Structure of Atom

1) 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

2) 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

3) 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:

3. proton,

4. neutron and

5. electron,

6. The proton and neutron form the nucleus at the centre of an atom. They are also called the nucleon of an atom.

7. The electron moves around the nucleus at a very high speed.

8. The nucleus is positively charged because of the presence of protons, which are positively charged. The neutrons are neutral.

9. The symbols, charge and relative masses of proton, neutron and electron are as below.

ParticleSymbolRelative chargeRelative mass

Proton

p

+1

1

Neutron

n

0

1

Electron

e

-1

1/1840

4) 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 protonNumber of electron

Charge

3

3

0

5

2

+3

9

10

-1

11

10

+3

16

18

-2

17

18

-1

20

18

+3

5) 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.

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 protonequal

Number of neutrondifference

Chemical propertiessame

Physical propertiesdifference

Electron Arrangement In Atom

1. We have learnt that electrons occupy orbits with definite energy level of an atom, as suggested by Neils Bohr.

2. These orbits with definite energy level are known as the shell.

3. Every single shell is capable of holding up to certain amount of electrons.

4. The first shell can hold up to two electrons. This is called a duplet.

5. The second shell can hold up to eight electrons. This is called an octet.

6. The third shell can hold up to eighteen electrons.

1. However, with the third shell, when eight electrons are present, extra stability is gained. The additional electrons go into the fourth shell before the third shell is completely filled.

2. The way in which the electrons are distributed in the shells of an atom is called the electron arrangement of the atom

3. The examples below show the electron arrangement of some elements:

Atom

Notes

Electrons Arrangement

1. Lithium has 3 protons and 3 neutrons and three electrons as well.

2. All the three electrons are arrange as follows:

3. Two electrons are filled in the first shell.

4. One electron is filled in the second shell.

5. The electron arrangement of carbon is 2.1

2.1

1. Chlorine has 17 protons and 18 neutrons and 17 electrons.

2. All the three electrons are arrange as follows:

3. Two electrons are filled in the first shell.

4. Eight electrons are filled in the second shell.

5. Seven electrons are filled in the third shell.

6. The electron arrangement of chlorine is 2.8.7.

2.8.7

1. Calcium has 20 protons and 20 neutrons and 20 electrons.

2. All the three electrons are arrange as follows:

3. Two electrons are filled in the first shell.

4. Eight electrons are filled in the second shell.

5. Eight electrons are filled in the third shell.

6. Two electrons are filled in the forth shell.

7. The electron arrangement of carbon is 2.8.8.2.

Valence Electrons

Valence electrons are the electrons in the outermost shell.

1. The electrons in the outermost shell of an atom are called valence electrons.

2. The valence electrons have great significance in determining the chemical properties of an atom.

3. Elements with the same number of valence electron have the same chemical properties.

Example: Given that a sodium atom has 11 protons 12 neutrons. Find the number of valence electron in a sodium atom. Answer:

For an atom,

Number of electrons = number of protons = 11

Electron arrangement of sodium = 2.8.1

Therefore, sodium has 1 valence electron.