Articles by "atoms"
Tampilkan postingan dengan label atoms. Tampilkan semua postingan

di 0
The atom comprises protons, neutrons and electrons. The protons and neutrons are enclosed in a central nucleus that is at the center of the atom, together they are collectively known as nucleons. All of the mass of an atom is said to be in the nucleus because electrons weigh little.

The table below shows some comparisons between protons, neutrons and electrons:


Sub-atomic particle
Relative Mass
Relative Charge
Proton
1
+1
Neutron
1
0
Electron
1/1836
-1


Calculations:


Finding protons, neutrons or mass number:-
Given the values of two of these variables one can transpose the following formula to find the third variable.

     Mass # (A) = # of protons (z) + # of neutrons
Note: The names may vary for e.g. number of protons can sometimes be called atomic number and mass number can be called nucleon number.

For example calculate the number of neutrons in this atom:


If you look at the diagram closely you will see that they have given you two value the mass number and the number of protons (in this example the term atomic number is used instead). Right away one can see that all is needed is to transpose the previous formula to find number of neutrons.

    Mass number = number of protons or atomic number + number of neutrons
                                                    
             19          =         9     +      N


When done transposing this formula for N you should get:

                      19 – 9 = 9 – 9 + N
                      19 – 9 = 0 + N
                            10 = N
                         =>N = 10

Finding the number of electrons:-


It is relatively easy to find the number of electrons because no calculation is needed. Atoms are neutral the positive charges from the protons will balance the negative charges of the electrons. Therefore the electrons would have to be the same number as that of the protons.
     Number of protons = Number of electrons.

Isotopes:
These are atoms of the same element with the same atomic number but different mass numbers. This is due to a difference in the amount of neutrons present in each atom; the protons however do not change. For example there are three isotopes of carbon namely: Carbon-12, Carbon-13 and carbon-14(the numbers represent mass) their protons remain the same being 6 but the neutrons vary being 6, 7 and 8 respectively.

Relative Atomic Mass:
The relative atomic mass of an element can be defined as the ratio of the mass of an atom of the element to one twelfth the mass of carbon-12.

The relative atomic mass can be found using the following formula:

     R.A.M = Mass of one atom of the element   × 12
                     Mass of one atom of Carbon-12
Continue reading

di 0
Over centuries many theories were developed but today only a few remain valid. One of the most “talked-about” theories was the one made by John Dalton, a physicist, meteorologist and a chemist. One of the main reasons why his work is so popular today is because some of his discoveries have helped in the development of the modern atomic theory. There are however cases were his theory is a little different from modern theories.

John Daltons Theory Stated that:
  1. All matter is made up of atoms.
  2. Atoms are indestructible and indivisible.
  3. Atoms of the same element have identical properties and mass.
  4. A chemical reaction is a rearrangement of atoms.
  5. A compound is formed by a combination of two or more different kinds of atoms.

    Cases where Daltons Theory is different:

    • John Dalton’s theory stated that atoms of the same element have identical masses but modern theories have proved that atoms of the same element can have different masses, these atoms are called isotopes however they do have the same chemical properties.
    • In modern research it was discovered that atoms can be destroyed by nuclear reactions but not by chemical reactions.
    The following table summarizes the changes in theories over time:

    Name Discovery/Theory
    Democritus, 460 BC    Came up with the idea of atoms
    John Dalton, 1808 After the idea of atoms was dismissed for over 2000 years, he reintroduced the idea of atoms and through many experiments he proved the existence of atoms.
    Joseph .J.Thompson, 1898 Stated that positively charged substances were distributed uniformly over the surface of the atom containing electrons embedded within them.
    Ernest Rutherford, 1910 From J.Thompson’s model and through his own experiments he was able to determine the true model of the atom. Instead of the positively charged particles being distributed over the atom they were gathered together in the nucleus.
    Niels Bohr, 1913
    He came up with these rules:
    1. Electrons can only orbit a fixed distance from the nucleus.
    2. Atoms lose energy when electrons go from higher energy to lower energy orbits and the opposite happens from lower energy to higher energy orbits.
    Wolfgang Pauli, 1924 Stated that electrons spin while orbiting the nucleus. He also came up with his own principle known as the Pauli Exclusion Principle which stated that no two electrons of a given atom can have the same set of quantum numbers.
    Continue reading
    Langganan: Postingan (Atom)