Particles in the Atom and Atomic Radius

Paper 1 & 2

Particles in the Atom and Atomic Radius

1: Atoms And Sub-Atomic Particles: 

Cambridge International Examinations Lawrie Ryan, Roger Norris

> All of the mass of an atom is concentrated (focused) in a very small region at the center of the atom known as the Nucleus.

> "Nucleons" are subatomic particles that are present in the Nucleus.

> Nucleons are of two types:

1. Protons

2. Neutrons

> There are particles outside of the Nucleus. Those particles are called Electrons.

> Electrons move around in regions of space called orbitals which are present within shells (energy levels) in the, empty, space around the nucleus.

2: Protons, Neutrons And Electrons:

Subatomic Particle
Relative Mass 
Relative Charge

>Note: the charge on a single electron is -1.602×10^-19 coulombs. The value of which is too small to be useful in discussing chemical properties.

3: Definitions Of Atomic Number Or Proton Number, Mass Number Or Nucleon Number:

a: Atomic Number or Proton Number: 

>It is the number of Protons in the Nucleus here represented by 'Z'

b: Mass Number or Nucleon Number:

> It is the total number of Protons and Neutrons here represented by A.

The numbers of Neutrons can be found through:

>Number of Neutrons=A-Z


(Cambridge International Examinations) Lawrie Ryan, Roger Norris 

4: Distribution Of Mass And Charge Within An Atom:

> The nucleus of the atom has the most relative Mass and hence is the densest part of the atom. 

> The electrons have negligible mass. 

> The protons in the atoms are positivity charged, the nucleons are neutral and the electrons are negatively charged subatomic particles.

5: Behavior Of Beams Of Protons, Neutrons And Electrons In Electric Fields:


> The electron which is negatively charged is attracted to the positive plate. 


>The neutron has no charge and is unaffected by a charged field. 


>The proton (positively charged) is attracted to the negative plate. It is deflected less than an electron as it has a larger mass.

6: How To Determine The Numbers Of Protons, Neutrons And Electrons Present In Both Atoms And Ions Given Atomic (Proton) Number, Mass (Nucleon) Number And Charge:

> A neutral atom has the same number of Protons (positively charged) in its nucleus and Electrons (negatively charged) outside of the nucleus. 

>When an atom gains or loses electrons, ions are formed, these ions are electrically charged.  

>For example:


(Cambridge International Examinations) Lawrie Ryan, Roger Norris

The chloride ion (Cl) has a single negative charge because there are 17 protons (+) and 18 electrons (–)

>Similarly another example:

 (Cambridge International Examinations) Lawrie Ryan, Roger Norris

The magnesium (Mg) ion has a charge of 2+ because it has 12 protons (+) but only 10 electrons (–).

7: Why Is There A Variation In The Atomic Radius And The Ionic Radius?

Across A Period:

Atomic Radius: 

> Atomic radius decreases as you move from left to right across a Period because the increased number of Protons helps create more positive charge attraction (nuclear charge) for electrons which are in the same shell with shielding of the same nature. 

Ionic Radius:

>There is an increasing number of Protons that exist from Na to Si but the same number of electrons. The effective nuclear attraction hence per electron therefore increases and ions get smaller. 

>Positive ions (Cations) are smaller compared to their atoms because it has one less shell of electrons and the ratio of Protons to Electrons has increased so there is a greater force on electrons that are left, holding them more closely. 

>There are increasing numbers of protons from Si to Cl but the number of electrons remains the same. The nuclear charge per electron therefore increases and ions get smaller. 

Down A Group:

Atomic Radius: 

>Atomic Radius increases down a group. This results in a larger atomic radius. This happens due to the number of energy levels which increase; hence there is a greater distance between the nucleus and the outermost orbital. 

>The electrons in lower energy levels create a shielding effect due to the electron repulsion that they have.

Ionic Radius:

>same as atomic radius

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