Atoms and Elements

Introduction to Elements

Elements are the fundamental substances from which all matter is constructed. They cannot be broken down into simpler substances by chemical means. Each element is represented by a unique chemical symbol, often derived from its English or Latin name.

• Element: A pure substance consisting of only one type of atom.

• Chemical Symbol: One- or two-letter abbreviation for an element (e.g., Al for aluminum).

• Example: Sulfur is an element with the symbol S.

Sources of Element Names

Element names originate from various sources, including planets, mythological figures, minerals, colors, geographic locations, and famous people.

• Uranium: Named after the planet Uranus.

• Titanium: Named after Titans in mythology.

• Chlorine: From Greek 'chloros' meaning greenish yellow.

• Gold: Symbol Au from Latin 'aurum'.

Chemical Symbols and Their Usage

Chemical symbols are standardized abbreviations. If two letters are used, only the first is capitalized.

• Co: Cobalt

• CO: Carbon and oxygen (compound)

Common Elements and Their Symbols

Health and Toxicity: Mercury

Mercury (Hg) is a liquid metal at room temperature. It is toxic and can enter the body via inhalation, skin contact, or ingestion. Mercury disrupts proteins and cell function, causing damage to the brain and kidneys.

• Testing: Blood, urine, and hair samples are used to detect mercury exposure.

The Periodic Table

Structure and Organization

The periodic table arranges 118 elements by increasing atomic number and groups elements with similar properties.

• Groups: Vertical columns (1–18 or 1A–8A for representative elements).

• Periods: Horizontal rows (1–7).

Group Names and Representative Elements

• Group 1A (1): Alkali metals (Li, Na, K, Rb, Cs, Fr)

• Group 2A (2): Alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra)

• Group 7A (17): Halogens (F, Cl, Br, I, At, Ts)

• Group 8A (18): Noble gases (He, Ne, Ar, Kr, Xe, Rn, Og)

Alkali Metals and Halogens

• Alkali Metals: Highly reactive, soft metals (Li, Na, K).

• Halogens: Reactive nonmetals (Cl2, Br2, I2).

Classification: Metals, Nonmetals, Metalloids

The periodic table is divided by a zigzag line:

• Metals: Left of the line; shiny, ductile, malleable, good conductors.

• Nonmetals: Right of the line; dull, brittle, poor conductors.

• Metalloids: Along the line; properties intermediate between metals and nonmetals.

Comparison Table: Metal, Metalloid, Nonmetal

Elements Essential to Health

Major Elements and Macrominerals

Four elements (O, C, H, N) make up 96% of body mass. Macrominerals (Ca, P, K, Cl, S, Na, Mg) are vital for bones, teeth, heart, blood vessels, muscle contraction, nerve impulses, and metabolism.

Microminerals (Trace Elements)

Trace elements (Fe, Si, Zn, Cu, Mn, I, Mo, As, Cr, Co, Se, V) are present in small amounts and are essential for various biological functions.

• Iron (Fe): Component of hemoglobin.

• Iodine (I): Required for thyroid function.

• Zinc (Zn): Needed for DNA synthesis and immune function.

The Atom

Atomic Structure and Subatomic Particles

Atoms are the smallest units of elements, retaining their properties. They consist of a nucleus (protons and neutrons) and electrons in surrounding space.

• Proton: Positive charge, mass ≈ 1 amu, in nucleus.

• Neutron: No charge, mass ≈ 1 amu, in nucleus.

• Electron: Negative charge, mass ≈ 0.00055 amu, outside nucleus.

Dalton’s Atomic Theory

John Dalton proposed that all matter is made of atoms, atoms of each element are identical, compounds are formed by combinations of atoms, and chemical reactions rearrange atoms.

Electrical Charges in Atoms

• Like charges repel; unlike charges attract.

J. J. Thomson’s Cathode Ray Experiment

Thomson discovered electrons and proposed the "plum-pudding" model, where electrons are embedded in a positively charged cloud.

Rutherford’s Gold Foil Experiment

Rutherford showed that atoms have a small, dense, positively charged nucleus surrounded by electrons.

Subatomic Particle Table

Atomic Number and Mass Number

Atomic Number

The atomic number is the number of protons in an atom and defines the element. It is shown above the element symbol in the periodic table.

Mass Number

The mass number is the sum of protons and neutrons in the nucleus. It applies to individual atoms, not the average atomic mass.

• Number of neutrons = mass number - atomic number

Composition Table for Elements

Isotopes and Atomic Mass

Isotopes

Isotopes are atoms of the same element with different mass numbers due to varying numbers of neutrons.

• Example: Carbon has three isotopes: C-12, C-13, C-14.

Atomic Symbol

The atomic symbol shows the mass number (upper left) and atomic number (lower left) for a specific isotope.

Atomic Mass

Atomic mass is the weighted average of all naturally occurring isotopes of an element, listed below the symbol in the periodic table.

Isotopes of Magnesium

Electron Energy Levels

Electromagnetic Radiation and Atomic Spectrum

Electromagnetic radiation includes radio waves, microwaves, visible light, and X-rays. The atomic spectrum is unique for each element and results from electron energy changes.

Electron Energy Levels

Electrons occupy specific energy levels (quantized), denoted by principal quantum numbers (n = 1, 2, 3...). Energy increases with distance from the nucleus.

Electron Arrangements for the First 20 Elements

Trends in Periodic Properties

Periodic Properties

Periodic properties such as atomic size, ionization energy, and metallic character change predictably across periods and groups.

Summary Table: Periodic Trends

Lewis Symbols

Lewis symbols represent valence electrons as dots around the element symbol.