Developing atomic theory

Learning goals We are learning how the atomic theory developed over time. We are learning of the contributions of various scientists to the atomic theory.

atoms the smallest part of an element that has all of the element’s properties modern term “atom” from Greek atomos meaning indivisible

Atomic theory the study of the nature of atoms and how they combine to form all types of matter. helps to understand why there are different kinds of atoms explains how atoms combine to form over 100 known elements and all other forms of matter, including compounds and mixtures.

Your task Become an expert of a scientist who contributed to the discovery of the atom and their contribution to the atomic theory. Share your acquired knowledge with your peers.

Early views Democritus (440 BCE) Aristotle All matter can be broken down into smaller and smaller pieces until single indivisible particle Atom – cannot be cut Different sizes, constant motion, separated by empty space Aristotle Earth made of 4 basic substances: earth, water, air, fire 4 qualities: dry, wet, cold, hot

dalton All matter is made of small, indivisible particles called atoms All the atoms of an element are identical in properties such as size and mass Atoms of different elements have different properties Atoms of different elements can combine in specific ways to form new substances

thompson researched idea that atoms might be made from a combination of particles discovered negative particles reasoned there must be positive particles too depicts the atom as a positive sphere with negative electrons scattered throughout it

rutherford discovered the nucleus, the centre of the atom tiny positively charged part of the atom also contains most of the atom’s mass determined nucleus to be very tiny compared to rest of atom electrons surrounded the nucleus and occupied most of the atom’s volume, but they contained only a small fraction of the atom’s total mass

chadwick refined concept of nucleus discovered nucleus contains neutral particles as well as positively charged particles called neutrons positively charged particles in the atom are called protons each neutron in an atom has about the same mass as each proton

bohr suggested electrons surround the nucleus in specific energy levels, called shells discovered electrons jump between shells by gaining or losing energy each shell can contain only a specific number of electrons max number of electrons that can exist in each of the first three shells is two, eight, and eight

Quantum mechanical model electrons do not exist as tiny points inside an atom electrons exist in specific energy levels, but they surround the positively charged nucleus in a form resembling a cloud

Atom summary all elements are composed of atoms, and one atom is the smallest unit of any element. atoms themselves are made of different kinds of smaller particles, called subatomic particles three subatomic particles are protons, neutrons, and electrons, and they have different properties

Properties of subatomic particles Name Symbol Relative Mass Electric Charge Location Proton p 1836 1+ nucleus Electron e 1 1- in energy levels surrounding nucleus Neutron n 1837

Watch! History of Atomic Theory

Learning goals We are learning how the atomic theory developed over time. We are learning of the contributions of various scientists to the atomic theory.