Chemistry: Elements and Atomic Theory — Quick Review chapter 4

Element Basics

Elements are the simplest substances; there are 118 known elements, many human-made with lifetimes shorter than milliseconds.
An element cannot be broken down into simpler substances.
Names of elements come from various sources (planets, mythology, colors, places, scientists).
- Examples: Uranium (named after the planet Uranus), Titanium (from Titan), Chlorine (from Greek chloros, greenish-yellow), Magnesium (from a mineral), Tennessee (location origin), Curie (Marie Curie), Copernicus (scientist).

Symbols are abbreviations, usually from the first letter or the first two letters of the name.
- Examples: Uranium → \mathrm{U}, Titanium → \mathrm{Ti}, Chlorine → \mathrm{Cl}, Magnesium → \mathrm{Mg}.
Some symbols come from Latin names (e.g., Silver → \mathrm{Ag} from Argentum, Gold → \mathrm{Au} from Aurum).
You should know the name and symbol for elements listed in the provided table; you may be asked to match one to the other.

The table has 18 groups (columns) and 7 periods (rows).
Elements in the same group share similar chemical properties; periods are horizontal sequences.
Group naming (common system): 1A, 2A (main-group); 3A–6A (main-group/nonmetals region); 7A (often halogens, with some context mentioning hydrogen); 8A (noble gases).
The main body is supplemented by two separate rows: lanthanides (elements 58-71) and actinides (elements 90-103). These are shown separately to keep the main table rectangular,
but their chemical properties are similar to the elements directly above them.
Each block shows the element symbol, full name, and the atomic number (top of the block).

Main-group elements: groups 1A-2A and 3A-8A (often labeled as A groups).
Transition metals: groups B (3B–8B, including 3B, 4B, 5B, 6B, 7B, 8B, with 1B and 2B at the ends).
1A (alkali metals): Li, Na, K, Rb, Cs, etc.; 2A (alkaline earth metals): Be, Mg, Ca, etc.
Metalloids: sit along the zigzag boundary between metals and nonmetals; typically have properties between metals and nonmetals; Silicon (Si) is the most famous metalloid (semiconductor).
Elements with sharp regional color coding: red (major elements in the body), blue (nonmetals), green (metalloids), gray (transition metals). The highlighted regions help identify common names: 1A = alkaline metals, 2A = alkaline earth metals, 7A = halogens, 8A = noble gases.

Metals: shiny, malleable, high density, high melting points; most are solid at room temperature (except Mercury, a liquid metal).
Nonmetals: poor conductors, often dull, can be powders; variable densities and melting points.
Metalloids: properties between metals and nonmetals; semiconductors; crucial for electronics (e.g., Silicon).
The table lists the symbols for the metalloids (roughly 12, 34, 56, 78 9 in the transcript’s shorthand; the actual metalloids are B, Si, Ge, As, Sb, Te, Po, etc.).

Major elements in the human body (most abundant): Hydrogen (H), Carbon (C), Nitrogen (N), Oxygen (O).
- About 70\% of body weight is water (H and O together); carbon is essential for organic chemistry; nitrogen is abundant in proteins.
Major minerals in body fluids (macro minerals): Sodium (Na), Magnesium (Mg), Potassium (K), Calcium (Ca), Phosphorus (P), Sulfur (S), Chlorine (Cl).
Trace (micro) minerals: Manganese (Mn), Iron (Fe), Cobalt (Co), Copper (Cu), Zinc (Zn) – required in small amounts; many act as cofactors in enzymes.
Ions and charge: ions carry either positive (cation) or negative (anion) charges.
- Common cations in the body: \mathrm{Na^+}, \mathrm{K^+}, \mathrm{Mg^{2+}}, \mathrm{Ca^{2+}}
- Common anions in the body: \mathrm{Cl^-}, \mathrm{PO_4^{3-}}, \mathrm{S^{2-}}
Balance: body remains electrically neutral through balanced positive and negative ions.

Atom: the smallest unit of an element that retains the properties of that element.
Subatomic particles inside the atom: Protons (positive charge), Neutrons (neutral), Electrons (negative charge).
- Proton: +1 charge; mass similar to neutron.
- Neutron: neutral; no charge.
- Electron: -1 charge; much smaller than protons/neutrons.
Dalton’s Atomic Theory (four key points):
1) Atoms are the basic, tiny particles of matter.
2) Atoms of a given element are identical; different from atoms of other elements.
3) Compounds are formed by chemical combinations of atoms of different elements.
4) In chemical reactions, atoms are rearranged to form new substances;
reactions are rearrangements of atoms into new molecules.

J. J. Thomson: discovered the electron; proposed the plum pudding model (positively charged 'pudding' with embedded electrons).
Rutherford’s Gold Foil Experiment: alpha particles directed at a thin gold foil; most pass through, some deflected, a few rebounded.
- Conclusion: atoms are mostly empty space with a very small, dense nucleus at the center.
- This nucleus contains protons (and neutrons) and is positively charged; electrons orbit around at a distance.

Atomic number: number of protons in the nucleus; defines the element.
Most atoms are neutral overall; electrons balance protons.
Transition from plum pudding to nuclear model marked a major shift in understanding atomic structure.
Periodic table organization (groups/periods) underpins element properties and reactivity.