2.1 Atomic Structure and the periodic Table

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.

 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

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

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.

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.

• 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 TableUse 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 ?

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.

Term

Definition

Atomic number

The number of protons in the nucleus of an atom.

Mass number

The sum of the number of protons and neutrons in the nucleus of an atom.

Isotopes

Atoms of the same element that contains the same number of protons and electrons but a different number of neutrons. Therefore, they have the same atomic number but a different mass number.

Relative atomic mass (Ar)

Weighted average mass of one atom of an element, taking into account the abundances of all the isotopes of that element. It is measured as a ratio to ½ of the mass of an atom of carbon-12.

Calculating Relative Atomic Mass

Isotopes:

Atoms of the same element that contains the same number of protons and electrons but different number of neutrons.

Equation:

     (% of isotope 1 x mass of isotope 1) + (% of isotope 2 x mass of isotope 2)

Ar =   _______________________________________________________________

                                                                     100

Example:

The table shows information about the isotopes in a sample of Rubidium

Isotope

Number of protons

Number of neutrons

Percentage of isotope in sample

1

37

48

72%

2

37

50

28%

Use information from the table to calculate the relative atomic mass of this sample of Rubidium. Give your answer to one decimal place.

         ( 72   x   85 )   +   ( 28   x   87 )

    ____________________________     =   85.6

                         100

Relative atomic mass = 85.6