For an element with atomic number 35, which one of the following will be the correct number of electrons in its valence shell based on Bohr's model of an atom?
- A.1
- B.3
- C.5
- D.7
Correct answer
Explanation
The correct answer is D, 7. The element with atomic number 35 is bromine, whose 35 electrons are arranged as 2, 8, 18, 7, leaving seven in the outermost shell.
In Bohr's model the shells K, L, M and N hold at most 2, 8, 18 and 32 electrons. Filling them in order, 35 electrons give 2 in K, 8 in L, 18 in M and the remaining 7 in N. Seven valence electrons place bromine in group 17, the halogens, along with fluorine, chlorine and iodine. Bromine needs one more electron to complete an octet, so it readily forms the bromide ion, Br⁻. It is the only non-metal that is a liquid at room temperature.
A is wrong because one valence electron belongs to alkali metals such as potassium (atomic number 19).
B is wrong because three valence electrons belong to group 13 elements such as aluminium and gallium (31).
C is wrong because five valence electrons belong to group 15 elements such as arsenic (33).
Exam tip: atomic number 35 is bromine, configuration 2, 8, 18, 7, a halogen and the liquid non-metal.
Practice Questions
View allWhich of the following statements about a fluid at rest in a cup is/are correct? 1. Pressure is same at all the points in the fluid 2. Pressure is exerted on the walls. 3. Pressure exists everywhere in the fluid. Select the correct answer using the code given below :
- A.1 and 2 only
- B.2 and 3 only
- C.1 only
- D.1, 2 and 3
Show answer
A fluid at rest presses on the walls and has pressure at every point, but that pressure is not the same at all depths.
Read the full explanation →The absolute zero temperature is 0 Kelvin. In °C unit, which one of the following is the absolute zero temperature?
- A.0 °C
- B.-100 °C
- C.-273.15 °C
- D.-173.15 °C
Show answer
Zero on the Kelvin scale lies 273.15 degrees below the freezing point of water. The Kelvin and Celsius scales have degrees of the same size, so T(K) = t(°C) + 273.15.
Read the full explanation →Dinitrogen (N₂) and dioxygen (O₂) are the main constituents of air but they do not react with each other to form oxides of nitrogen because
- A.the reaction requires initiation by a catalyst
- B.oxides of nitrogen are unstable
- C.the reaction is endothermic and requires very high temperature
- D.the stoichiometry of N₂ and O₂ in air is not ideal for the reaction to take place
Show answer
Nitrogen and oxygen combine only when a lot of energy is supplied. The nitrogen molecule is held by a very strong triple bond, so breaking it takes much energy, and the reaction N₂ + O₂ → 2NO absorbs heat.
Read the full explanation →Hydrogenation of vegetable oils using nickel catalyst is an example of
- A.Substitution reaction.
- B.Elimination reaction.
- C.Addition reaction.
- D.Free-radical polymerization.
Show answer
Hydrogen adds across the carbon–carbon double bonds of the oil. Vegetable oils are unsaturated: their fatty acid chains contain C=C double bonds, which keep them liquid.
Read the full explanation →A positive charge is moving towards south in a space where magnetic field is pointing in the north direction. The moving charge will experience :
- A.a deflecting force towards north direction.
- B.a deflecting force towards east direction.
- C.a deflecting force towards west direction.
- D.no deflecting force.
Show answer
The charge moves exactly opposite to the field, and a magnetic field exerts no force on a charge moving along its line.
Read the full explanation →