Atomic Structure
9 marks · HardThe Periodic Table symbol for the element bromine (B r) is shown below.
| Symbol | Mass number | Name | Atomic number |
|---|---|---|---|
| Br | 80 | Bromine | 35 |
Bromine has two main isotopes: B r-79 and B r-81.
State the meaning of atomic number and mass number.
For a neutral atom of bromine-80, calculate the number of:
protons
neutrons
electrons
Describe the relative mass and charge of each sub-atomic particle found in a bromine atom.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Atomic number: the number of protons in the nucleus of an atom. Mass number: the total number of protons and neutrons in the atom. · must show atomic number = number of protons, mass number = number of protons + neutrons
(b(i)) 35 · must show atomic number from table
(b(ii)) 45 · must show mass number - atomic number
(b(iii)) 35 · must show neutral atom, electrons = protons
(c) Proton: relative mass 1, charge +1. Neutron: relative mass 1, charge 0. Electron: relative mass 1/1836, charge -1. · must show proton mass 1, charge +1, neutron mass 1, charge 0, electron mass 1/1836, charge -1
Common mistakes: Confusing atomic number and mass number definitions; Incorrectly calculating neutrons; Mixing up relative charges
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correct meaning of atomic number: number of protons
B2 (a) · 1 mark · Correct meaning of mass number: total protons and neutrons
A1 (b(i)) · 1 mark · Correctly identifies 35 protons in bromine-80
M1 (b(ii)) · 1 mark · Uses formula: neutrons = mass number - atomic number (follow-through)
A2 (b(ii)) · 1 mark · Gives 45 neutrons for bromine-80
A3 (b(iii)) · 1 mark · Correctly identifies 35 electrons for neutral atom
B3 (c) · 1 mark · Describes relative mass and charge of proton
B4 (c) · 1 mark · Describes relative mass and charge of neutron
B5 (c) · 1 mark · Describes relative mass and charge of electron
Worked solution
(a) Atomic number is the number of protons in an atom's nucleus. Mass number is the total number of protons and neutrons in the nucleus.
(b) For bromine-80 (B r-80):
(i) Number of protons = atomic number = 35
(ii) Number of neutrons = mass number - atomic number = 80 - 35 = 45
(iii) Number of electrons = number of protons = 35 (in a neutral atom)
(c) Sub-atomic particles in a bromine atom:
| Particle | Relative Mass | Relative Charge |
|---|---|---|
| proton | 1 | +1 |
| neutron | 1 | 0 |
| electron | 1/1836 | -1 |