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Atoms &Molecules.

posted Jul 24, 2016, 2:25 PM by Upali Salpadoru   [ updated Aug 28, 2017, 12:51 AM ]






Fig.1 Mr.Hydrogen with a positive charge attracts Miss Chlorine with a negative charge.

An example of chemical bonding.


Hydrogen and chlorine are atoms. When charged they are called ions.

An atom would have a charge if the electron number differs from the Proton number.




An atom can be described as the smallest part of an element.

An atom has 3 kinds of subatomic particles.
They are shown in this chart.











Fig. 2  The structure of two atoms.
            Hydrogen and Helium.




                                           
 

The speed of an electron in an orbit.

. " A calculation shows that the electron is traveling at about 2,200 kilometers per second. It's fast enough to get it around the Earth in just over 18 seconds." Author:Carl Zorn, Detector Scientist

Chart no.1.  Sub atomic particles.
 Particle Electrical  charge Mass Where placed.
 Electron
     e-
 Negative Negligible. In orbits away from nucleus.
 Proton
     p+
 Positive 1 amu. Inside the nucleus.
 Neutron
   n 
 No charge. 1 amu. Inside the nucleus.

Drawing the structure of atoms.

The essential facts required for this are the Atomic no. and the Mass no. of the atoms.
Atomic No:-  Number of protons =Number of electrons.
Mass No:-   This gives the total number of Nucleons. Nucleons constitute of Protons + Neutrons. 


Chart. no. 2.   Proton number and electron distribution in orbits.

Atomic symbol

Atomic No.

P+

Electron distribution    e-


P+


K-shell

L- shell

M- shell

H

1

1

1



He.

2

2

2



Li

3

4

2

1


Be

4

4

2

2


B

5

5

2

3


C

6

6

2

4


N

7

7

2

5


O

8

8

2

6


F

9

9

2

7


Ne

10

10

2

8


Na

11

11

2

8

1

Mg

12

12

2

8

2

Al

13

13

2

8

3

Si

14

14

2

8

4

P

15

15

2

8

5

S

16

16

2

8

6

Cl

17

17

2

8

7

Ar

18

18

2

8

8

Valency electrons are in red.


Chart No.3.  The structure of atoms 1 to 18.


The chemical nature of an element depends mainly due to the number of valency electrons.

Formation of Compounds by the combination of atoms.

There are two main methods of combining atoms.
They are Ionic bonding and co-valent bonding.

Ionc bonding.
There are 2 methods for an atom to be an ion.
i. By donating electrons to get an octet (8)
ii. By accepting electrons to get the octet.
The electrons that can donate or accept are the valency electrons.

How Hydrogen gets Oxygen to form a molecule of water..

 Fig.3. Forming of water.

Hydrogen atom is not stable with only 1 valency electron. It donates this acquiring a positive charge.
Oxygen has 6 Valency e-. For it to be stable it must get 2 extra e-.
As one H atom can donate only 1e- it goes after two H atoms to get 2 e-. This is illustrated in the diagram.


How Carbon combines.- Co-valent bonding.
Carbon, with 4 valency electrons do not form ions. Carbon can share the valency electrons with other atoms.
 

Fig.4. Transfering a Lewis diagram to a line diagram.

The sharing of a pair of electrons can be shown by a short line. According to this method methane can be shown like this.



Chart No.4 How carbon combines.
The sharing pair of electrons is indicated by a line. So usually gets 4 lines.












This chart will help you to get the formulae of various molecules. Especially that of ionic compounds.

Chart No.5.   Negative and Positive ions.


Electro valency

Positive ions.  (Cations)

+1

H+  Li+  KNa+  Ag+   NH4+ Ammonium)

+2

Ca2+ Mg2+ Zn 2+  Cu2+    Pb2+ (Lead)  Fe2+ (Iron ll)   Ba2+

+3

Al 3+  Fe3+(Iron lll )

+4

Pb4+ (LeadIV)  Sn3+

 

Negative ions.  (Anions)

-1

F-  Cl - Br- I- OH- (Hydroxide) NO3-  HCO3     CH3COO-(Acetate)

-2

O— (Oxide)  S2-(Sulphide)

-3

PO43- (Phosphate).   N 3+(Nit ride)


 Getting Molecular Formulae

This is a simple thing if you know the atomic symbols and their electrovalencies.

The chart below shows how the charges cancel.

Co valent compounds.


Ionic compounds

Ions

Br-

S2-

PO43-

H+

H+  +  Br-

HBr

Hydrogen bromide

H+, H+    + S2-H2S

Hydrogen sulphide

H+,H+,H+ + PO43-

H3 PO4

Phosphoric acid.

Ca2+

Ca2++  Br- ,Br-

CaBr2

Calcium bromide

Ca2++ S2-

Ca S

Calcium sulphide

Ca2+,Ca2+,Ca2+ + PO43-,PO43-

Ca3(PO4)2

Calcium phosphate.

Al3+

Al3++  Br,Br,Br

AlBr3

Aluminium bromide

Al3+,Al3++ S2-S2-,S2-

Al2S3

Aluminium sulphide

Al3+PO43-

AlPO4

Aluminium phosphate






















 Covalent compounds.

Each bond line shows a pair of sharing electrons.  Dots in dinitrogen tetroxide shows valency electrons.


Q.1.0


     What could these be:-

  1. It has a mass but no charge. It is the smallest mass that can show all the properties of a particular elememt.

  2. It has negligible mass but very energetic.

  3. Smallest neutral particle showing the properties of a pure substance.

  4. A subatomic particle having a charge and a mass of 1amu.

  5. An atom or a molecule that has donated electrons.

  6. A cluster of atoms bonded together.

Marks 5x6 =30

Q.2.0

X,Y and Z are names asigned for three atoms. Their particulars are given here in 2 columns.

Atomic no.

Atomic Mass

Protons

Electrons

Neutrons

X  H

1

2

Y  C

6

14

Z  O

8

16

                                                                                           Marks  2x9=18

 1.1  Complete the chart by inserting their number of sub atomic particles.

 1.2   Which could form a positive ion?                                    Marks            2   

 1.3   Which could form a negative ion?                                                       2

 1.4   What is the formula of the molecule that can form from X and Y? (use given names)  

                                                                                            marks           3


 Q. 3.0

 Using Chart No.3 give the molecular formula of the following and state whether they wold be ionic or covalent.

1. H and F,  2.  Mg and Cl. 3. B and O.  4. Si and F.   5.  P and O.     

Marks  5 X 5 = 25.



Q. 4.0



Ions

Chlorides

Cl-

Hydroxides

OH-

Carbonate

CO32-

Nitrate

NO3-

Hydrogen   H+

HCl

HOH

H2CO3

HNO3

Sodium

Na+





Calcium

Ca++





Aluminium

Al +++





Ammonium

NH4 +





Silver Ag +






Marks 20.

 For answers click Answers Sc.

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