Subatomic Structure of Atoms & Ions
- All matter is composed of atoms, which are the smallest parts of an element that can take place in chemical reactions
- Atoms are mostly made up of empty space
- The mass of an atom is concentrated in the nucleus, because the nucleus contains the heaviest subatomic particles (the neutral neutrons and positive protons)
- The mass of the electron is negligible
Relative mass & charge of subatomic particles table
- The nucleus is also positively charged due to the protons
- Negatively charged electrons orbit the nucleus of the atom, contributing very little to its overall mass, but creating a ‘cloud’ of negative charge
- The electrostatic attraction between the positive nucleus and negatively charged electrons orbiting around it is what holds an atom together
The mass of the atom is concentrated in the positively charged nucleus which is attracted to the negatively charged electrons orbiting around it
- 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
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
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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
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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, or cation, has lost electrons and therefore has fewer electrons than protons
- A negatively charged ion, or anion, 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
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- However, the 2+ charge in Mg2+ ion suggests it has lost two electrons
- It only has 10 electrons left now
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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
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- The neutral atom of element X therefore also has 29 electrons
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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
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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
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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
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