Download Atomic structure and periodic table

Atomic structure and periodic table
General objectives
 Learners should acquire an understanding of the structure
and characteristics of atoms
 Learners should be aware of the periodic table as a method of
classifying elements
The Atom
 An atom is the smallest particle of an element that can take part in a chemical reaction.
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P
N
e
 An atom consists of a nucleus containing nucleons namely protons and neutrons.
 Electrons move around the nucleus in pathways called orbits or shells
Proton
Neutron
Electron
Relative mass
Relative charge
Location
In an atom of an element, the number of protons always equals the number of electrons
Atomic number (proton number)
 The number of protons in the nucleus of an atom
 Atomic number is characteristic of each element
 No two elements can have the same atomic number
Mass number (nucleon number)
 The sum of protons and neutrons in an atom
Atomic number and mass number are often written together with the symbol for an atom
Mass number
Symbol
Atomic number
Symbol for an atom of carbon
atomic number =
Mass number =
Atomic structure and the periodic table
 A periodic table is a way of organising the known elements according to their similarities
 In the table, elements are arranged in order of their atomic number
Build up of electrons in shells
 The horizontal rows are called periods (numbered from 1-7)
 Each period represents the filling of one electron shell
 Shell number 1 can contain only 2 electrons, therefore period one has only 2 elements,
hydrogen and helium.
 Shell number 2 can contain 8 electrons.
 Period 2 has 8 elements going from Li to Ne
 Shell number 3 can contain 18 electrons. However, after only 8 electrons it appears to be full
and then shell number 4 begins the fill
 The number of the period is also the number of the shells in the atoms of that period.
Groups
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The vertical columns in the periodic table are called groups
The groups are numbered from l to Vll
The last group is numbered Vlll or 0
From group l to Vll, all the members of the group have the same number of outermost
electrons. This is the same as the group number
The noble gas configuration
 All members of group 0, except helium have 8 outermost electrons.
 This is a very stable arrangement called a stable octet, and these elements do not take part in
chemical reactions
 Atoms tend to attain noble state/structure by loosing, gaining or sharing of electrons
Valency Electrons
 When electrons take part in chemical reaction, it is only the outermost electrons that actually
take part
 The outermost electrons are called valency electrons (because they are involved in bonding)
 Valency is the combining power of an atom
Example
Lithium has a valency of 1. By loosing 1 electron it can achieve the noble gas electronic structure of
helium
Li
He
Fluorine also has a valency of 1. By gaining 1 electron it can have the electronic structure of Neon
F
Ne
The electronic structure of elements 1 to 20
 The significance of valency electrons is to take part in a chemical reaction.
 The table below summaries how atoms in each group achieve a
noble gas configuration
Group Outermost electrons
l
ll
lll
lV
V
Vl
Vll
To achieve noble gas configuration Valency
Isotopes
 Isotopes are atoms of the same elements having the same number of protons but different
number of neutrons
 Isotopes of the same element usually have identical chemical properties but they have
different physical properties
 Many elements consist of two or more isotopes
Hydrogen
There are three different types of hydrogen atom
H
H
H
Hydrogen
Deuterium
Tritium
In any sample of hydrogen, both deuterium and tritium are present
in extremely small quantities
Chlorine
There are two isotopes of chlorine
Cl
Cl
In any sample of chlorine gas about 25% of the atoms in the sample of the 37 Cl type while 75% are
of the 35 Cl type. This gives the average mass of 35.5
Carbon
There are two common isotopes of carbon
C
C
Because carbon 14 is radioactive, its presence can be easily detected. The amount of this isotope
in bone can be used to tell how old the bone is. The process is called carbon dating.