have the same number of valence (outer
shell) electrons.
• The rows of elements are called periods.
• Atoms of elements in the same period (row) have the same number of "shells" of electrons.
Helium
Helium is an exception for its group which have 8 eight electrons in their outer shell. Although
helium atoms have only two electrons, the outer shell is completely full (1st shell only holds two
electrons) and thus helium is more like neon, argon and the other elements of Group 0 than the
elements of Group II.
Ions
Atoms become ions by gaining or losing electrons to attain the electronic structure of the nearest
group 0/18/noble gas/inert gas element.
eg. Sodium loses one electron to become a +1 ion.
Na: 2,8,1

Na+: 2,8
Oxygen gains two electrons to become a –2 ion.
O: 2,6

O2-: 2,8
TASIS 9th GradeScience