Atomic Structure and Chemical Bonding

The One Thing

Atomic structure determines how electrons are arranged, especially in the outermost shell. Atoms form ionic or covalent bonds by losing, gaining, or sharing valence electrons to achieve a more stable electronic configuration.

Reactions, Processes and Experiments

Key Terms

• Atom: The smallest unit of an element that retains the chemical properties of that element.
• Nucleus: The dense central part of an atom containing protons and neutrons.
• Proton: A positively charged subatomic particle present in the nucleus.
• Neutron: An electrically neutral subatomic particle present in the nucleus.
• 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; it is represented by Z.
• Mass number: The total number of protons and neutrons in an atom; it is represented by A.
• Electronic configuration: The arrangement of electrons in the different shells of an atom.
• Valence electrons: Electrons present in the outermost shell of an atom.
• Valency: The combining capacity of an atom, usually related to the number of electrons lost, gained, or shared to achieve stability.
• Octet rule: The principle that atoms generally become stable when their outermost shell contains eight electrons, except for the first shell, which is stable with two.
• Ion: An electrically charged atom or group of atoms formed by the loss or gain of electrons.
• Cation: A positively charged ion formed when an atom loses one or more electrons.
• Anion: A negatively charged ion formed when an atom gains one or more electrons.
• Chemical bond: The force of attraction that holds atoms or ions together in a compound or molecule.
• Electrovalent or ionic bond: A bond formed by the complete transfer of electrons from one atom to another, followed by attraction between oppositely charged ions.
• Covalent bond: A bond formed when two atoms share one or more pairs of electrons.
• Single covalent bond: A covalent bond formed by sharing one pair of electrons.
• Double covalent bond: A covalent bond formed by sharing two pairs of electrons.
• Triple covalent bond: A covalent bond formed by sharing three pairs of electrons.
• Lewis dot structure: A representation of an atom or molecule using dots to show valence electrons and shared electron pairs.
• Molecule: The smallest independent particle of a substance that can exist and shows the properties of that substance.
• Isotopes: Atoms of the same element having the same atomic number but different mass numbers because they contain different numbers of neutrons.

Additional relationships and rules:

• Atomic number: $Z=$ number of protons $=$ number of electrons in a neutral atom.
• Mass number: $A=$ number of protons $+$ number of neutrons.
• Number of neutrons: $A-Z$.
• The outermost shell is the valence shell.
• Atoms with completely filled outer shells, such as noble gases, are generally chemically unreactive.
• Metals usually form cations by losing valence electrons, whereas non-metals usually form anions by gaining electrons.
• Ionic compounds contain oppositely charged ions arranged in a crystal lattice.
• Covalent compounds generally have low melting and boiling points and usually do not conduct electricity because they lack free ions.
• A coordinate bond is a covalent bond in which both shared electrons are provided by the same atom.
• Chemical formulae show the elements present and the ratio or number of their atoms in a compound.

Easily Confused

• Atomic number and mass number: Atomic number $Z$ is the number of protons, whereas mass number $A$ is the total number of protons and neutrons.
• Cation and anion: A cation is formed by electron loss and is positive; an anion is formed by electron gain and is negative.
• Ionic and covalent bonding: Ionic bonding involves complete electron transfer and electrostatic attraction, whereas covalent bonding involves electron sharing.
• Valence electrons and valency: Valence electrons are the electrons in the outermost shell; valency is the atom’s combining capacity.
• Octet rule and duplet rule: The octet rule concerns stability with eight outer-shell electrons, whereas the duplet rule applies to the first shell, which is complete with two electrons.
• Molecule and ion: A molecule is the smallest independent particle of a substance, whereas an ion carries an electrical charge.
• Single, double, and triple covalent bonds: These involve the sharing of one, two, and three pairs of electrons respectively.
• Ionic and covalent compound properties: Ionic compounds generally conduct electricity when molten or dissolved because their ions can move; covalent compounds generally do not because they lack free ions.
• Atomic identity and electron arrangement: An element’s identity is determined by its number of protons, while its chemical properties and valency are largely determined by its electron arrangement.

What Gets Asked

• Questions may require calculation of protons, electrons, or neutrons using $Z$, $A$, and $A-Z$. The mark-losing slip is confusing atomic number with mass number.
• Questions may ask for an electronic configuration using the shell capacity rule $2n^2$, the K, L, M, and N shells, and the filling order from lower to higher energy. The specific issue is exceeding the stated outer-shell limit for the first 20 elements.
• Questions may ask why atoms form bonds or how metals and non-metals form ions. Marks depend on identifying electron loss for cations and electron gain for anions.
• Questions may require explaining sodium chloride formation. The essential details are that sodium forms Na+ by losing one electron, chlorine forms Cl− by gaining one electron, and the ions are held by electrostatic attraction.
• Questions may ask students to distinguish single, double, triple, and coordinate covalent bonds using H2, O2, N2, and NH4+. The common error is confusing the number of shared electron pairs or failing to identify that both electrons in a coordinate bond come from one atom.
• Questions may compare ionic and covalent compounds. The required distinctions concern crystal lattices, melting and boiling points, and electrical conductivity when ionic compounds are molten or dissolved.
• Questions may involve writing formulae using element symbols, valencies, and the criss-cross method. The main risk is producing an incorrect ratio of atoms or failing to use valencies correctly.