Physical & Chemical Properties of Matter: Atomic Structure Models
Physical Properties of Matter
A physical property is a characteristic that can be observed or measured without changing the substance’s composition. These properties are often revealed during physical processes like changes in state, temperature, or pressure.
Examples include color, hardness, density, boiling point, and melting point.
Physical properties are categorized as:
- Electrical
- Magnetic
- Optical
- Thermal
- Mechanical
Physical properties can be general or specific:
- General properties apply to various substances, such as mass, volume, color, and texture.
- Specific properties are unique to each substance, such as density, specific gravity, boiling point, and melting point.
Chemical Properties of Matter
Chemical properties describe how a substance interacts with other substances, resulting in chemical reactions that change the original substances into new ones.
Some chemical properties include reactivity, heating value, and acidity.
Extensive and Intensive Properties
Extensive properties depend on the amount and form of matter, such as weight, volume, length, potential energy, and heat.
Intensive properties are related to the internal chemical structure and do not depend on the amount of matter. Examples include temperature, melting point, boiling point, specific heat, concentration, and refractive index. Intensive properties are useful for identifying and characterizing pure substances.
Atomic Structure Models
Rutherford’s Model
Based on his experiments demonstrating the existence of the atomic nucleus, Rutherford proposed that:
- Almost all of an atom’s mass is concentrated in a tiny, positively charged nucleus.
- Electrons with negligible mass and negative charge orbit the nucleus in circular paths.
- The positive charge of the nucleus and the negative charge of the electrons balance, making the atom electrically neutral.
Bohr’s Model
Bohr postulated that:
- Electrons orbit the nucleus at high speeds in specific energy levels.
- Electrons can absorb energy to jump to a higher energy level and emit energy to return to a lower level.
- The energy of a photon emitted or absorbed during an electron transition corresponds to a specific wavelength and frequency, producing a characteristic spectral line.
Dalton’s Atomic Theory
Dalton’s theory introduced the idea of the discontinuity of matter:
- Matter is composed of indivisible and unchangeable particles called atoms.
- All atoms of the same element are identical.
- Atoms of different elements have different masses and properties.
- Compounds are formed when atoms combine in simple, whole-number ratios.
Note: Current scientific understanding has refined some of Dalton’s postulates. Atoms can be divided and altered, and isotopes exist within elements.
Thomson’s Model
Thomson’s model, also known as the “plum pudding” model, depicted the atom as a sphere of positive charge with negatively charged electrons embedded within it, like raisins in a pudding.
Thomson’s experiments with cathode ray tubes measured the charge-to-mass ratio of electrons.