Atoms and Elements: Structure, Properties, and Periodic Trends

Atoms and Elements

Introduction

Understanding atoms and elements is fundamental to chemistry. Atoms are the basic building blocks of matter, and elements are pure substances composed of only one type of atom. The arrangement and properties of atoms determine the behavior of elements and their compounds.

Classification of Matter

Pure Substances vs. Mixtures

Pure substances consist of only one type of particle and include elements and compounds. Mixtures contain two or more substances physically combined.

• Element: A pure substance that cannot be broken down into simpler substances.

• Compound: A pure substance composed of two or more elements chemically combined.

• Mixture: A combination of two or more substances that retain their individual properties.

Elements and Their Symbols

Element Names and Sources

Element names are derived from various sources, including planets, mythology, minerals, colors, geographic locations, and famous people.

Element	Symbol	Source of Name
Uranium	U	The planet Uranus
Titanium	Ti	Titans (mythology)
Chlorine	Cl	Chloros: "greenish yellow" (Greek)
Iodine	I	Iodes: "violet" (Greek)
Magnesium	Mg	Magnesia, a mineral
Californium	Cf	California
Curium	Cm	Marie and Pierre Curie
Copernicium	Cn	Nicolas Copernicus

Chemical Symbols

Chemical symbols are one- or two-letter abbreviations for element names, often derived from English or Latin names.

Name	Symbol	Name	Symbol
Aluminum	Al	Gold	Au
Carbon	C	Oxygen	O
Chlorine	Cl	Silver	Ag
Iron	Fe	Hydrogen	H
Mercury	Hg	Platinum	Pt
Potassium	K	Sodium	Na
Calcium	Ca	Zinc	Zn

Examples of Elements

• Carbon (C): Exists in several forms, including graphite, diamond, buckminsterfullerene, and nanotubes.

• Mercury (Hg): A liquid metal at room temperature, toxic to humans.

The Periodic Table

Organization and Structure

The periodic table arranges elements by increasing atomic number and groups elements with similar properties into columns (groups) and rows (periods).

• Groups: Vertical columns with similar properties.

• Periods: Horizontal rows numbered 1–7.

Group Names and Types

• Alkali Metals (Group 1A): Highly reactive metals including Li, Na, K, Rb, Cs.

• Halogens (Group 7A): Reactive nonmetals including F, Cl, Br, I.

• Representative Elements: Groups 1A–8A.

• Transition Elements: Groups 3B–12B.

Elements Essential to Life

Twenty elements are essential for human health, with oxygen, carbon, hydrogen, and nitrogen making up 96% of body mass. Macrominerals such as Ca, P, K, Cl, S, Na, and Mg are vital for physiological functions.

Structure of the Atom

Subatomic Particles

Atoms consist of protons, neutrons, and electrons. Protons and neutrons are located in the nucleus, while electrons occupy the space around the nucleus.

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

Dalton’s Atomic Theory

• Atoms are tiny particles of matter.

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

• Atoms combine to form compounds.

• Atoms are rearranged in chemical reactions but are not created or destroyed.

Electrical Charges in Atoms

Protons are positively charged, electrons are negatively charged, and neutrons are neutral. Like charges repel, unlike charges attract.

Rutherford’s Gold Foil Experiment

This experiment demonstrated the existence of a small, dense, positively charged nucleus in the atom.

Atomic Mass Unit (amu)

One atomic mass unit is defined as one-twelfth the mass of a carbon-12 atom. Protons and neutrons have masses close to 1 amu; electrons are much lighter.

Atomic Number and Mass Number

Definitions and Calculations

• Atomic Number: Number of protons in an atom; unique to each element.

• Mass Number: Sum of protons and neutrons in the nucleus.

• Neutral Atoms: Number of protons equals number of electrons.

Element	Symbol	Atomic Number	Mass Number	Protons	Neutrons	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

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons and thus different mass numbers. They are represented by atomic symbols showing mass and atomic numbers.

Isotopes of Magnesium

Atomic Symbol	Name	Protons	Electrons	Mass Number	Neutrons	Mass (amu)	Percent Abundance
$^{24}_{12}\text{Mg}$	Mg-24	12	12	24	12	23.99	78.70
$^{25}_{12}\text{Mg}$	Mg-25	12	12	25	13	24.99	10.13
$^{26}_{12}\text{Mg}$	Mg-26	12	12	26	14	25.98	11.17

Calculating Atomic Mass

The atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes, based on their percent abundance.

• Formula: $\text{Atomic mass} = \sum (\text{isotope mass} \times \text{percent abundance})$

Isotope	Mass (amu)	Abundance (%)	Contribution (amu)
$^{35}\text{Cl}$	34.97	75.76	26.49
$^{37}\text{Cl}$	36.97	24.24	8.962
Atomic mass of Cl (weighted average mass)			35.45

Electron Energy Levels

Energy Levels, Subshells, and Orbitals

Electrons occupy energy levels (shells) around the nucleus, which are divided into subshells and orbitals. Each energy level can hold a maximum number of electrons given by $2n^2$.

• Energy Levels: n = 1, 2, 3, 4, ...

• Subshells: Regions within shells with electrons of similar energy.

• Orbitals: Regions within subshells where electrons are most likely found; each orbital holds up to 2 electrons.

Electron Configurations and Orbital Diagrams

Electron configuration describes the distribution of electrons among subshells. Orbital diagrams show electron arrangement in orbitals.

• Example: Oxygen: 1s22s22p4

• Example: Sulfur: 1s22s22p63s23p4

• Example: Calcium: 1s22s22p63s23p64s2

Periodic Trends

Valence Electrons and Group Number

Valence electrons are the electrons in the outermost energy level. The group number for representative elements indicates the number of valence electrons.

Group Number	Number of Valence Electrons
1A (1)	1
2A (2)	2
3A (13)	3
4A (14)	4
5A (15)	5
6A (16)	6
7A (17)	7
8A (18)	8

Electron-Dot Symbols (Lewis Structures)

Electron-dot symbols represent valence electrons as dots around the element symbol.

Atomic Size

Atomic size increases down a group and decreases across a period due to changes in energy levels and nuclear charge.

Ionization Energy

Ionization energy is the energy required to remove an electron. It decreases down a group and increases across a period.

Metallic Character

Metallic character refers to the tendency of an element to lose electrons. It increases down a group and decreases across a period.

Summary of Periodic Trends

Periodic Property	Top to Bottom (Group)	Left to Right (Period)
Valence Electrons	Remains the same	Increases
Atomic Size	Increases	Decreases
Ionization Energy	Decreases	Increases
Metallic Character	Increases	Decreases

Concept Map

The concept map below summarizes the relationships between elements, atomic structure, periodic table organization, and periodic properties.