Structure Of The Atom

The atomic number of Magnesium is 12. Hence, the distribution of electrons in the different shells will be as follows:

Kth shell - 2, Lth shell -8 and Mth shell-2

Rutherford designed an experiment in which the fast-moving alpha particles were made to strike a very thin gold foil. Alpha particles were deflected by sub-atomic particles in the gold atoms.

The mass number of the element is 31. The element can be represented as X ₁₅³¹ where 31 is the mass number of the element. 15 is the atomic number of the element.

Dalton’s atomic theory was based on laws of chemical combination. His theory provided an explanation for the law of conservation of mass and the law of definite proportions.

According to Rutherford’s model of an atom, there is a positively charged center called nucleus. Almost all the mass of an atom resides in the nucleus. The electrons revolve around the nucleus in well-defined orbits which can be compared to the solar system.

All the atoms of an element have the same atomic number. Atomic number is defined by the number of protons present in an element. Also, a number of protons is the same as the number of electrons in an element. Therefore, statement 4 is correct. Mass of an atom is due to the protons and neutrons alone. Hence, mass number = number of protons +number of neutrons.

J J Thomson proposed the model of an atom in which the atom consisted of a positively charged sphere. The electrons are embedded in the sphere. The negative and the positive charges are equal in magnitude. Hence, the atom is electrically neutral.

Rutherford’s - scattering experiment concluded that mass and positive charge of an atom is concentrated in the nucleus. This is because very few alpha particles were deflected by 180 degrees from their path. Most of the alpha particles passed straight through the gold without getting deflected. This indicates that most of the space in an atom is empty.

The number of protons in an atom= Mass number- the number of neutrons. Therefore, an element has 13 protons and 13 electrons. To become a positive ion with 3 positive charges the atom will have to lose 3 electrons. Hence, the number of remaining electrons in the ion will be 10.

Magnesium atom has 12 protons and 12 electrons. The Mg²⁺ ion, after the loss of 2 electrons has 10 electrons remaining in the outermost shell which is shown in (d).

The atoms of same elements which have the same atomic numbers but different mass numbers are known as isotopes. Isotopes have same chemical properties but different physical properties.

In order to achieve an octet in the outermost shell, atoms would react. This is done by sharing, gaining or losing electrons. The number of electrons gained, lost or shared in the outermost shell determines the valency or combining capacity of the element. Both metals or non-metals can have valency 1 either by losing or gaining electrons. Metals usually form positive ions by losing an electron. Non-metals form negative ions by gaining an electron.

J.J. Thomson conducted an experiment in 1900. He identified in his experiment that the atom was not a simple, indivisible particle but contained at least one subatomic particle called electron.

In order to achieve an octet in the outermost shell, atoms would react. This is done by sharing, gaining or losing electrons. In this case, an atom has one electron in the outermost shell. The atom will lose 1 electron and hence, valency is 1.

The atomic number of aluminium is 13. The distribution of electrons in its shells will be as follows:

K-2, L-8 and M-3

According to Bohr’s model of an atom, orbits or shells of an atom are represented by K,L, M,N….. or the numbers 1,2,3, 4… respectively. The maximum number of electron in K shell is equal to 2n² where n is the orbit number. Hence, first orbit or K-shell will be = 2 × 12 = 2 electrons . Second orbit or L-shell will be = 2 × 22 = 8 electrons.

An atom as a whole is neutral. It is neither positive nor negative. The number of electrons are always equal to the number of protons. The mass of the atom is the sum of the masses of protons and neutrons.

J.J. Thomson identified in his experiment that the atom was not a simple, indivisible particle but contained at least one subatomic particle called electron. Rutherford put forward the model of an atom which had a positively charged centre called nucleus. It also had the electrons which revolved around the nucleus in well-defined orbits. According to Bohr’s model of an atom, only certain or special orbits of electrons were allowed in an atom. During revolving around these orbits, the electrons do not radiate energy.

It is possible for the atom of an element to have one electron, one proton and no neutron. The name of this element is Hydrogen.

Hydrogen can be represented as:

Discovery of protons and electrons supported the fact that atoms are divisible. Dalton’s atomic theory suggested that atoms were indestructible and indivisible. But the discovery of two fundamental particles- protons and electrons inside the atom, led to the failure of this aspect of Dalton’s atomic theory. It was necessary to find out the arrangement of electron and protons within the atom. There were a lot experiments done to prove the presence of these particles within the atom. J.J. Thomson proposed the model of atom as a positively charged sphere in which the electrons were embedded. Further experiment by Rutherford proved that the atom has a positively charged nucleus at the center in which the mass of an atom resides. The negatively charged electrons revolve around the nucleus.

Cl³⁵ and Cl³⁷ will have same valencies. These are two isotopes of Chlorine. The atoms of same elements which have the same atomic numbers but different mass numbers are known as isotopes. Isotopes have same chemical properties but different physical properties. Both Cl³⁵ and Cl³⁷ have same atomic number, hence the number of electron in both of these atoms will be same. This will result in similar valencies.

Rutherford selected gold foil during theα–ray scattering experiment. He needed a very thin foil of metal. It is possible to make a very thin sheet of gold metal due to its high malleability. He used a gold foil which was about 1000 atoms thick.

(a) In order to achieve an octet in the outermost shell, atoms react. This is done by gaining, sharing or losing electrons. The number of electrons gained, lost or shared so as to make the octet of electrons in the outermost shell, gives the combining capacity of the element which is called the valency of an atom. The valency of atom (a) is 0. The outermost shell of this atom is complete and has 8 electrons. Since, the outermost shell of this atom is complete, it does not have to lose or gain any electron. This results in zero valency.

(b) The valency of this atom is 1. The outermost shell of this atom has seven electrons. It needs one more electron to complete its octet. In this case, this atom will gain an electron and will change into negative ion with valency 1.

In order to achieve an octet in the outermost shell, atoms react. This is done by gaining, sharing or losing electrons. When an electron is removed from this atom, it will change into a positive ion. The charge on this ion will be + 1. This is because on losing one electron, the number of protons will still remain the same. As a result, there will be one extra proton in the nucleus which will result in changing the atom into a positive ion.

The Atomic number of chlorine atom is 17. This means it has 17 protons. In an atom, the number of protons is always equal to the number of electrons. Hence, the electronic configuration of chlorine atom is following:

K shell -2, L shell -8 and M shell-7. The L shell has eight electrons.

In order to achieve an octet or to achieve the nearest noble gas configuration, the atoms react. This is done by gaining, sharing or losing electrons. In this atom, 6 electrons are present in the outermost shell. In order to complete an octet, it has to gain 2 more electrons. This will result in the formation of a negative ion. The ion thus formed will have charge equal to -2.

The atomic number of element X is 5. Its mass number is 5 Protons + 6 Neutrons= 11. Since, there are 3 electrons in the outermost shell, its valency is 3. Atom X can lose 3 electrons to attain the nearest noble gas configuration.

The atomic number of atom Y is 8. Its mass number is 8 protons + 10 neutrons = 18. It has 6 electrons in the outermost shell. In order, to attain the nearest noble gas configuration, it has to gain 2 electrons. Therefore, the valency of atom Y is 2.

The atomic number of Z is 15. The mass number is equal to 15 protons + 16 neutrons = 31. The outermost shell has 5 electrons. To attain the nearest noble gas configuration, atom Z can either gain or lose electrons. It can gain 3 electrons and become negative ion with -3 charge or it can also lose 5 electrons and result in a positive ion with +5 charge. As a result, atom Z can show a valency of both 3 and 5.

This statement is incorrect. In an atom, the number of protons is always equal to the number of electrons. The electrons are present in the orbits or shells of an atom. The protons and neutrons are present in the nucleus of an atom. They are also called nucleons. The mass of an atom resides in its nucleus.

The atomic number of element X is 15. This means it has 15 protons. The mass number of element X is 31. Mass number = No. of protons + No. of neutrons = 31

∴ The number of neutrons = 31– number of protons = 31–15 = 16

Element X has 16 neutrons.

(a) Ernest Rutherford - (iii) Concept of nucleus

Rutherford’s alpha scattering experiment gave the model of the atom with a positively charged nucleus in the center.

(b) J.J.Thomson - (iv) Discovery of electrons

J.J. Thomson identified in his experiment that the atom was not a simple, indivisible particle but contained at least one subatomic particle called electron.

(c) Dalton - (i) Indivisibility of atoms

Dalton’s atomic theory suggested that atoms were indestructible and indivisible. But the discovery of two fundamental particles- protons and electrons inside the atom, led to the failure of this aspect of Dalton’s atomic theory.

(d) Neils Bohr - (ii) Stationary orbits

According to Bohr’s model of an atom, only certain or special orbits of electrons were allowed in an atom. During revolving around these orbits, the electrons do not radiate energy.

(e) James Chadwick - (vi) Neutron

James Chadwick discovered another subatomic particle known as Neutron which had no charge. It has a mass nearly equal to that of a proton. Neutrons are present in the nucleus of all atoms, except hydrogen.

(f) E. Goldstein - (vii) Canal rays

E. Goldstein discovered the presence of new radiations in a gas discharge and called them canal rays. These rays were positively charged radiations and led to the discovery of proton.

(g) Mosley - (v)Atomic number

Mosley developed the concept of Atomic number. Atomic number of is the number of protons in an atom.

Atoms of different elements which have the same mass number but have different atomic numbers are called isobars. For example, calcium, atomic number-20 and argon, atomic number 18 are isobars. Both these elements have same mass number which is equal to 40. Therefore, the total number of nucleons is the same in both the elements.

The mass number of chlorine is 35 and atomic number is 17.

Number of neutrons is equal to the mass number - number of protons.

It means it has 35-17= 18 Neutrons.

Similarly, in case of carbon atomic number is 6 and its mass number is 12. hence, the number of protons is 6 and the number of neutrons is also 6.

Bromine has atomic number 35 which means it has 35 protons and 46 neutrons.

Atomic number of helium is 2. Its electronic configuration is K-2. The maximum number of electrons K shell can accommodate is 2. It means that its duplet is complete. Helium atom does not have to gain or lose any electron since, its outermost shell is complete with 2 electrons. Hence, the valency of helium atom is zero.

(a) The Rutherford’s α-particle scattering experiment led to the discovery of the atomic nucleus.

Rutherford’s alpha scattering experiment gave the model of the atom with a positively charged nucleus in the center.

(b) Isotopes have same atomic number but different mass number.

For example, Hydrogen has three isotopes, protium (H¹₁), deuterium (H²₁ or D²₁) and tritium (H³₁ or T³₁). The chemical properties of isotopes are similar but their physical properties are different.

(c) Neon and chlorine have atomic numbers 10 and 17 respectively. Their valencies will be 0 and 1 respectively.

Neon has 8 electrons in its outermost shell. Its octet is complete. It does not have to gain or lose any electron. Hence, its valency is zero.

In case of chlorine, it has 7 electrons in its outermost shell. It can gain an electron to complete its octet. Hence, the chlorine atom gains an electron to become an ion with -1 charge. Therefore, the valency of chlorine atom is 1.

(d) The electronic configuration of silicon is 2, 8, 4 and that of sulphur is — 2, 8, 6.

The atomic number of Silicon is 14 and the atomic number of sulphur is 16.

Atomic number of this element is 2. Its electronic configuration is K-2. The maximum number of electrons K shell can accommodate is 2. It means that its duplet is complete. It does not have to gain or lose any electron since, its outermost shell is complete with 2 electrons. Hence, the valency of this element is zero.

Atomic number of helium is 2. Its electronic configuration is K-2. The maximum number of electrons K shell can accommodate is 2. It means that its duplet is complete. Helium atom does not have to gain or lose any electron since, its outermost shell is complete with 2 electrons. Hence, the valency of helium atom is zero.

Both Argon and Neon have 8 electrons in their outermost or their valence shells. It means their octet is complete. As their octets are complete, these elements do not have a tendency to gain or lose any electron. They do not tend to combine with other elements. This results in having a zero valency.

(i) Volume of the sphere =

Let R be the radius of the atom and r be that of the nucleus.

Volumes of nucleus = Nucleus of an atom is assumed to be a sphere.

Ratio of the size of atom to that of nucleus =

⇒ R = 10⁵ r

Hence, the radius of earth is equal to 10⁵ the radius of the hydrogen atom.

(ii) If the atom is represented by the planet earth (R = 6.4×106m) then the radius of the nucleus would be equal to,

‘Re’= Radius of earth and ‘Rn’=radius of nucleus

Therefore, if the atom is represented by planet Earth, the size of its nucleus will be 64m.

Rutherford designed an experiment to know about the arrangement of electrons within an atom. He took a gold foil on which the fast-moving alpha particles were made to fall. He took a very thin layer of gold foil which was about 1000 atoms thick.

Rutherford concluded the following from the α-particle scattering experiment:

(i) Most of the alpha particles passed through the gold foil without getting deflected. This concluded that most of the space inside the atom is empty. This observation was totally unexpected.

(ii) Some of the α-particles were deflected by the foil by small angles. The number of particles which were deflected was very small. This shows that the positive charge of the atom occupies very little space.

(iii) Surprisingly one out of every 12000 particles appeared to rebound meaning they were deflected by 180 degrees. This concluded that all the positive charge and the mass of the atom were concentrated in a very small volume within the atom.

From the data, he also calculated that the radius of the nucleus is about 105 times less than the radius of the atom. Based on this data Rutherford put forward the model of an atom which had a positively charged center called nucleus. It also had the electrons which revolved around the nucleus in well defined orbits.

Rutherford put forward the model of an atom which had a positively charged center called nucleus. It also had the electrons which revolved around the nucleus in well-defined orbits. He also proposed that the size of the nucleus is very small as compared to the size of the atom and nearly all the mass of an atom is centred in the nucleus. From the data, he also calculated that the radius of the nucleus is about 105 times less than the radius of the atom.

On the other hand, Thomson proposed the model in which an atom consists of a positively charged sphere and the electrons are embedded in it. Both the negative and the positive charges are equal in magnitude. So, the atom as a whole is electrically neutral. Although Thomson’s model explained that atoms are electrically neutral, the results of experiments carried out by other scientists could not be explained by this model.

Rutherford’s model of an atom was able to explain a lot about the structure of atom. Although it has a few drawbacks as well. The major drawback were it could not explain the stability of the atom. According to him, the electrons travelled in spiral orbits around the nucleus. But, if electrons followed the spiral path, then the electrons would loose the energy continuously and then, slowly they would fall in the nucleus. But, this does not happen, because atom is all stable and the electrons keep revolving in the circular orbits around the nucleus.

To overcome the drawbacks of the Rutherford’s model , Neil Bohr came forward with his postulates about the model of an atom, which are given below:

(i) The electrons revolve around in circular definite orbits.

(ii) Each orbit corresponds to a fixed energy.

(iii) These orbits are characterized by energy levels which are represented by integers.

(iv) These orbits are represented by the letters: K,L,M,N,… or the numbers, n =1,2,3,4,….

(v) The electrons can move from the lower energy orbits to the higher energy orbits by absorbing the energies and an electron can jump from higher energy levels to the lower energy level by losing energy.

Sodium atom has atomic number 11. Its electronic configuration is 2, 8, 1. It has one electron in its outermost shell. To achieve the noble gas configuration, sodium atom has to lose one electron. After losing one electron sodium atom changes into sodium ion with +1 charge. The electronic configuration of the sodium ion will be 2 and 8 which is the noble gas configuration of Neon, Neon being a noble gas. The atomic number of an element is equal to the number of protons in its atom. Since, sodium atom and sodium ion contain the same number of protons, therefore, the atomic number of both is 11.

Percentage of α-particles deflected by more than 50 degrees =1% of α-particles.

Percentage of α-particles deflected less than 50 degrees =100–1 = 99%

Number of α-particles bombarded = 1 mole = 6.022×1023 particles

Number of particles that deflected at an angle less than 50⁰

= 5.96 × 10²³

Hence, the number of particles that deflected at an angle less that 50°is equal to 5.9610²³