Atoms and Elements: Structure, Properties, and the Periodic Table

Elements and Chemical Symbols

Definition and Properties of Elements

Elements are pure substances from which all other substances are built. They cannot be broken down into simpler substances by chemical means. Each element is represented by a unique chemical symbol, which consists of one or two letters, always starting with a capital letter.

• One-letter symbols: Examples include C (carbon), N (nitrogen), F (fluorine), O (oxygen).

• Two-letter symbols: Examples include Co (cobalt), Ca (calcium), Al (aluminum), Mg (magnesium).

• Latin names: Some elements have symbols derived from Latin names, such as Ag (silver, argentum) and Au (gold, aurum).

Examples of Elements

Elements can exist in various physical forms, such as metals, powders, or solid objects.

The Periodic Table

Organization and Structure

The periodic table organizes the 118 known elements into groups with similar properties and arranges them in order of increasing atomic mass. Vertical columns are called groups, and horizontal rows are called periods. Group numbers may be written as 1A–8A for representative elements and 3B–12B for transition elements, or as numbers 1–18 across the table.

Group Names and Classifications

Groups have common names, such as alkali metals (Group 1A), alkaline earth metals (Group 2A), halogens (Group 7A), and noble gases (Group 8A).

Metals, Nonmetals, and Metalloids

Classification on the Periodic Table

The periodic table is divided by a heavy zigzag line that separates metals (left), nonmetals (right), and metalloids (along the line). Each category has distinct physical and chemical properties.

Characteristics of Metals, Nonmetals, and Metalloids

• Metals: Shiny, ductile, good conductors of heat and electricity, mostly solid at room temperature (except mercury).

• Nonmetals: Dull, brittle, poor conductors, often good insulators, low densities and melting points.

• Metalloids: Exhibit properties of both metals and nonmetals, used as semiconductors and insulators.

Metalloids on the Periodic Table

Elements Essential to Health

Major, Macrominerals, and Microminerals

Elements play vital roles in biological systems. Major elements in the human body include H, C, N, and O. Macrominerals (Na, Mg, K, Ca, P, S, Cl) are required in larger amounts, while microminerals (Si, V, Cr, Mn, Fe, Co, Cu, Zn, As, Se, Mo, I) are needed in trace amounts.

The Atom

Definition and Structure

An atom is the smallest particle of an element that retains its chemical properties. Atoms consist of a nucleus (containing protons and neutrons) and electrons that occupy the space around the nucleus.

Dalton’s Atomic Theory

• Atoms are tiny particles of matter.

• Atoms of the same element are similar; those of different elements are different.

• Atoms combine to form compounds.

• Atoms are rearranged in chemical reactions but are never created or destroyed.

Subatomic Particles

Types and Properties

Atoms contain three main subatomic particles:

• Protons: Positive charge (+), located in the nucleus.

• Neutrons: No charge (neutral), located in the nucleus.

• Electrons: Negative charge (−), located outside the nucleus.

Models of the Atom

J. J. Thomson’s "plum pudding" model described atoms as having protons and electrons scattered throughout.

Rutherford’s gold foil experiment demonstrated that atoms have a dense nucleus.

Structure and Mass of the Atom

The nucleus contains most of the atom’s mass, while electrons occupy a large volume around it. The mass of subatomic particles is measured in atomic mass units (amu), where 1 amu is defined as 1/12 the mass of a carbon-12 atom.

Subatomic Particle Table

Properties of Subatomic Particles

The following table summarizes the properties of protons, neutrons, and electrons:

Particle	Symbol	Charge	Mass (amu)	Location in Atom
Proton	p or p+	+1	1.007	Nucleus
Neutron	n or n0	0	1.008	Nucleus
Electron	e−	−1	0.00055	Outside nucleus

Atomic Number and Mass Number

Atomic Number

The atomic number is a whole number specific to each element and equals the number of protons in the atom. It appears above the element symbol in the periodic table.

For neutral atoms, the number of protons equals the number of electrons, resulting in a net charge of zero.

Mass Number

The mass number represents the total number of protons and neutrons in the nucleus. It is always a whole number and is specific to each isotope of an element.

• Number of protons = atomic number

• Number of neutrons = mass number − atomic number

• Number of protons + neutrons = mass number

Composition of Atoms Table

Element	Symbol	Atomic Number	Mass Number	Number of Protons	Number of Neutrons	Number of Electrons
Hydrogen	H	1	1	1	0	1
Nitrogen	N	7	14	7	7	7
Oxygen	O	8	16	8	8	8
Chlorine	Cl	17	37	17	20	17
Iron	Fe	26	58	26	32	26
Gold	Au	79	197	79	118	79

Isotopes and Atomic Mass

Definition and Properties of Isotopes

Isotopes are atoms of the same element with different mass numbers. They have the same number of protons but different numbers of neutrons. Isotopes are distinguished by their atomic symbols, which show the mass number and atomic number.

Atomic Symbols and Subatomic Particles

Given an atomic symbol, you can determine the number of protons, neutrons, and electrons:

• Number of protons = atomic number

• Number of neutrons = mass number − atomic number

• Number of electrons = number of protons (for neutral atoms)

Isotope	Atomic Number	Mass Number	Number of Protons	Number of Neutrons
O, 16O	8	16	8	8
P, 31P	15	31	15	16
Zn, 65Zn	30	65	30	35

Key Equations

• Atomic number: $\text{Atomic number} = \text{Number of protons}$

• Mass number: $\text{Mass number} = \text{Number of protons} + \text{Number of neutrons}$

• Number of neutrons: $\text{Number of neutrons} = \text{Mass number} - \text{Atomic number}$

Summary Table: Metals, Nonmetals, Metalloids

Category	Location	Properties
Metals	Left of zigzag line	Shiny, ductile, good conductors, mostly solid
Nonmetals	Right of zigzag line	Dull, brittle, poor conductors, low density
Metalloids	Along zigzag line	Properties of both, semiconductors

Additional info: Academic context was added to clarify definitions, properties, and relationships between atomic structure and periodic table organization.