Study Guide: Atoms, Elements, and Chemical Bonding

Atoms and Elements

Atomic Structure

Atoms are the basic units of matter, consisting of a nucleus (protons and neutrons) surrounded by electrons in energy levels or shells.

• Protons: Positively charged particles found in the nucleus.

• Neutrons: Neutral particles found in the nucleus.

• Electrons: Negatively charged particles found in energy levels around the nucleus.

Valence electrons are the electrons in the outermost shell and are important in chemical bonding.

• The number of valence electrons determines an element's chemical properties.

• Elements in the same group (vertical column) of the periodic table have the same number of valence electrons.

Example: Sodium (Na) has 1 valence electron; Chlorine (Cl) has 7 valence electrons.

Electron Configuration and the Periodic Table

The arrangement of electrons in an atom is called its electron configuration. The periodic table is organized so that elements with similar electron configurations are in the same group.

• Each period (row) corresponds to a principal energy level.

• Group 1 elements (alkali metals) have 1 valence electron.

• Group 18 elements (noble gases) have 8 valence electrons (except Helium, which has 2).

Example: The Lewis dot diagram for chlorine (Cl) shows 7 dots around the symbol, representing its 7 valence electrons.

Chemical Bonding

Ionic Bonds

Ionic bonds form when electrons are transferred from one atom to another, resulting in the formation of ions.

• Cation: A positively charged ion (loses electrons).

• Anion: A negatively charged ion (gains electrons).

• Ionic compounds are formed between metals (which lose electrons) and nonmetals (which gain electrons).

Example: Sodium (Na) transfers one electron to chlorine (Cl), forming Na+ and Cl-, which combine to make NaCl.

Formula for Ionic Compounds: The total positive charge must balance the total negative charge.

• Example: The formula for lithium oxide is Li2O because two Li+ ions balance one O2- ion.

Covalent Bonds

Covalent bonds form when two atoms share one or more pairs of electrons.

• Usually occurs between nonmetal atoms.

• Each shared pair of electrons forms one covalent bond.

Example: Two hydrogen atoms share electrons with one oxygen atom to form H2O (water).

Lewis Dot Structures

Lewis dot structures are diagrams that show the valence electrons around atoms and how they are shared or transferred in bonds.

• Dots represent valence electrons.

• Lines or pairs of dots between symbols represent shared electron pairs (covalent bonds).

Example: The Lewis structure for O2 shows a double bond (two shared pairs) between two oxygen atoms.

Periodic Properties of the Elements

Groups and Periods

The periodic table is arranged in groups (columns) and periods (rows) based on atomic number and electron configuration.

• Group 1: Alkali metals (very reactive, 1 valence electron).

• Group 17: Halogens (very reactive nonmetals, 7 valence electrons).

• Group 18: Noble gases (inert, 8 valence electrons except Helium).

Example: Elements in Group 1 react vigorously with water; elements in Group 18 are generally unreactive.

Octet Rule

Atoms tend to gain, lose, or share electrons to achieve a full set of 8 valence electrons (octet), similar to noble gases.

• This rule explains the formation of most ionic and covalent bonds.

Additional Concepts

Energy Levels and Electron Shells

Electrons occupy energy levels (shells) around the nucleus. Each shell can hold a specific maximum number of electrons:

• First shell: 2 electrons

• Second shell: 8 electrons

• Third shell: 18 electrons

Electrons fill the lowest energy levels first (Aufbau principle).

Classification of Elements

• Metals: Good conductors of heat and electricity, tend to lose electrons.

• Nonmetals: Poor conductors, tend to gain electrons.

• Metalloids: Have properties of both metals and nonmetals.

Sample Table: Maximum Number of Electrons in Each Shell

Shell (n)	Maximum Electrons
1	2
2	8
3	18
4	32

Key Equations and Concepts

• Electron Configuration: Follows the order of filling subshells: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, etc.

• Octet Rule: Atoms are stable with 8 valence electrons.

• Ionic Compound Formula: $\text{Total positive charge} = \text{Total negative charge}$

Examples and Applications

• Formation of NaCl: $\text{Na} \rightarrow \text{Na}^+ + e^-$; $\text{Cl} + e^- \rightarrow \text{Cl}^-$; $\text{Na}^+ + \text{Cl}^- \rightarrow \text{NaCl}$

• Lewis Structure for Water: Oxygen shares electrons with two hydrogens, forming two single covalent bonds.

Additional info: Some questions in the file reference organic compounds and enzymes, which are more relevant to introductory biochemistry or biology, but the majority of the content is focused on atomic structure, periodic properties, and chemical bonding, which are core General Chemistry topics.