Science Answer the questions.[Che-5M]

1.  All protons are present inside the nucleus, which is situated at the center of the atom.
2. Electrons reside outside the nucleus and revolve around the nucleus in well-defined orbits.
3. The size of the nucleus is very small in comparison to the size of an atom. As per Rutherford’s calculations, the size of the nucleus is 105 times smaller than an atom.
4. As the mass of the electron is negligible in comparison to the mass of the proton, almost all the mass of the atom is concentrated in the nucleus.

Drawbacks of Rutherford’s nuclear model:

• It could not explain line spectra of atom.
• It could not explain stability of an atom because this model does not obey Maxwell law of electrodynamics. According to Maxwell electron should continuous emit radiation and thus and gradually lose energy, so its distance from nucleus should become shorter and finally it should fall into the nucleus. However this never happen

1. Atomic no. of the element = 18
2. Element Z is a non-metal
3. Element Z will not form any ion because its outermost shell is complete.
4. Outermost electronic shell of the atom of this element is completely filled with electrons.
5. Name of element Z is Argon Symbol is Ar.
6. Z belongs to the noble gases group or the zero group of the periodic table.

2. The maximum number of electrons present in a shell is given by the formula 2n², where W is the orbit number or energy level index, 1,2, 3…. Hence, the maximum number of electrons in different shells are as follows.

First orbit (K shell) will be = 2 × 1² = 2, second orbit (or L shell) will be = 2 × 2² = 8, third orbit (M shell) will be = 2 × 3² =18 and so on.

3. The maximum number of electrons that can be accomadated in the outermost orbit is 8.
4. Electrons are not accomodated in a given shell, unless the inner shells are filled. That is the shells are filled in a step-wise manner.
5. While revolving in discrete orbits the electrons do not radiate energy.

On the other hand, if the number of electrons in the outermost shell of the atom of an element is greater than 4, then the valency of that element is determined by subtracting the number of electrons in the outermost shell from 8.

The distribution of electrons in chlorine, sulphur, and magnesium atoms are 2, 8, 7; 2, 8, 6 and 2, 8, 2 respectively. Therefore, the number of electrons in the outer most shell of chlorine, sulphur, and magnesium atoms are 7, 6, and 2 respectively.

Thus,

The valency of chlorine = 8 −7 = 1

The valency of sulphur = 8 − 6 = 2

The valency of magnesium = 2

1. Atomic number: The atomic number of an element is the total number of protons present in the atom of that element. For example, nitrogen has 7 protons in its atom. Thus, the atomic number of nitrogen is 7.

2. Mass number: The mass number of an element is the sum of the number of protons and neutrons present in the atom of that element. For example, the atom of boron has 5 protons and 6 neutrons. So, the mass number of boron is 5 + 6 = 11.

3. Isotopes: They are atoms of the same element and have same atomic number but different mass number/ atomic mass. For example:

Carbon: 6C, 12 and 6C, 14

Two uses of isotopes are as follows:

1. An isotope of uranium is used as a fuel in nuclear reactors.
2. An isotope of cobalt is used in the treatment of cancer.

4. Isobars: They are atoms of different elements having same mass number but different atomic number. For example calcium, atomic number 20 and argon, atomic number 18 The number of electrons in these atoms is different, but the mass number of both these elements is 40. That is, the total number of neutrons is the same in the atoms of this pair of elements.

1. An atom consists of positively charged, dense and very small nucleus containing all the protons and neutrons. Almost all the mass of atom is concentrated in the nucleus.
2. The nucleus is surrounded by negatively charged electrons. The electrons are revolving at very high speed round the nucleus in fixed circular orbits.
3. The electrostatic attraction between the positively charged nucleus and negatively charged electrons keep the atom held together.
4. An atom is electrically neutral.
5. Most of the space in an atom is empty.

1. An atom is made up of three particles, namely electrons, protons and neutrons.
2. The protons and neutrons are located in the small nucleus at the center of atom.
3. Electrons revolve round the nucleus in fixed circular orbits.
4. Maximum no. of electrons for any given shell is fixed. Any shell cannot exceed that maximum value.
5. Each given shell is associated with fixed amount of energy.
6. There is no change in energy of electrons as long as they keep revolving in the same energy level, and the atom remains stable.

1. Figure 9 indicates the mass number.
2. Figure 4 indicates the atomic number.
3. No. of protons in atom X = 4
4. No. of neutrons in atom X = 9 - 4 = 5
5. No. of electrons in atom X = 4
6. Electrons in the outermost shell of an atom of element X = 2
7. X²⁺