Atomic Structure
Introduction to AS Chemistry Unit 1
This unit covers atomic structure, bonding, and an introduction to organic chemistry. It builds upon O-level
concepts with some additional AS-level material.
Atomic Structure Basics
Components of an Atom
Nucleus: Contains protons and neutrons.
Electrons: Revolve around the nucleus.
Subatomic Particles
Particle Mass (Relative) Charge
Neutron 1 0
Proton 1 +1
Electron 1/1840 -1
Atomic Number and Mass Number
Atomic Number (Z): The number of protons in the nucleus. It is also equal to the number of
electrons in a neutral atom. This is the smaller number found on the periodic table.
Mass Number (A): The total number of protons and neutrons in the nucleus. This is the larger
number found on the periodic table.
Number of Neutrons: Mass Number (A) - Atomic Number (Z).
Example: Fluorine (F) has atomic number 9 and mass number 19.
Number of protons = 9
Number of electrons = 9
Number of neutrons = 19 - 9 = 10
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, Ions
Cation: An atom that has lost electrons, becoming positively charged (more protons than electrons).
Anion: An atom that has gained electrons, becoming negatively charged (more electrons than
protons).
−
Example: Fluoride ion (F ).
It has the same number of protons (9) and neutrons (10) as a fluorine atom.
It has gained one electron, so it has 10 electrons.
Electron configuration: 2 electrons in the first shell, 8 electrons in the second shell.
Elements, Compounds, and Mixtures
Element: Contains only one type of atom. Atoms can be joined together or exist alone, but they are
all the same type.
Compound: Formed when two or more elements are chemically combined in a fixed ratio.
Mixture: Formed when two or more substances are added together. They can be separated by simple
physical processes.
Isotopes
Isotopes are atoms of the same element that have the same number of protons but different numbers of
neutrons.
Example: Hydrogen has three isotopes:
Protium (¹H): 1 proton, 0 neutrons
Deuterium (²H): 1 proton, 1 neutron
Tritium (³H): 1 proton, 2 neutrons
All hydrogen isotopes have 1 proton and 1 electron, but they differ in their mass number due to the varying
number of neutrons.
Question Example: Which of the following represents a pair of isotopes?
Carbon and Nitrogen (Different elements)
Sulfur and Sulfur²⁻ (Atom and ion of the same element)
O₂ and O₃ (Different molecules of the same element)
Correct Answer: Two atoms of the same element with different mass numbers (e.g., $^{12}Cand
^{13}$C).
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Introduction to AS Chemistry Unit 1
This unit covers atomic structure, bonding, and an introduction to organic chemistry. It builds upon O-level
concepts with some additional AS-level material.
Atomic Structure Basics
Components of an Atom
Nucleus: Contains protons and neutrons.
Electrons: Revolve around the nucleus.
Subatomic Particles
Particle Mass (Relative) Charge
Neutron 1 0
Proton 1 +1
Electron 1/1840 -1
Atomic Number and Mass Number
Atomic Number (Z): The number of protons in the nucleus. It is also equal to the number of
electrons in a neutral atom. This is the smaller number found on the periodic table.
Mass Number (A): The total number of protons and neutrons in the nucleus. This is the larger
number found on the periodic table.
Number of Neutrons: Mass Number (A) - Atomic Number (Z).
Example: Fluorine (F) has atomic number 9 and mass number 19.
Number of protons = 9
Number of electrons = 9
Number of neutrons = 19 - 9 = 10
1/5
, Ions
Cation: An atom that has lost electrons, becoming positively charged (more protons than electrons).
Anion: An atom that has gained electrons, becoming negatively charged (more electrons than
protons).
−
Example: Fluoride ion (F ).
It has the same number of protons (9) and neutrons (10) as a fluorine atom.
It has gained one electron, so it has 10 electrons.
Electron configuration: 2 electrons in the first shell, 8 electrons in the second shell.
Elements, Compounds, and Mixtures
Element: Contains only one type of atom. Atoms can be joined together or exist alone, but they are
all the same type.
Compound: Formed when two or more elements are chemically combined in a fixed ratio.
Mixture: Formed when two or more substances are added together. They can be separated by simple
physical processes.
Isotopes
Isotopes are atoms of the same element that have the same number of protons but different numbers of
neutrons.
Example: Hydrogen has three isotopes:
Protium (¹H): 1 proton, 0 neutrons
Deuterium (²H): 1 proton, 1 neutron
Tritium (³H): 1 proton, 2 neutrons
All hydrogen isotopes have 1 proton and 1 electron, but they differ in their mass number due to the varying
number of neutrons.
Question Example: Which of the following represents a pair of isotopes?
Carbon and Nitrogen (Different elements)
Sulfur and Sulfur²⁻ (Atom and ion of the same element)
O₂ and O₃ (Different molecules of the same element)
Correct Answer: Two atoms of the same element with different mass numbers (e.g., $^{12}Cand
^{13}$C).
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