Cambridge IGCSE CHEMISTRY 0620 LESSON 1 EXPLANATION




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Protons, Neutrons and Electrons  
 
1- State the relative charges and approximate relative masses of protons, neutrons and electrons
If you tried to cut anything , what do you think the smallest part that you will get if you just keep cutting.
The Atom. The name comes from   (a - tom_ meaning can not be divided.

 All of the substances around us or even us goes back to the same building units, (The atoms).

 Please meet your first atom

Chemists usually draw atoms as circles and write its name inside or a symbol abbreviating its name
Let's concentrate now on the atoms and their structure.

 Atoms are very very tiny, they cannot be seen by the naked eye.

 A lot of theories were done about the structure of the atom and how the particles inside it are arranged until we reached to the well-known structure that will explain now.

 The Atom is electrically neutral, as a whole it has no charge.


At the centre  it contains a nucleus which is very very very tiny when compared to the whole size of the atom.



Unlike the atom the nucleus is not electrically neutral, but due to the its content the nucleus is positively charged.


The space between the atom borders (if it has borders) and its nucleus is filled with electrons which are moving around the nucleus.
 if we go to check the nucleus and its content and why it is positively charged we will find it contains two types of particles
which we can call Subatomic particles

The Protons which are positively charged
and The neutral Neutrons 


so the sum of the two particles Positive + Neutral = Positive 

 This gives us a positive charge and that is why the total charge of the nucleus is positive
If we sum the positively charged nucleus with the negatively charged electrons 
around  the nucleus, we will have an explanation of why the whole atom is neutral


or in another imaginary diagram


Remember 


Next 


Subatomic particles Masses

if we compare the mass of the atom and the particles inside it, we can relatively give a mass for them 
like if the proton has a mass of 1 unit
the neutron will also have a mass of 1 unit




The electrons, on the contrary(which  as we agreed have a negative charge) have a very small  mass which is about 1/2000 of the mass of the protons and the neutrons.


 This is a negligible mass compared to the total mass of the atom. that is why we can say now that the total mass of the atom is concentrated in its nucleus. 



The following comparative chart, which is also for the sake of understanding and not very accurate will show you how large is the mass of the electrons compared to other constituents of the atom.


The following pictures shows a comparison of the atom components and there relative masses , and charges.






Answer the following fast quizzes to check your understanding





And now check this fine quiz






so to summarize
  • Elements are made of tiny particles of matter called atoms.
  • Each atom is made of subatomic particles called protons, neutrons and electrons.
  • Their size is so tiny that we can’t really compare their masses in conventional units such as kilograms or grams, so a unit called the relative atomic mass is used.
  • One relative atomic mass unit is equal to 1/12 the mass of a carbon-12 atom.
  • All other elements are measured relative to the mass of a carbon-12 atom and since these are ratios, the relative atomic mass has no units.
  • Hydrogen for example has a relative atomic mass of 1, meaning that 12 atoms of hydrogen would have exactly the same mass as 1 atom of carbon.(Read more about this if you wish)
Defining Proton Number Define proton number (atomic number) as the number of protons in the nucleus of an atom
  • The atomic number (or proton number) is the number of protons in the nucleus of an atom. The symbol for this number is Z.


  • It is also the number of electrons present in an atom 
  • Why ? remember the following :


Defining Nucleon Number
  • Nucleon number (or mass number) is the total number of protons and neutrons in the nucleus of an atom.
  • The symbol for this number is A.

Since Both Protons and Neutrons lies inside the Nucleus we can call them "Nucleons" 




Don't feel lost 
if you feel lost make sure you solve the following before you continue

















  • The nucleon number minus the proton number gives you the number of neutrons of an atom.
  • Note that protons and neutrons can collectively be called nucleons.
  • The atomic number and mass number for every element is on the Periodic Table.
Electrons (symbol e–)
  • These subatomic particles move very fast around the nucleus.
  • They move in orbital paths called shells.
  • The mass of the electron is negligible, hence the mass of an atom is contained within the nucleus where the neutron and proton reside.
 You can find Both the atomic number and the mass number are given on the Periodic Table 

 Check any element and find its symbol , the names are written in a special abbreviated way
and there are two numbers around it

 The Mass Number A and the Atomic Number Z
you can find the numbers switching places in different Periodic tables , although there is an agreement about all the details of the periodic table. ,but you may find it drawn in different ways.

 So how not to get confused about them 

 Remember always the bigger number is the mass number
The smaller one is the Atomic number

 why is that ?

 



The Basis of the Periodic Table

 

Use proton number and the simple structure of atoms to explain the basis of the Periodic Table with special reference to the elements of proton number 1 to 20
  • Elements are arranged on the Periodic table in order of increasing atomic number where each element has one proton more than the element preceding it. ( The atomic Number is the like ID of the element : it never changes , for Carbon it is always 6 , and for Hydrogen always 1 , it will never ever change )

  • Hydrogen has 1 proton, helium has 2 protons, lithium has 3 etc.
  • The table is arranged in vertical columns called Groups numbered I – VIII and in rows called Periods.
  • Elements in the same group have the same amount of electrons in their outer shell, which gives them similar chemical properties.
The proton number is unique to each element and could be considered as an element’s identity card.
Electrons Number can change during chemical processes but the proton number never change.
Defining Isotopes
  • Define isotopes as atoms of the same element which have the same proton number but a different nucleon number
As we agreed the number of protons in the atom is fixed , while the number of electrons can be changed during chemical reactions

 What about Neutrons number ?
The number of neutrons in a certain element can be different from atom to atom
those atoms for the same element but with different neutrons number are called isotopes 
  • Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.
Let's have a look at the following isotopes


All the previous are atoms of the same element , they all have a fixed number of protons 
(identity of element as agreed ) but the number of neutrons differ
The first one has no neutrons at all
The second has 1 neutron 
The third has 2 neutrons

That's why we can call them isotopes

  • The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number.
  • So C-14 is the isotope of carbon which contains 6 protons, 6 electrons and 14 – 6 = 8 neutrons.
How many isotopes are available for the element carbon ?



Types of Isotope
  • State the two types of isotopes as being radioactive and non-radioactive
  • Isotopes can be divided into two categories: radioactive and non-radioactive.
  • Radioactive isotopes (radioisotopes) are unstable due to the imbalance of neutrons and protons, which causes the nucleus to decay over time through nuclear fission and emit radiation. Examples of radioisotopes include tritium and carbon-14.
  • Radioactive isotopes have numerous medical and industrial uses.
  • Non-radioactive isotopes are stable atoms which really only differ in their mass.

Uses of Radioactive Isotopes

  • State one medical and one industrial use of radioactive isotopes
Medical uses
  • Radiation is extremely harmful and kills cells so isotopes are used to treat cancer. The isotope cobalt-60 is frequently used for this purpose.
  • Medical tracers as certain parts of the body absorb isotopes and others do not. In this way an isotope can be injected into the blood and its path through the body traced with a radiation-detecting camera, revealing the flow of blood through bodily systems.

  • Medical instruments and materials are routinely sterilized by exposure to radiation, which kills any bacteria present.
Industrial uses
  • Radioactive dating uses the carbon-14 isotope to date carbon-containing materials such as organic matter, rocks and other artefacts. The half-life of C-14 is 5730 years and so this technique is often used to date very old historical objects.

  • Similar to the medical use, radioactive tracers are deployed for detecting leaks in gas or oil pipes.

  • The radioactive isotope uranium-235 is used as fuel for controlled fission reactions in nuclear power plants.

Why Isotopes Share Properties
  • Understand that isotopes have the same properties because they have the same number of electrons in the outer shell
  • Isotopes of the same element display the same chemical characteristics.
  • This is because they have the same number of electrons in their outer shells and this is what determines an atoms chemistry.
  • The difference between isotopes is the neutrons which are neutral particles within the nucleus and add mass only.
Radioactive decay is a random process which occurs inside the nucleus and is independent of temperature, pressure, pH etc.
It is a nuclear process and is not considered a chemical reaction.
Understanding Review
Have you Been Paying attention ?
  • What is the charge of Proton , Electron , Neutron?
  • What are the relative masses of each of the previous subatomic particles?
  • where are these particles located in the atom?
  • A and B are two atoms of the same element , is it possible for them to have?
a- different Proton Number
b- different Mass Number
c- different Number of electrons if they are neutral
  • What is the Size of Nucleus compared to the atom size?
  • If the two atoms A and B have the same number of protons but different number of neutrons what do we call these 2 atoms ?
  • How can we calculate the number of neutrons of the atom from the periodic table
  • according to what basis atoms are placed in the periodic table
  • if one isotope has a proton number which when checked a few days later was found to have changed , is this isotope Radioactive or not?
  • Using Isotopes to treat cancer is a medical use or a side effect of their radioactivity?
  • If I can know where the radioactive isotope comes out from a broken gas pipe , can this be considered as an industrial use for the isotopes?

Extended Only

Why Isotopes Share Properties

  • Isotopes of the same element display the same chemical characteristics
  • This is because they have the same number of electrons in their outer shells and this is what determines an atom’s chemistry
  • The difference between isotopes is the neutrons which are neutral particles within the nucleus and add mass only


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