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Chemical bond

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José Vicente Pacheco

on 3 August 2016

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Transcript of Chemical bond

Chemical Bond
Why can atoms bond
to each other?

Understanding chemical bond
is important because...
Properties of every substance (melting point, boiling point, density, colour, conductivity,...) are determined by the kind of bond in them.
Elements can't bond each other in any proportion (e. g.: Salt is NaCl; not Na2Cl or NaCl2 or anything else). Theories of chemical bond explain why two specific elements bond in a specific proportion.
Properties of matter and bond
Molecules and crystals
When atoms bond to each other, two kinds of structures can be produced:
MOLECULES: they are formed by a small group of atoms.
CRYSTALS: they are formed by a large number of particles in a regular three-dimensional structure.
Three kinds of chemical bond
Not all atoms can bond with others:
Atoms of elements in group 18 (noble gasses) never exist in molecules with other atoms.
Noble gasses'
electron arrangement
Looking for stability
Most atoms bond others because they are more stable when they are bonded than isolated.
A hydrogen molecule has less energy than two isolated hydrogen atoms

Some lonely atoms
s2 p6: A stable arrangement
Helium has only two electrons in its first shell that is full. It hasn't room for more electrons.
The other noble gasses have 8 valence electrons (s2 p6).
s2 p6 is a specially stable arrangement.
Why do never noble gasses form compounds?
What makes them special?
Atoms of all other elements (Groups 1 to 17) join together with other atoms.
Many of them bond with others because they tend to get a noble gas arrangement.
This tendency is called THE OCTET RULE.
The "magic" of number 8
How to get an octet.
Many atoms tend to gain one or more electrons in order to get an octet (s2p6)
They become negative ions (anions).
Others tend to lose one or more valence electrons.
They become positive ions (cations)
Forming anions
A chloride atom has 17 electrons:
Cl: 1s2 2s2 2p6 3s2 3p5
If it gains 1 more electron in its valence shell, it gets a gas noble arrangement:
Cl + 1e- -> Cl-
Cl-: 1s2 2s2 2p6 3s2 3p6.
Forming cations

A sodium atom has 11 electrons:
Na: 1s2 2s2 2p6 3s1
If it loses one electron, it gets the gas noble arrangement (s2p6) in its second shell.
Na - 1 e- -> Na+
Na+ -> 1s2 2s2 2p6
Elements that form anions
All of them have 7 valence electrons (s2p5)
They get the gas noble arrangement if they gain one extra electron.
They tend to form anion with charge -1
Group 17
F, Cl, Br, I and At
Elements that form anions
Group 16
O, S, Se, Te and Po
All of them have 6 valence electrons (s2p4)
They get the gas noble arrangement if they gain two extra electrons.
They tend to form anion with charge -2
Elements that form anions
Group 15
N, P, As, Sb and Bi
All of them have 5 valence electrons (s2p3)
They get the gas noble arrangement if they gain three extra electrons.
They tend to form anion with charge -3
Elements that form cations
Group 1
Li, Na, K, Rb, Cs and Fr
All of them have an only valence electron (s1)
They get the gas noble electron arrangement if they lose their only valence electron.
They tend to form cations with charge +1.
Elements that form anions
Group 2
Be, Mg, Ca, Sr, Ba and Ra
All of them have two valence electrons (s2)
They get the gas noble electron arrangement if they lose their two valence electron.
They tend to form cations with charge +2.
Elements that for anions
Metals of Group 13
Al, Ga, In and Tl
All of them have three valence electrons (s2p1)
They get the gas noble electron arrangement if they lose their three valence electron.They tend to for anions with charge +3.
Ionic bond
Covalent bond
Metallic bond

The ionic bond occurs due to the attractive force between positive an negative ions.
In the ionic bond:
one of the elements is a metal (that tends to lose electrons).
the other one is a nonmetal (that tends to gain electrons)
Two steps:
Transferring one or more electrons. As a result of this, atoms become ions.
Ions with opposite sign attract each other.
A simple example: formation of ions in sodium chloride (common salt)
Ionic crystals
In ionic compounds every cation attracts all anions around it and vice versa. Ionic bond doesn't affect to an only pair of opposite ions.
Ionic compounds are always CRYSTALS. In them, every cation has several anions around it and vice versa.
Properties of ionic compounds
They are solid at room temperature, their melting and boiling points are high.
They are highly soluble in water
They are hard, in other words, they are difficult to scratch
They are not electrical conductors when they are solid.
Ionic solutions and melted ionic compounds are really good electrical conductors.
Covalent bond occurs when two atoms share one or more pair of electrons.
Nonmetal atoms bond to each other by covalent bond.
According to the number of shared pairs of electrons, covalent bond can be single, double or triple.
H2: Hydrogen (1s1) need one additional electron to get 1s2 (like Helium). Two atoms of H share 1 pair of electrons.
Two atoms of Cl (2s2 2p5) also bond with each other by sharing 1 pair of electrons.
In the O2 molecule two pairs of electrons are shared (double covalent bond), due to the electronic arrangement of an atom of oxygen (2s2 2p4)
COVALENT BOND. More examples
In a nitrogen molecule (N: 1s2 2s2 2p3) three pairs of electrons are shared. it is a triple covalent bond.
In water (H2O) the atom of oxygen shares one pair of electron with each atom of hydrogen. There are two single covalent bonds.
In a carbon dioxide molecule the atom of carbon (1s2 2s2 2p2) shares two pairs of electrons with each atom of oxygen (1s2 2s2 2p4)
Covalent substances
There are two kinds of covalent substances:
Molecular substances
Covalent crystals
Water and oxygen are molecular substances
Quartz and diamond are covalent crystals
Molecular substances
They are made of molecules with a few atoms in every of them.
These substances have low melting and boiling points, so most of them are liquids or gasses.
Molecules are not strongly bonded to each other.
Molecular substances are not electric conductors.
Electrons belong to a specific pair of atoms, so they can't move.
They have lots of atoms that form a lattice.
Every atom joins to a specific number of nearby atoms by covalent bonds.
These substances are solids whose melting points are very high.
Atoms are very strongly bonded.
They can't conduct electricity:
Every electron belongs to a specific pair of atoms.
In metallic bond, positive ions of metal share a "cloud" of electrons.
The electrons of the cloud, that also are named conductivity electrons, belong to all atoms in a piece of metal.
The positive ions are arranged in a three-dimensional lattice.
EXAMPLE 1: A sodium crystal.
Sodium's electron arrangement (1s2 2s2 2p6 3s1)
In a crystal, there are:
A lattice of ions Na+ (1s2 2s2 2p6).
A cloud with every valence electron of every sodium atom. (As many electrons as ions there are)
They are solids at room temperature (the only exception is mercury which is a liquid).
They conduct easily heat and electricity;
That's because there are many free electrons.
They are ductile and malleable, in other words, threads and foils can be made of them.
They have metallic shine
EXAMPLW 2: A Magnesium crystal.
Magnesium's electron arrangement
1s2 2s2 2p6 3s2
In a crystal, there are:
A lattice of ions Mg2+ (1s2 2s2 2p6).
A cloud with every valence electron of every magnesium atom. (twice as many electrons as ions)
One more sample of metallic bonding
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