Modern Atomic Model

History of Development of the Model of Atom

Table below shows the scientists that contribute to the development of the Model of Atom.
John Dalton
  • Five main points of Dalton’s Atomic Theory
  • All matter is composed of extremely small particles called atoms.
  • All atoms of a given element are identical, having the same size, mass, and chemical properties. Atoms of a specific element are different from those of any other element.
  • Atoms cannot be created, divided into smaller particles, or destroyed.
  • Different atoms combine in simple whole-number ratios to form compounds.
  • In a chemical reaction, atoms are separated, combined, or rearranged.
Weakness
  • Atoms consist of even smaller particles called electrons, protons and neutrons.
  • Atoms can be created and destroyed in the nuclear reactions such as nuclear fusion and nuclear fission..
  • Atoms of the same element can have different physical properties, for example, isotopes of hydrogen.
J.J. Thomson The electrons were positioned uniformly throughout the atom.
Ernest Rutherford
  • the atom is mostly empty space,
  • most of the atom’s mass concentrated in a tiny center, the nucleus and electrons being held in orbit around it by electrostatic attraction.
  • The nucleus was around 10-15 meters in diameter, in the centre of a 10-10 metre diameter atom.
  • Those alpha particles that had come into close proximity with the nucleus had been strongly deflected whereas the majority had passed at a relatively great distance to it.
Neils Bohr
  • Electrons in an atom of an element are not randomly distributed around the atomic nucleus.
  • Electrons move around the nucleus in fixed orbits.
  • Each orbit forms a circle and has a fixed distance from the nucleus.
James Chadwick
  • Chadwick discovered the presence of neutrons in the nucleus.
  • He concluded that the nucleus contains another tiny particle known as a neutron that has no charge.
  • The neutron mass is almost similar to the proton mass.
  • All nuclei contain protons and neutrons, except for the hydrogen which contains protons. only
 

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Modern Atomic Model

According to the modern atomic model,
  1. The central nucleus consists of protons and neutrons. It containing almost all the mass of the atom.
  2. the nucleus of an atom is very small compared to the size of the atom
  3. the electrons are orbiting outside the nucleus in the electron shells
  4. the electrons are moving in electron shells at a very high speed and we cannot determine the position of the electrons at a particular time
 

The Subatomic Particles of an Atom

  1. Atoms are made up of tiny particles called subatomic particles.
  2. An atom contains three types of subatomic particles:
    1. proton,
    2. neutron and
    3. electron,
  3. The proton and neutron form the nucleus at the centre of an atom. They are also called the nucleon of an atom.
  4. The electron moves around the nucleus at a very high speed.
  5. The nucleus is positively charged because of the presence of protons, which are positively charged. The neutrons are neutral.
  6. The symbols, charge and relative masses of proton, neutron and electron are as below.
 
Particle Symbol Relative charge Relative mass
Proton
p
+1
1
Neutron
n
0
1
Electron
e
-1
1/1840

The Charge of Particles

  1. A neutral atom contains the same number of electrons as the protons.
  2. The positive and negative charges of the protons and electrons respectively neutralise each other, for example, (+4) + (-4) = 0
  3. If the number of protons is greater than the number of electron, the particle is positively charge.
  4. If the number of protons is greater than the number of electron, the particle is positively charge.
Example:
Number of proton Number of electron
Charge
3
3
0
5
2
+3
9
10
-1
11
10
+3
16
18
-2
17
18
-1
20
18
+3

Proton Number and Nucleon Number

  1. Proton number = the number of protons
  2. Nucleon number = Number of protons + Number of neutrons

Proton Number

  1. The proton number (Z) represent the number of protons found in the nucleus of an atom.
  2. Proton number = the number of protons
  3. The proton number is also known as the atomic number.
  4. In an atom of neutral charge, the number of electrons also equals the atomic number.
  5. Hence, the proton number of an atom can also represent the number of electrons.

Nucleon Number

  1. The nucleon number (A), also called atomic mass number or mass number, is the number of protons plus the number of neutrons in an atomic nucleus. (Nucleon number = Number of protons + Number of neutrons)
  2. The nucleon number of an atom is about the same as the mass of the atom because the mass of an electron is very small and can be ignored.
Atom
Proton Number
Nucleon Number
Amount of Proton
Amount of electron
Amount of Neutron
Helium
2
4
2
2
2
Oxygen
8
16
8
8
8
Sodium
11
23
11
11
12
Chlorine
17
35
17
17
18
 
[Notes: In ion, the amount of protons IS NOT equal to the amount of electrons]

Isotopes

Isotopes are atoms of certain elements which have the same number of protons but different number of neutrons in the nucleus of the atoms.

It can also be defined as atoms of certain elements with the same proton numbers but with different nucleon numbers.

Properties of Isotope
Number of proton equal
Number of neutron difference
Chemical properties same
Physical properties difference

Example:
Element Name Symbol Proton Number Nucleon Number Number of proton Number of neutron
Hydrogen Hydrogen
11H
1
1
1
0
Deuterium
21H
1
12
1
1
Tritium
31H
1
23
1
2
Oxygen Oxygen-16
168O
8
16
8
8
Oxygen-17
178O
8
17
8
9
Oxygen-18
188O
8
18
8
10
Carbon Carbon-12
126C
6
12
6
6
Carbon-13
136C
6
13
6
7
Carbon-14
146C
6
14
6
8
Chlorine Chlorine-35
3517Cl
17
35
17
18
Chlorine-37
3717Cl
17
37
17
20
Sodium Sodium-23
2311Na
11
23
11
12
Sodium-24
2411Na
11
24
11
13