A105 General and Physical Chemistry Lesson 1: Mendeleev’s Table (Revision)
Atomic Structure, Number and Mass
Atoms
Electrons
Nucleus
Neutrons
Protons
Positively charged
Nuclear Charge = Total charge of all protons
Negatively charged
No. of electrons = No. of protons
Are distributed in electron shells surrounding the nucleus
Electronic configuration
For example:
A hydrogen atom has 1 electron.
Its electron configuration is 1.
An oxygen atom has 8 electrons.
Its electron configuration is 2,6.
For example:
Nuclear charge of hydrogen atom is +1.
Nuclear Charge of Oxygen is +8
1st electron shell can occupy up to 2 electrons
2nd electron shell can occupy up to 8 electrons
Attractive interaction between them
Electrons at the outermost occupied shell are called valence electrons
Valence electrons take part in the chemical reactions
Octet rule
The tendency of atoms to prefer to have eight electrons in the valence shell
A stable arrangement is attained when the atom is surrounded by eight electrons.
This octet can be made up by own electrons and some electrons which are shared.
Atoms will either lose, gain or share electrons to attain a filled outermost shell with 8 electrons.
Duplet Rule
Isotopes & Relative Atomic Mass
Atomic number = Proton number
Relative atomic mass
An average of the atomic masses of all the different isotopes in a sample
Contains the same number of atoms as the number of atoms in 12 g of carbon-12 ((12/())𝐶).
Periodic Table of Elements
Isotopes
Same atomic number
Different number of neutrons
Relative atomic mass of oxygen
= (16 x 99.76 %) + (17 x 0.04 %) + (18 x 0.20 %) = 16.00
A stable arrangement is attained when the atom is surrounded by two electrons.
+Valence Shells = +Atomic radius
+No. of protons = -Atomic radius
Electronegativity
The ability of an atom to draw shared electron towards itself
High electronegativity = Greater tendency to attract electron density
Electronegativity increases as moving across the period
Electronegativity decreases as moving down the group
Nuclei and Nuclear Reaction
Types of emission
Alpha
A nucleus breaks up into two chunks: a pair of protons bound to a pair of neutrons
Beta
Positron
All nuclei contain protons and neutrons except hydrogen ((1/1)𝐻)
The ratio of protons and neutrons determines the stability of a given nuclei
Some nuclei are unstable and undergo radioactive decay, i.e. emitting particles and/or electromagnetic radiation
Helium-4 nuclei are emitted
(226/88)Ra breaks down to (222/86)Rn + (𝟒/𝟐)He
One of the neutrons in the nucleus suddenly changes into a proton, causing an increase in the atomic number of an element
High speed electrons are emitted
(90/37)Rb breaks down to (90/38)Sr + (𝟎/−𝟏)e
Stabilizes the nucleus by removing a positive charge through the conversion of a proton into a neutron
A positron has the same mass as an electron but opposite charge
(50/25)Mn breaks down into (50/24)Cr + (𝟎/𝟏)e
Electron capture
An electron is captured by the nucleus
(179/74)W + (𝟎/−𝟏)e becomes (179/73)Ta