Structure of the Atom

The One Thing

An atom’s identity is determined by its number of protons, while its chemical behaviour depends largely on the arrangement of electrons in shells, especially the outermost shell. The nucleus contains nearly all the atom’s mass, whereas electrons occupy most of its volume.

Reactions, Processes and Experiments

Key Terms

• Atom: The smallest particle of an element that can take part in a chemical reaction.
• Nucleus: The tiny, dense central region of an atom containing protons and neutrons.
• Proton: A positively charged subatomic particle found in the nucleus; its relative mass is approximately 1 u.
• Neutron: A subatomic particle with no charge found in the nucleus; its relative mass is approximately 1 u.
• Electron: A negatively charged subatomic particle that moves around the nucleus in shells or energy levels.
• Atomic number: The number of protons in the nucleus of an atom, represented by Z; in a neutral atom, it also equals the number of electrons.
• Mass number: The total number of protons and neutrons in an atom, represented by A. A = number of protons + number of neutrons.
• Electronic configuration: The distribution of electrons in different shells around the nucleus.
• Shell or energy level: A region around the nucleus where electrons are arranged; shells are commonly named K, L, M and N.
• Valence electrons: Electrons present in the outermost shell of an atom.
• Valency: The combining capacity of an atom, usually determined by the number of electrons lost, gained or shared to achieve a stable outer shell.
• Isotopes: Atoms of the same element having the same atomic number but different mass numbers because they contain different numbers of neutrons.
• Rutherford’s nuclear model: A model stating that an atom has a small, positively charged nucleus and that most of the atom is empty space.
• Bohr’s model: A model stating that electrons revolve around the nucleus in fixed shells or energy levels and do not lose energy while remaining in an allowed shell.
• Stable electronic arrangement: An arrangement in which the outermost shell is complete, commonly 2 electrons for the first shell or 8 electrons for many later shells.
• Ion: A charged atom or group of atoms formed by loss or gain of electrons.

Easily Confused

• Atomic number and mass number: Atomic number is the number of protons; mass number is the total number of protons and neutrons.
• Protons and electrons: Protons are positively charged and located in the nucleus; electrons are negatively charged and occupy shells around the nucleus.
• Neutrons and electrons: Neutrons have no charge and are located in the nucleus; electrons have a negative charge and occupy shells.
• Valence electrons and valency: Valence electrons are the electrons in the outermost shell; valency is the atom’s combining capacity.
• Rutherford’s nuclear model and Bohr’s model: Rutherford’s model establishes the small nucleus and empty space; Bohr’s model specifies fixed electron shells or energy levels.
• Isotopes and ions: Isotopes differ in neutron number and mass number; ions differ in electron number and therefore carry a charge.
• Chemical and physical properties of isotopes: Isotopes have nearly identical chemical properties because their electronic configurations are the same, but their physical properties may differ.

What Gets Asked

• Identify subatomic particles from their charge, location and relative mass. The mark-losing error is assigning electrons a relative mass of approximately 1 u instead of 1/1836 u.
• Calculate atomic structure from atomic and mass numbers. Questions require the number of protons, electrons or neutrons; the key error is forgetting that number of neutrons = mass number - atomic number.
• Interpret nuclear notation, ^A_ZX. The specific slip is reversing A and Z: A is the mass number, while Z is the atomic number.
• Write electronic configurations using the shell capacities and given examples. The configuration must be filled from inner to outer shells; the stated examples include hydrogen = 1, carbon = 2,4, oxygen = 2,6, sodium = 2,8,1 and chlorine = 2,8,7.
• Determine valency and explain ion formation. The common error is failing to distinguish elements with 1–4 valence electrons, whose valency is often the number of valence electrons, from those with 5–7, whose valency is often 8 minus that number.
• Distinguish isotopes using atomic number and mass number. Isotopes must have the same number of protons but different numbers of neutrons and mass numbers; examples include hydrogen-1, hydrogen-2, hydrogen-3, chlorine-35, chlorine-37, carbon-12 and carbon-14.