Teachoo Questions

Class 9
Chapter 4 Class 9 - Structure of Atom

## Neutrons are found in the nucleus of all atoms except ____

1. Hydrogen
2. Chlorine
3. Oxygen
4. Helium

1. Hydrogen

Explanation: Hydrogen’s atomic number is 1. The nucleus is the smallest and cannot accommodate a heavy neutron along with the proton. Thus it doesn’t have a neutron.

## Which of the following does not have a neutron?

1. Tritium
2. Deuterium
3. Protium
4. None of the above

1. Protium

Explanation: Protium does not have a neutron and it is considered as the most stable isotope of hydrogen. So, when we talk about hydrogen, we take protium into consideration.

## What is the absolute mass and charge of an electron?

Absolute mass of an electron = 9 x 10 -28 kg.

Absolute charge of an electron = 1.6 x 10 -19 C.

## What information is furnished about the nucleus of an atom by the alpha particle scattering experiment of Rutherford?

Important information furnished about nucleus in Rutherford’s alpha-particles scattering experiment is:

1. Nucleus of an atom is positively charged.
2. Nucleus of an atom is very hard and dense.
3. Nucleus of an atom is very small as compared to the size of an atom as a whole.
4. Nearly the entire mass of an atom is contained in the nucleus.

## How was it shown that an atom has a lot of empty space within it?

Most of the alpha-particles passed straight through the gold foil without any deflection in Rutherford’s alpha-particle scattering experiment, this shows that most of the space in an atom is empty.

The isotopes which are not stable due to presence of extra neutrons in their nuclei and emit various types of radiations are called radioactive isotopes or radio-isotopes.

For example: Carbon - 14 and Arsenic - 74

## Give any two uses of radioactive isotopes.

The two uses of radioactive isotopes are as follows:-

1. They are used in the treatment of cancer.

2. Radioactive isotopes are used as ‘ tracers ’ in medicine to detect the presence of tumors and blood clots in the human body.

## What is the relation between the valency of an element and the number of valence electrons in its atoms? Explain with examples.

The valency of an element is either equal to the number of valence electrons in its atom or equal to the number of electrons required to complete eight electrons in the valence shell.

Valency of elements with 1, 2 or 3 valence electrons = Number of valence electron in its atom

Valency of elements with 7, 6 or 5 valence electrons = 8 - Number of valence electrons in its atom.

For example:

• Sodium atoms have 1 valence electron so the valency of sodium is one.
• Chlorine has 7 valence electrons so it needs one more electron to complete 8 electrons hence the valency of Chlorine is 1 .

In the above figures we can see that Sodium has one electron and Chlorine has 7 electrons in their valence shells respectively. So Sodium (Na) loses one electron and Chlorine (Cl) gains 1 electron in accordance with octet rule. So valencies of Na and Cl are +1 and -1 respectively.