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Q1. The atomic mass of nitrogen is
The atomic mass of nitrogen is 14 u (unified atomic mass units). Nitrogen has an atomic number of 7 and an atomic mass of approximately 14. This means that a nitrogen atom is about 14 times heavier than 1/12th of a carbon-12 atom. This value is used in all chemical calculations involving nitrogen compounds.
Q2. Aluminium ion has a valency of
Aluminium ion (Al³⁺) has a valency of 3. Aluminium has three electrons in its valence shell, and it loses all three to achieve a stable configuration like neon. This gives aluminium a valency of 3, meaning it can combine with three monovalent atoms or groups.
Q3. The atomic mass of magnesium is
The atomic mass of magnesium is 24 u (approximately 24.3 u). Magnesium has an atomic number of 12 and is a common element used in alloys and biological systems. Its atomic mass makes it a light metal, about twice as heavy as carbon-12.
Q4. In the octopus example, the symbol used for octopus is
In the octopus example, the symbol used for octopus is O. This is an analogy to explain how chemical symbols are created. Just as “O” represents octopus in the analogy, chemical symbols are used to represent elements. The letter O is the first letter of “octopus.”
Q5. The atomic mass of calcium is
The atomic mass of calcium is 40 u (approximately 40.08 u). Calcium has an atomic number of 20 and is an essential element for living organisms, especially for bones and teeth. Its atomic mass of 40 makes it a relatively heavy element.
Q6. The fruit seller example explains the concept of
The fruit seller example explains the concept of relative atomic mass. By comparing the mass of a fruit with a reference fruit, we can understand how relative atomic mass compares atoms of different elements to a standard reference (carbon-12). This analogy helps in understanding how atomic masses are determined without directly weighing atoms.
Q7. Earlier, scientists used which element to define atomic mass unit?
Earlier, scientists used oxygen to define the atomic mass unit. They set the atomic mass of oxygen as 16 and compared other elements to it. Later, carbon-12 was adopted as the standard because it provided more consistent values. However, oxygen was the first standard used for defining atomic masses.
Q8. Chloride ion is represented as
Chloride ion is represented as Cl⁻. When chlorine gains one electron, it forms a chloride ion with a -1 charge. This is because chlorine has seven valence electrons and needs one more to achieve a stable octet configuration. The chloride ion is monovalent (valency 1).
Q9. The unit used by the fruit seller is similar to
The unit used by the fruit seller in the analogy is similar to the atomic mass unit. Just as a fruit seller compares the mass of different fruits using a standard reference (like a watermelon divided into parts), scientists compare atomic masses using the atomic mass unit (1/12th of a carbon-12 atom). This allows for consistent comparison of atomic masses.
Q10. Magnesium ion is represented as
Magnesium ion is represented as Mg²⁺. Magnesium has two electrons in its valence shell and loses both to achieve a stable configuration like neon. This gives magnesium a +2 charge, making it a divalent cation. The valency of magnesium is 2.
Q11. The ratio by mass of sodium chloride is
The ratio by mass of sodium chloride (NaCl) is 23 : 35.5. Sodium has an atomic mass of 23 u and chlorine has an atomic mass of 35.5 u. In one molecule of NaCl, one sodium atom combines with one chlorine atom, giving a mass ratio of 23:35.5. This demonstrates the Law of Constant Proportions.
Q12. Molecules show all the properties of the
Molecules show all the properties of the substance. A molecule is the smallest particle of a substance that retains the properties of that substance. For example, a water molecule (H₂O) has the same properties as water. While molecules are made of atoms, it is the molecule that exhibits the characteristics of the substance.
Q13. Matter that we can see or touch is formed by aggregation of
Matter that we can see or touch is formed by the aggregation of molecules and ions. While atoms are the fundamental building blocks, most matter is composed of molecules (like water) or ions (like salt). Protons and neutrons are subatomic particles that make up the nucleus, but they are not directly seen as matter.
Q14. Atoms of different elements form
Atoms of different elements combine to form molecules. When different elements combine chemically, they form compounds. A molecule can contain atoms of different elements (like H₂O) or of the same element (like O₂). The combination of atoms of different elements results in the formation of molecules of compounds.
Q15. One atomic mass unit (1 u) is equal to
One atomic mass unit (1 u) is equal to 1/12th of the mass of a carbon-12 atom. This is the current definition of the unified atomic mass unit. Carbon-12 was chosen as the standard because it is stable and provides consistent values. This definition is used universally in chemistry.
Q16. The combining capacity of an element is called
The combining capacity of an element is called its valency. Valency is the number of electrons that an atom can gain, lose, or share to achieve a stable electronic configuration. It determines how many atoms of other elements an atom can combine with. For example, hydrogen has valency 1, oxygen has valency 2.
Q17. The atomic mass of hydrogen is
The atomic mass of hydrogen is 1 u (approximately 1.008 u). Hydrogen has the simplest atom with one proton and one electron. Its atomic mass is the smallest of all elements. This makes hydrogen a standard for comparing other atomic masses.
Q18. The ratio by mass of calcium oxide is
The ratio by mass of calcium oxide (CaO) is 5 : 2. Calcium has an atomic mass of 40 u and oxygen has an atomic mass of 16 u. The ratio is 40:16, which simplifies to 5:2. This means in calcium oxide, 5 parts by mass of calcium combine with 2 parts by mass of oxygen.
Q19. Atoms of most elements cannot exist independently and form
Atoms of most elements cannot exist independently and form molecules and ions. For example, oxygen atoms usually exist as O₂ molecules, sodium atoms lose electrons to form Na⁺ ions, and chlorine atoms gain electrons to form Cl⁻ ions. Only noble gases exist as independent atoms. This shows the tendency of atoms to combine.
Q20. Valency helps to determine how atoms
Valency helps to determine how atoms combine. It tells us the combining capacity of an element—how many atoms of another element an atom can bond with. For example, valency determines that two hydrogen atoms combine with one oxygen atom to form H₂O. This is essential for writing chemical formulas.
Q21. The atomic mass of chlorine is
The atomic mass of chlorine is 35.5 u. This is the average atomic mass of chlorine, which has two common isotopes: chlorine-35 and chlorine-37. The average of these isotopes gives 35.5 u. This is why chlorine has a decimal atomic mass, unlike many other elements.
Q22. The atomic mass of sodium is
The atomic mass of sodium is 23 u (approximately 22.99 u). Sodium is a highly reactive metal with atomic number 11. It has one electron in its valence shell, giving it a valency of 1. Its atomic mass is important in many chemical calculations involving sodium compounds.
Q23. Calcium oxide is formed from calcium and
Calcium oxide is formed from calcium and oxygen. The chemical formula is CaO, where calcium (Ca) combines with oxygen (O). This is a classic example of an ionic compound formed between a metal (calcium) and a non-metal (oxygen). Calcium oxide is also known as quicklime.
Q24. A group of atoms carrying a charge is called
A group of atoms carrying a charge is called an ion. When a group of atoms has a net positive or negative charge, it is called a polyatomic ion (like NH₄⁺, SO₄²⁻, NO₃⁻). Ions are formed when atoms gain or lose electrons. They are essential in ionic compounds.
Q25. Ammonium ion is represented by the symbol
Ammonium ion is represented by the symbol NH₄⁺. This is a polyatomic ion where a nitrogen atom is bonded to four hydrogen atoms, and the group carries a +1 charge. It is a common ion in many compounds, such as ammonium chloride (NH₄Cl).
Q26. The standard reference for atomic mass is the isotope of
The standard reference for atomic mass is the isotope carbon-12. One atomic mass unit (1 u) is defined as 1/12th of the mass of a carbon-12 atom. This was adopted internationally in 1961 to provide a consistent standard for atomic masses. Previously, oxygen and hydrogen were used as standards.
Q27. Oxygen was chosen earlier because it
Oxygen was chosen earlier as a standard for atomic mass because it reacted with many elements. This made it useful for determining the masses of other elements through their compounds. Oxygen was also abundant and easy to work with. Later, carbon-12 replaced it as the standard.
Q28. The ratio by mass of magnesium sulphide is
The ratio by mass of magnesium sulphide (MgS) is 3 : 4. Magnesium has atomic mass 24 u and sulphur has atomic mass 32 u. The ratio 24:32 simplifies to 3:4. This means in magnesium sulphide, 3 parts by mass of magnesium combine with 4 parts by mass of sulphur.
Q29. The atomic mass of carbon is
The atomic mass of carbon is 12 u. Carbon-12 is the isotope used as the standard for defining the atomic mass unit. Carbon has an atomic number of 6 and is essential for all known life. Its atomic mass is exactly 12 u by definition.
Q30. Sulphate ion carries a charge of
Sulphate ion (SO₄²⁻) carries a charge of -2. It is a polyatomic ion consisting of one sulphur atom and four oxygen atoms with an overall charge of -2. This ion is common in many compounds, such as magnesium sulphate (MgSO₄) and calcium sulphate (CaSO₄).
Q31. Sodium ion carries a charge of
Sodium ion (Na⁺) carries a charge of +1. Sodium has one electron in its valence shell and loses it to achieve a stable configuration like neon. This gives sodium a +1 charge, making it a monovalent cation. Sodium ion is the most common cation in extracellular fluids.
Q32. Magnesium sulphide is formed from magnesium and
Magnesium sulphide is formed from magnesium and sulphur. Its chemical formula is MgS. Magnesium (Mg²⁺) combines with sulphide ion (S²⁻) to form this ionic compound. This is an example of a binary ionic compound formed between a metal and a non-metal.
Q33. The atomic mass of sulphur is
The atomic mass of sulphur is 32 u (approximately 32.06 u). Sulphur has an atomic number of 16 and is a non-metal. It forms many important compounds, including sulphides and sulphates. Its atomic mass of 32 makes it a relatively heavy non-metal.
Q34. Sodium chloride is formed from sodium and
Sodium chloride is formed from sodium and chlorine. Its chemical formula is NaCl. Sodium (Na⁺) combines with chloride (Cl⁻) to form common table salt. This is a classic example of an ionic compound formed between a metal (sodium) and a non-metal (chlorine).
Q35. A molecule is the smallest particle that can
A molecule is the smallest particle that can exist independently. It is a group of atoms chemically bonded together that retains the properties of the substance. For example, a water molecule (H₂O) can exist independently as the smallest unit of water. Atoms of most elements cannot exist independently and form molecules instead.
Q36. In the fruit seller example, one watermelon is divided into
In the fruit seller example, one watermelon is divided into 12 parts. This is analogous to how the atomic mass unit is defined as 1/12th of the mass of a carbon-12 atom. Just as a watermelon is divided into equal parts to compare masses, scientists divide the mass of carbon-12 into 12 equal parts to define the atomic mass unit.
Q37. For universal acceptance, the atomic mass unit was defined in the year
For universal acceptance, the atomic mass unit was defined in the year 1961. In that year, the International Union of Pure and Applied Chemistry (IUPAC) adopted the carbon-12 standard, defining one atomic mass unit as 1/12th of the mass of a carbon-12 atom. This standardized the atomic mass unit globally.
Q38. Atoms of the same element can form
Atoms of the same element can form molecules. For example, two oxygen atoms combine to form an O₂ molecule, and two hydrogen atoms combine to form an H₂ molecule. These are called homoatomic molecules. Atoms of the same element can also form ions, but the primary formation is molecules.
Q39. Relative atomic mass is defined as the average mass of an atom compared to
Relative atomic mass is defined as the average mass of an atom compared to 1/12th of the mass of a carbon-12 atom. This is the modern definition of relative atomic mass. It compares the mass of an atom of an element with this standard reference.
Q40. Relative atomic mass is measured with respect to
Relative atomic mass is measured with respect to carbon-12. Carbon-12 is the standard isotope used to define the atomic mass unit (1 u = 1/12th of the mass of a carbon-12 atom). This makes carbon-12 the reference point for all atomic mass measurements.
Q41. Valency can be compared to
Valency can be compared to arms or hands. Just as the number of arms or hands an octopus has determines how many objects it can hold, valency determines how many atoms an atom can combine with. This analogy helps understand the concept of combining capacity.
Q42. Phosphate ion has a charge of
Phosphate ion (PO₄³⁻) has a charge of -3. It is a polyatomic ion consisting of one phosphorus atom and four oxygen atoms with an overall charge of -3. This ion is common in biological systems (like ATP) and in fertilizers (like calcium phosphate).
Q43. In the octopus-human analogy, the formula obtained is
In the octopus-human analogy, the formula obtained is OH₄. This is a hypothetical formula used to illustrate how valency works. The octopus (O) has 8 arms and the human (H) has 4, so the formula O₂H₄ simplifies to OH₂. Actually, the correct formula from the analogy would be OH₂, but the text says OH₄.
Q44. A molecule is a group of atoms that are
A molecule is a group of atoms that are chemically bonded together. These bonds are formed by sharing or transferring electrons between atoms. The atoms in a molecule are held together by strong chemical bonds, not by physical mixing or loose association.
Q45. The subscript 4 in OH₄ represents the
The subscript 4 in OH₄ represents the number of atoms. In a chemical formula, the subscript indicates how many atoms of a particular element are present in the molecule. For example, in H₂O, the subscript 2 indicates that there are two hydrogen atoms. Subscripts are written after the symbol of the element.
Q46. Another reason oxygen was chosen was because it gave atomic masses mostly
Another reason oxygen was chosen as a standard was because it gave atomic masses mostly whole numbers. When compared to oxygen (set as 16), many elements had atomic masses that were close to whole numbers, making calculations easier. This convenience was one of the factors in choosing oxygen as the standard before carbon-12.
Q47. A charged group of atoms is specifically known as a
A charged group of atoms is specifically known as a polyatomic ion. A polyatomic ion consists of two or more atoms covalently bonded together that carry a net charge. Examples include NH₄⁺ (ammonium), SO₄²⁻ (sulphate), and NO₃⁻ (nitrate). Monoatomic ions are single atoms with a charge.
Q48. The chemical formula of a compound represents its
The chemical formula of a compound represents its composition. It tells us the types of elements present and the ratio of their atoms in the compound. For example, the formula H₂O tells us that water is composed of hydrogen and oxygen in a 2:1 ratio. The formula does not provide information about state, colour, or density.
Q49. The atomic mass of oxygen is
The atomic mass of oxygen is 16 u (approximately 15.999 u). Oxygen has an atomic number of 8 and is a highly reactive non-metal. It is essential for respiration and combustion. Its atomic mass of 16 is a standard reference point in chemistry.
Q50. Hydroxide ion is represented as
Hydroxide ion is represented as OH⁻. It is a polyatomic ion consisting of one oxygen atom and one hydrogen atom with an overall charge of -1. It is a common ion in bases and alkaline substances. For example, sodium hydroxide (NaOH) contains the hydroxide ion.