Atomic Structure, Periodic Table Trends, and Element Classification

Atomic Structure and Isotopes

Understanding Atomic Structure

Atoms are composed of three main subatomic particles: protons, neutrons, and electrons. The number of protons defines the element, while the sum of protons and neutrons gives the atomic mass. Isotopes are atoms of the same element with different numbers of neutrons.

• Protons (p+): Positively charged particles found in the nucleus. The atomic number equals the number of protons.

• Neutrons (n0): Neutral particles in the nucleus. Neutron number = mass number - atomic number.

• Electrons (e-): Negatively charged particles orbiting the nucleus. In a neutral atom, electrons = protons.

• Isotopes: Atoms with the same number of protons but different numbers of neutrons.

Example Table: Atomic Structure and Isotopes

Atomic #	Mass #	Symbol	Protons	Neutrons	Electrons	Nuclear Symbol	Name of Isotope
8	18	O	8	10	8	$^{18}_{8}\text{O}$	Oxygen-18
17	35	Cl	17	18	17	$^{35}_{17}\text{Cl}$	Chlorine-35
35	79	Br	35	44	35	$^{79}_{35}\text{Br}$	Bromine-79
27	59	Co	27	32	27	$^{59}_{27}\text{Co}$	Cobalt-59
15	31	P	15	16	15	$^{31}_{15}\text{P}$	Phosphorus-31
1	1	H	1	0	1	$^{1}_{1}\text{H}$	Hydrogen-1

Classification of Elements

Metals, Non-metals, and Metalloids

Elements are classified based on their physical and chemical properties:

• Metals: Good conductors of heat and electricity, malleable, ductile, and usually solid at room temperature.

• Non-metals: Poor conductors, not malleable or ductile, can be solid, liquid, or gas at room temperature.

• Metalloids: Have properties intermediate between metals and non-metals.

Examples by Periodic Table Groups

• Group 14, Period 3: Silicon (Si) – Metalloid

• Group 2, Period 6: Barium (Ba) – Metal

• Noble Gas, Period 2: Neon (Ne) – Non-metal (Noble gas)

• Halogen, Period 4: Bromine (Br) – Non-metal (Halogen)

Rutherford's Nuclear Model of the Atom

Key Conclusions from Rutherford's Experiments

Ernest Rutherford's gold foil experiment led to the modern understanding of atomic structure. His main conclusions were:

1. The atom is mostly empty space.

2. Most of the mass and all of the positive charge are concentrated in a small, dense nucleus.

3. Electrons orbit the nucleus at relatively large distances.

Periodic Trends

Atomic Radius

The atomic radius is the distance from the nucleus to the outermost electron shell. It generally increases down a group and decreases across a period.

• Example: Na (Sodium) has a larger atomic radius than K (Potassium) because atomic radius increases down a group.

• Example: Sr (Strontium) has a larger atomic radius than Ba (Barium) because atomic radius increases down a group.

Ionization Energy

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

• Example: Br (Bromine) has a higher ionization energy than I (Iodine) or Te (Tellurium) because it is higher up and further right on the periodic table.

Metallic Character

Metallic character refers to how easily an atom can lose electrons. It increases down a group and decreases across a period.

• Example: Cs (Cesium) is the most metallic among C, Li, Cr, Cs, and Xe.

Summary Table: Periodic Trends

Trend	Across a Period (Left to Right)	Down a Group (Top to Bottom)
Atomic Radius	Decreases	Increases
Ionization Energy	Increases	Decreases
Metallic Character	Decreases	Increases

Key Formulas

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

• Nuclear Symbol: $^{A}_{Z}\text{X}$, where A = mass number, Z = atomic number, X = element symbol