science 2

• All forms of matter are made up of particles.
• building blocks of matter

  Scientists call this characteristic as particulate nature of matter
• These particles, described as the building blocks of matter, are called atoms.
• • All elements are composed of particles called atoms
• All atoms of the same element are identical
• Atoms of different elements combine to form compounds
• • Atoms of one element can never be changed into another element
• Thales thought that all matter was made of water.
• Anaximenes thought that matter was made of air.
• Heraclitus believed that fire made up all matter on earth.
• •Aristotle (Greek 384-322 BC)

  • Rejected the theory of atoms

  • Believed in 4 core elements fire, air, water, earth.
• • Democritus (Greek 460-370 BC)

  •1st to use the word atomos

  •Believed that atoms were indivisible & indestructible •No scientific support.
• The word "atom" is derived from the Greek word, “atomos” or indivisible
• Atom is the smallest unit of matter that retains the identity of the substance
• Atom is thought to be the smallest particle of a given element.
• The Atomic Theory

  • 2000 years later, it became a theory when John Dalton put together many observations and results of several experiments done by other scientists.
• john dalton

  The postulates of his theory, which is found in his book entitled A New System of Chemical Philosophy published in 1808.
• Dalton’s Atomic Theory

  1. All matter is made of indestructible atoms, which are tiny, hard spheres.
  2. 2. All the atoms of the same element are alike; the atoms of various elements are different and possess different properties (including different masses)
• Dalton’s Atomic Theory

  3. When matter undergoes physical or chemical change, the atoms are merely rearranged. No atom is created or destroyed.

  4. Atoms of different elements combine in a definite ratio of small whole numbers when they form compounds.
• All samples of an element, regardless of size or source, have the same properties.
• Compounds have a definite composition, regardless of how the samples were prepared or where they originated.
• • For example, table sugar, and sucrose from sugarcane have the same proportion of carbon, hydrogen, and oxygen as that obtained from sugar beets. This is called the law of definite proportion.
• Conservation of mass

  • During a physical or chemical change, the total mass of all substances before and the total mass of all substances after the change are the same.
• According to the atomic theory, during changes, only rearrangements of atoms take place.
• • The total mass remains the same since no new atoms are created and none are destroyed. This is called the law of conservation of mass.
• Law of Definite Proportion

  • Joseph Proust –French Chemist / 1797

  • A compound is always composed of the same elements, in the same proportion by mass.
• • Antoine Lavoisier -French Chemist / 1789

  • Matter is not created or destroyed during a chemical reaction
• • Following Dalton's atomic theory, there was a surge in scientific interest in:

  • Discovering new subatomic particles

  • Developing methods and equipment to study matter in greater detail
• • Today, Dalton's Atomic Theory undergoes a significant modification. Atoms are divisible because of the discovery of the Subatomic Particles.
• Cathode Ray Experiment (1897)

  • A series of experiments conducted by scientists including Joseph John Thomson in 1897 played a pivotal role in the discovery of the electron, the fundamental building block of atoms with a negative charge.
• • Based on these observations, scientists like Thomson concluded that the cathode ray was a stream of negatively charged particles, later named electrons.
• Thomson's Raisin Bread Model of the Atom by revising Dalton's atomic theory.
• raisin bread model

  • Proposed a positively charged sphere as the basic structure of the atom.
• • Electrons, negatively charged particles, were scattered throughout the sphere like raisins in bread.
• Radioactivity and Subatomic Particles

  •Henri Becquerel discovered a mineral (pitchblende) emitting invisible rays (Becquerel rays), hinting at the existence of even smaller atomic components.
• Radioactivity and Subatomic Particles

  • Pierre and Marie Curie built on this discovery by finding elements (polonium and radium) that emitted similar rays.
• • They proposed these rays came from the breakdown of atoms themselves (radioactive decay).
• •Alpha (α) particles:

  • Tiny, positively charged particles with a lot of mass (like helium nuclei without electrons).
• Beta (β) particles:

  • High-energy electrons, confirming their presence within all atoms.
• Gamma (γ) radiation: • High-energy waves similar to X-rays, not particles.