Which one of the following is the most characteristic property of an element?
- A.Density
- B.Boiling point
- C.Mass number
- D.Atomic number
Correct answer
Explanation
The correct answer is D, Atomic number. The number of protons in the nucleus defines an element, and no two elements share it.
The atomic number (Z) is the number of protons in an atom's nucleus, and in a neutral atom it also equals the number of electrons, which decide chemical behaviour. Every atom with 6 protons is carbon and every atom with 8 protons is oxygen, whatever else changes. Henry Moseley showed this in 1913 with X-ray studies, and the modern periodic table is arranged in order of increasing atomic number rather than atomic mass, as Mendeleev had done.
A, density, and B, boiling point, change with temperature, pressure and the physical form of a substance; diamond and graphite are both carbon but differ greatly in density. C, mass number, is the total of protons and neutrons, and it varies between isotopes of the same element, for example carbon-12 and carbon-14.
Exam tip: atomic number = protons = identity of the element; Moseley's modern periodic law is based on it.
Practice Questions
View allIn plant cells, RNA is present in
- A.cytoplasm only
- B.nuclei and cytoplasm only
- C.nuclei, cytoplasm, mitochondria, chloroplast and endoplasmic reticulum
- D.nuclei, cytoplasm, mitochondria, chloroplast and ribosomes
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In plant cells RNA is found in the nucleus, the cytoplasm, mitochondria, chloroplasts and ribosomes. RNA is made in the nucleus, where DNA is copied into messenger, transfer and ribosomal RNA; the nucleolus in particular is rich in ribosomal RNA.
Read the full explanation →Cell wall is absent in which one of the following organisms ?
- A.Bacteria
- B.Diatom
- C.Mushroom
- D.Tapeworm
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The tapeworm is an animal, and animal cells have no cell wall, only a cell membrane. A rigid cell wall lies outside the cell membrane in plants, fungi, most bacteria and many protists, giving the cell shape and protection.
Read the full explanation →Methanol is toxic because
- A.methanol coagulates the protoplasm
- B.methanol gets oxidised to methanal in liver which coagulates the protoplasm
- C.methanol gets oxidised to acetic acid in liver which coagulates the protoplasm
- D.methanol gets oxidised to CO in liver which coagulates the protoplasm
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In the liver, methanol is oxidised to methanal (formaldehyde), which coagulates protoplasm and damages cells. Methanol, CH3OH, is the simplest alcohol and is sometimes mixed into illicit liquor.
Read the full explanation →In which one of the following types of connective tissues in animals does fat get stored?
- A.Adipocyte
- B.Chondrocyte
- C.Osteocyte
- D.Reticulocyte
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Adipocytes are the fat cells of adipose tissue, a loose connective tissue that stores fat in animals. Adipose tissue lies mainly beneath the skin and around organs such as the kidneys and heart.
Read the full explanation →Which one of the following agents does not contribute to propagation of plants through seed dispersal?
- A.Wind
- B.Fungus
- C.Animal
- D.Water
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Seeds are carried away from the parent plant by wind, water and animals, but fungi play no part in seed dispersal.
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