Atomic Structure Calculations
- An atom is neutral and has no overall charge
- Ions on the other hand are formed when atoms either gain or lose electrons, causing them to become charged
- The number of subatomic particles in atoms and ions can be determined given their atomic (proton) number, mass (nucleon) number and charge
Properties
- Isotopes have similar chemical properties but different physical properties
Chemical properties
- Isotopes of the same element display the same chemical characteristics
- This is because they have the same number of electrons in their outer shells
- Electrons take part in chemical reactions and therefore determine the chemistry of an atom
Physical properties
- The only difference between isotopes is the number of neutrons
- Since these are neutral subatomic particles, they only add mass to the atom
- As a result of this, isotopes have different physical properties such as small differences in their mass, density, melting point and boiling point
- For example samples of uranium hexafluoride gas, UF6, can undergo uranium enrichment by using a gas centrifuge cylinder
- This is because nuclear reactors require higher amounts of the lighter U235 isotope
- UF6 gas is placed in a gas centrifuge cylinder and rotated at a high speed
- This rotation creates a strong centrifugal force so that the heavier gas molecules (UF6 containing the heavier U238 isotope) move towards the outside of the cylinder
- The lighter gas molecules (containing the U235 isotope) collect closer to the centre
Protons
- The atomic number of an atom and ion determines which element it is
- Therefore, all atoms and ions of the same element have the same number of protons (atomic number) in the nucleus
- E.g. lithium has an atomic number of 3 (three protons) whereas beryllium has atomic number of 4 (4 protons)
- The number of protons equals the atomic (proton) number
- The number of protons of an unknown element can be calculated by using its mass number and number of neutrons:
Mass number = number of protons + number of neutrons
Number of protons = mass number - number of neutrons
Worked example
Determine the number of protons of the following ions and atoms:
- Mg2+ ion
- Carbon atom
- An unknown atom of element X with mass number 63 and 34 neutrons
Answers
Answer 1: The atomic number of a magnesium atom is 12 indicating that the number of protons in the magnesium element is 12
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- Therefore the number of protons in a Mg2+ ion is also 12
Answer 2: The atomic number of a carbon atom is 6 indicating that a carbon atom has 6 protons in its nucleus
Answer 3: Use the formula to calculate the number of protons
Number of protons = mass number - number of neutrons
Number of protons = 63 - 34
Number of protons = 29
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- Element X is therefore copper
Electrons
- An atom is neutral and therefore has the same number of protons and electrons
- Ions have a different number of electrons to their atomic number depending on their charge
- A positively charged ion has lost electrons and therefore has fewer electrons than protons
- A negatively charged ion has gained electrons and therefore has more electrons than protons
Worked example
Determine the number of electrons of the following ions and atoms:
- Mg2+ ion
- Carbon atom
- An unknown atom of element X with mass number 63 and 34 neutrons
Answers
Answer 1: The atomic number of a magnesium atom is 12 suggesting that the number of protons in the neutral magnesium atom is 12
-
- However, the 2+ charge in Mg2+ ion suggests it has lost two electrons
- It only has 10 electrons left now
Answer 2: The atomic number of a carbon atom is 6 suggesting that the neutral carbon atom has 6 electrons orbiting around the nucleus
Answer 3: The number of protons of element X can be calculated by:
Number of protons = mass number - number of neutrons
Number of protons = 63 - 34
Number of protons = 29
-
- The neutral atom of element X therefore also has 29 electrons
Neutrons
- The mass and atomic numbers can be used to find the number of neutrons in ions and atoms:
Number of neutrons = mass number (A) - number of protons (Z)
Worked example
Determine the number of neutrons of the following ions and atoms:
- Mg2+ ion
- Carbon atom
- An unknown atom of element X with mass number 63 and 29 protons
Answers
Answer 1: The atomic number of a magnesium atom is 12 and its mass number is 24
Number of neutrons = mass number (A) - number of protons (Z)
Number of neutrons = 24 - 12
Number of neutrons = 12
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- The Mg2+ ion has 12 neutrons in its nucleus
Answer 2: The atomic number of a carbon atom is 6 and its mass number is 12
Number of neutrons = mass number (A) - number of protons (Z)
Number of neutrons = 12 - 6
Number of neutrons = 6
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- The carbon atom has 6 neutrons in its nucleus
Answer 3: The atomic number of an element X atom is 29 and its mass number is 63
Number of neutrons = mass number (A) - number of protons (Z)
Number of neutrons = 63 - 29
Number of neutrons = 34
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- The neutral atom of element X has 34 neutrons in its nucleus