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All questions of Kinetic Energy for Grade 8 Exam

Which one of the following is the modern symbol of Gold?
  • a)
    Gl
  • b)
    Go
  • c)
    Si
  • d)
    Au
Correct answer is option 'D'. Can you explain this answer?

Arvind Singh answered
Gold is a chemical element with symbol Au (from Latin: aurum) and atomic number 79, making it one of the higher atomic number elements that occur naturally. In its purest form, it is a bright, slightly reddish yellow, dense, soft, malleable, and ductile metal.
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One atomic mass unit is a mass unit equal to exactly one twelfth (1/12th) the mass of one atom of ___________.
  • a)
    Carbon-12
  • b)
    Nitrogen -14
  • c)
    Carbon-1
  • d)
    4.Silicon-14
Correct answer is option 'A'. Can you explain this answer?

Jyoti Kapoor answered
An atomic mass unit (symbolized AMU or amu) is defined as precisely 1/12 the mass of an atom of carbon-12. The carbon-12 (C-12) atom has six protons and six neutrons in its nucleus. In imprecise terms, one AMU is the average of the proton rest mass and the neutron rest mass.

NaCl molecule is made of which of the following ions?
  • a)
    Na cation and Cl anion
  • b)
    Cl cation and Na anion
  • c)
    Both Na and Cl cation
  • d)
    Both Na and Cl anion
Correct answer is option 'A'. Can you explain this answer?

Jyoti Kapoor answered
An oxygen molecule (O2) is a good example of a molecule with a covalent bond. Ionic bonds occur when electrons are donated from one atom to another. Table salt (NaCl) is a common example of a compound with an ionic bond.

Valency of Calcium element is:
  • a)
    4
  • b)
    5
  • c)
    2
  • d)
    3
Correct answer is option 'C'. Can you explain this answer?

Amit Sharma answered
The valency of Calcium of 2+. Calcium has 2 electrons in its outermost shell, its electronic configuration being (2,8,8,2). In order to attain stability it loses 2 electrons to become stable according to the octet rule.

Valency of hydrogen is 1 and that of sulphur is 2. What should be the formula of hydrogen sulphide?​
  • a)
    HS
  • b)
    H2S2
  • c)
    HS2
  • d)
    H2S
Correct answer is option 'D'. Can you explain this answer?

Jyoti Kapoor answered
Hydrogen sulfide is the chemical compound with the chemical formula H2S. It is a colorless gas with the characteristic foul odor of rotten eggs. It is very poisonous, corrosive, and flammable.

Can you explain the answer of this question below:

Which of the following is correct pair of elements and its symbol?

  • A:

    Silver – Si

  • B:

    Sodium – So

  • C:

    Potassium – Pt

  • D:

    Sulphur – S

The answer is d.

Prachi Rathore answered
Here are the righ pairs .......... ... silver-Ag ....... sodium-Na ...... potassium- K...... and sulphur - s so in the given options (D) sulphur - s. is right answer

21H and 31H are _______ .​
  • a)
    isotopes
  • b)
    isobars
  • c)
    isotones
  • d)
    isochores
Correct answer is option 'A'. Can you explain this answer?

Saikat Iyer answered
Atoms of the same element having the same atomic numbers but different mass numbers are termed as isotopes.

Which of the following element is used in the treatment of cancer?
  • a)
    Uranium
  • b)
    Cobalt
  • c)
    Nickel
  • d)
    Iodine
Correct answer is option 'B'. Can you explain this answer?

Jyoti Kapoor answered
Elements Used in Radiation Therapy. Radiation therapy is an important cancer treatment along with chemotherapy and surgery. The most famous type is probably gamma ray irradiation by radioactive cobalt-60.

Which of the following elements has 17 electrons and 17 protons?
  • a)
    Sodium
  • b)
    Sulphur
  • c)
    Chlorine
  • d)
    Aluminium
Correct answer is option 'C'. Can you explain this answer?

Sarita Reddy answered
Isotopes of chlorine. Chlorine atoms contain 17 protons and 17 electrons. About 75 per cent of chlorine atoms have 18 neutrons, while about 25 per cent have 20 neutrons.

The two isotopes of chlorine are 3517Cl and 3517Cl in the ratio of 3:1. What is the average atomic mass of the chlorine atom?​
  • a)
    72 u
  • b)
    37 u
  • c)
    35.5 u
  • d)
    35 u
Correct answer is option 'C'. Can you explain this answer?

Hrishikesh Saha answered
Understanding Isotopes of Chlorine
Chlorine has two isotopes: 35Cl and 37Cl. The average atomic mass of an element is calculated based on the relative abundance of its isotopes.
Isotope Details
- 35Cl: This isotope has an atomic mass of approximately 35 u.
- 37Cl: This isotope has an atomic mass of approximately 37 u.
Given Ratios
- The ratio of the isotopes is 3:1, which means:
- For every 4 atoms of chlorine, 3 are 35Cl and 1 is 37Cl.
Calculating Average Atomic Mass
To find the average atomic mass, we use the formula:
Average Atomic Mass = (Fraction of 35Cl × Mass of 35Cl) + (Fraction of 37Cl × Mass of 37Cl)
- The total parts in the ratio = 3 + 1 = 4
- Fraction of 35Cl = 3/4
- Fraction of 37Cl = 1/4
Now, substituting the values:
Average Atomic Mass = (3/4 × 35 u) + (1/4 × 37 u)
- Calculation:
- (3/4 × 35) = 26.25 u
- (1/4 × 37) = 9.25 u
Adding these together:
Average Atomic Mass = 26.25 u + 9.25 u = 35.5 u
Conclusion
The average atomic mass of chlorine, considering the isotopes and their ratios, is 35.5 u. Therefore, the correct answer is option 'C'.

Neon occurs as two isotopes of mass numbers 20 and 22. Its relative atomic mass is 20.2. What is the percentage of 20Ne in naturally occurring neon?
  • a)
    90
  • b)
    20
  • c)
    80
  • d)
    10
Correct answer is option 'A'. Can you explain this answer?

C K Academy answered
Let the relative abundance% of Ne -20 = x1
So, relative abundance% of Ne -22 =  x2
Now, we know that atomic mass = sum of relative abundance in atomic mass of each isotope / 100% 
20.2 = 20 x1 + 22 x2 / x1 +x2 
20.2 x1 + 20.2x2 = 20x1 + 22x2
0.2x1 =  1.8 x2 
x1 / x2  = 1.8/ 0.2 = 9/1   ( relative abundance) 
x1%  =  9/10 * 100  = 90%  lighter isotopes
x2%  =  1/ 10* 100  = 10%   heavier isotope 

An atom has atomic number 11. The number of protons are ________.
  • a)
    11
  • b)
    12
  • c)
    23
  • d)
    44
Correct answer is option 'A'. Can you explain this answer?

Imk Pathshala answered
- The atomic number of an atom is the number of protons in its nucleus.
- An atom with an atomic number of 11 has 11 protons.
- The correct answer is A: 11.

Which one of the following statements is correct about isotopes?
  • a)
    Each isotope of an element is a pure substance.
  • b)
    The chemical properties of isotopes are similar.
  • c)
    The physical properties of isotopes are different.
  • d)
    All of above.
Correct answer is option 'D'. Can you explain this answer?

Ananya Sharma answered
Each atomic number identifies a specific element, but not the isotope; an atom of a given element may have a wide range in its number of neutrons. The number of nucleons (both protons and neutrons) in the nucleus is the atom's mass number, and each isotope of a given element has a different mass number.

An element B has 5 protons and 6 neutrons. What will be the symbol of the element B?​
  • a)
    116B
  • b)
    11B
  • c)
    115B
  • d)
    511B
Correct answer is option 'C'. Can you explain this answer?

Gagandeep Kaur, 14 9rose answered
Atomic number of B = number of protons
= number of electron in element
= 5

mass number = number of protons + number of
neutrons
=5+6
=11.

so, c part is correct option.

The atomic number of an element having mass number 23 and number of neutron 12 is ________ .
  • a)
    23
  • b)
    11
  • c)
    12
  • d)
    35
Correct answer is option 'B'. Can you explain this answer?

Ananya Sharma answered
A neutral atom will have the same number of electrons is does protons. Since it has an atomic number of 11 it must have 11 protons. Since it has an atomic mass of 23 it must have 12 neutrons.

The total number of neutrons in 1224Mg2+is:​
  • a)
    24
  • b)
    12
  • c)
    10
  • d)
    14
Correct answer is option 'B'. Can you explain this answer?

Raghavendra Chavan answered
Neutrons are subatomic particles that have no charge and are found in the nucleus of an atom. They contribute to the mass of the atom but not to its charge. The number of neutrons in an atom can be determined by subtracting the atomic number (number of protons) from the atomic mass (mass number).

In the given question, the element is 1224Mg2. The atomic number of magnesium (Mg) is 12, which means it has 12 protons. The superscript "2" indicates the charge of the magnesium ion, which means it has gained 2 electrons and has an overall charge of +2.

To find the number of neutrons, we need to know the atomic mass of magnesium. The atomic mass of magnesium is approximately 24.31 atomic mass units (amu). However, the given question states that the atomic mass is 1224Mg2, which can be interpreted as 12 + 24 + 2 = 38 amu.

Therefore, the number of neutrons can be calculated as:

Number of neutrons = Atomic mass - Atomic number
= 38 - 12
= 26

Hence, the correct answer is option B) 12 neutrons.

Why do isotopes have different physical properties?
  • a)
    Isotopes of an element have different electronic configuration.
  • b)
    Isotopes have same atomic number.
  • c)
    Isotopes of an element have different mass number.
  • d)
    Isotopes of an element have different atomic number.
Correct answer is option 'C'. Can you explain this answer?

Ishita Kumar answered
They have similar chemical properties because isotopes of an element have the same number of electrons as an atom of that element. The electron arrangement is the same owing to same chemical properties. However they have different numbers of neutrons, which affects the mass number. Mass number determines the physical properties such as boiling/melting/density etc

The atomic number and the mass number of an element A are 14 and 28 respectively. The number of neutrons are:
  • a)
    42
  • b)
    12
  • c)
    28
  • d)
    14
Correct answer is option 'D'. Can you explain this answer?

Sunidhi Rai answered
Given,Z=14..... nd A=28.... we know that Z=p... =>p=14........ and A=p+n.... => 28= 14+n.... =>n=28-14.. =>n= 14........ Hence, no of neutrons are 14........

An atom is represented by symbol 2314M. What does the number 23 represent?​
  • a)
    Mass number
  • b)
    Atomic number
  • c)
    Number of protons
  • d)
    Number of neutrons
Correct answer is option 'A'. Can you explain this answer?

Anita Menon answered
Nucleons reside inside the nucleus and are denoted by the mass number is denoted as “A.” All atoms of a particular chemical element will have the same atomic number, number of protons present in the nucleus of an atom, but different mass numbers.

Which of the following is the correct IUPAC symbol for the element Cobalt?
  • a)
    Co
  • b)
    Cb
  • c)
    C
  • d)
    none of these
Correct answer is option 'A'. Can you explain this answer?

Jyoti Kapoor answered
The wrong thing in writing cobalt as CO is that we can't write o as O because O represent Oxygen and symbols representing an element with two letters have first one as capital and next one should be in small letter.
Therefore we should always write cobalt as..Co not CO!

The isotopic composition of a certain element X is 80% 24X, 10% 25X and 10% 26X. The relative-atomic mass of X is _______ .​
  • a)
    24.67
  • b)
    24.25
  • c)
    24.33
  • d)
    24.30
Correct answer is option 'D'. Can you explain this answer?

Arvind Singh answered
We can use the formula :


Average atomic mass of X =


[(Atomic mass of X1) x (% of X1 )] + [(Atomic mass of X2) x (% of X2 )]

                         100                      100


                               +          [(Atomic mass of X3) x (% of X3 )]


                                                                              100


                                      = (24 x 0.8) + (25 x 0.10) + (26 x 0.10)


                                      = 19.2 + 2.5 + 2.6


                                      = 24.3 u


We can use the formula :


Average atomic mass of X = [(Atomic mass of X1) x (% of X1 )] + [(Atomic mass of X2) x (% of X2 )] +


                                                                              100                                               100


                                         [(Atomic mass of X3) x (% of X3 )]


                                                                              100


                                      = (24 x 0.8) + (25 x 0.10) + (26 x 0.10)


                                      = 19.2 + 2.5 + 2.6


                                      = 24.3 u

An neutral element A has 3 electrons, the atomic number of an element is _______.
  • a)
    1
  • b)
    2
  • c)
    5
  • d)
    3
Correct answer is option 'D'. Can you explain this answer?

Rudraksh Saha answered
If the number of electrons is 1, 2, or 3 then the atomic number will respectively be 1, 2, or 3.
if the number of electrons is 5, 6, or 7 then the atomic number will respectively be 5, 6, or 7.
if the number of electrons is 4 then the atomic number will respectively be 4.
if the number of electrons is 8 then the atomic number will respectively be 0.

Find the symbol of an element X, if its mass number is A and atomic number is Z respectively.
  • a)
    AXZ
  • b)
    AZX
  • c)
    ZAX
  • d)
    ZXA
Correct answer is option 'B'. Can you explain this answer?

Parth Khanna answered
Answer:
The symbol of an element X, given its mass number A and atomic number Z, is represented as AZX. Let's break down the symbol to understand why option 'B' is the correct answer.

Understanding the symbol:
- A represents the mass number of the element, which is the sum of the protons and neutrons in the nucleus of an atom.
- Z represents the atomic number of the element, which is the number of protons in the nucleus of an atom.
- X represents the chemical symbol of the element, which is a one or two-letter abbreviation used to represent an element.

Explanation of the correct answer:
Option 'B' is the correct answer because it follows the correct order of the elements in the symbol.

- A is the mass number, which represents the total number of protons and neutrons in the nucleus of the atom. It should come before Z.
- Z is the atomic number, which represents the number of protons in the nucleus of the atom. It should come after A.
- X is the chemical symbol, which represents the element. It should come at the end.

Therefore, the correct symbol of the element X, given its mass number A and atomic number Z, is AZX.

Explanation of incorrect options:
- Option 'A' (AXZ) is incorrect because it does not follow the correct order of the elements in the symbol. The mass number should come first, followed by the atomic number, and then the chemical symbol.
- Option 'C' (ZAX) is incorrect because it reverses the order of the elements in the symbol. The atomic number should come before the chemical symbol.
- Option 'D' (ZXA) is incorrect because it also reverses the order of the elements in the symbol. The atomic number should come before the chemical symbol.

Conclusion:
The correct symbol of an element X, given its mass number A and atomic number Z, is represented as AZX. It is important to follow the correct order of the elements in the symbol to accurately represent the element.

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