Matter, Elements, & Atomic Structure

Matter Matter is anything that has mass and takes up space. Ex: buildings, a fork, air, your clothes, etc… The ancient Greeks believed that all matter is made up of four elements: air, earth, fire, and water. Much later, scientists realized that matter is composed of many different elements.

Elements Elements are the simplest pure substances, and they cannot be broken down into any other substances. Elements are often called the building blocks of matter because all matter is composed of one element or a combination of two or more elements.

What is an atom? An atom is the smallest particle of an element (or unit of matter) that retains the identity of the substance.

Atomic Structure Atoms are composed of 2 regions: Nucleus: the center of the atom that contains the mass of the atom Electron cloud: region that surrounds the nucleus that contains most of the space in the atom Nucleus Electron Cloud

What’s in the Nucleus? The nucleus contains 2 of the 3 subatomic particles: Protons (p+) : positively charged subatomic particles Neutrons (n0): neutrally charged subatomic particles

What’s in the Electron Cloud? The 3rd subatomic particle resides outside of the nucleus in the electron cloud. Electron (e-): the subatomic particle with a negative charge and relatively no mass

How do these particles interact? Protons and neutrons live compacted in the tiny, positively charged nucleus accounting for most of the mass of the atom. The negatively charged electrons are small and have a relatively small mass but occupy a large volume of space outside the nucleus.

How do the subatomic particles balance each other? In an uncharged atom: The protons = the electrons If 20 protons are present in an atom then 20 electrons are there to balance the overall charge of the atom—atoms are neutral or uncharged. The neutrons have no charge; therefore they do not have to equal the number of protons or electrons.

How do we know the number of subatomic particles in an atom? Atomic number: this number indicates the number of protons in an atom. Ex: Hydrogen’s atomic number is 1 So hydrogen has 1 proton Ex: Carbon’s atomic number is 6 So carbon has 6 protons **The number of protons identifies the atom. Ex. 2 protons = He, 29 protons = Cu

How do we know the number of subatomic particles in an atom? Mass number: the number of protons and neutrons in the nucleus. Ex: hydrogen can have a mass of 3. Since it has 1 proton it must have 2 neutrons # of neutrons = mass # - atomic #

Determining the number of protons and neutrons Li has a mass number of 7 and an atomic number of 3 Protons = 3 (same as atomic #) Neutrons= 7-3 = 4 (mass # - atomic #) Ne has a mass number of 20 and an atomic number of 10 Protons = 10 Neutrons = 20 - 10= 10

What about the electrons? The electrons are equal to the number of protons (in an uncharged/neutral atom). So e- = p+ = atomic # Ex: Na has a mass # of 23 and an atomic # of 11 p+ = 11 no = 12 e- = 11

Where Do You Find “these” Numbers? Atomic Number Symbol Name Atomic Mass (this is an average, always round) protons electrons Atomic number equals the number of _________or __________. protons neutrons Atomic mass equals the number of _________ + ___________.

Determine the number of subatomic particles in the following: Cl has a mass # of 35 and an atomic # of 17 p+ = ____, no = ____, e- = ____ K has a mass # of 39 and an atomic # of 19 P+ = ___, no = ___ e- = ____

Early Ideas about Atoms!!! Democritus- a Greek Philosopher who lived about 440 B.C. He thought you could“cut” down matter into pieces, until you reached the smallest possible piece He named these pieces atoms which is Greek for “uncuttable”.

Early Ideas about Atoms!! John Dalton- a British school teacher In 1802 proposed an Atomic Theory. He carried out experiments and concluded that atoms have different characteristics. Atomic Theory grew as a series of models that developed from experimental evidence. As more evidence was collected, the theory and models were revised. To this day, scientists have found few exceptions to Dalton’s theories.

Dalton’s Atomic Theory Atoms can’t be broken into smaller pieces. Atoms of the same element are exactly the same. Atoms of different elements are different Atoms of 2 different elements combine to form compounds. Atoms of each element have a unique mass. The masses of the elements in a compound are always in a constant ratio. Ex. Water- H2O 2 Hydrogen 1 Oxygen

J.J. Thomson J.J. Thomson was a British scientist who found that atoms contain negatively charged particles in 1897. Because scientists knew that atoms didn’t have an electrical charge, Thomson reasoned that atoms also contained some sort of positive charge. Thomson described an atom as having negative charges scattered throughout a ball of positive charge.

Earnest Rutherford In 1911, one of Thomson’s students found evidence that went against his model. Rutherford’s research team aimed a beam of positively charged particles at a thin sheet of gold foil. They found that some particles passed through without problem, but some deflected strongly. Since like charges repel, Rutherford inferred there had to be a cluster of positive charges in a tiny region (called a nucleus).

Earnest Rutherford Scientists knew that electrons had almost no mass, so they reasoned that nearly all of an atom’s mass must be located in the tiny, positively charged nucleus. Later it was suggested that the nucleus was made of one or more positively charged particles. Rutherford named those particles – protons.

What is the structure of an atom? Bohr Model “Planetary Model” Bohr model – electrons are in specific energy levels Schrödinger Model “Electron Cloud Model” Electron cloud model – electrons are in a random cloud

James Chadwick In 1932, British scientist James Chadwick discovered another particle in the nucleus of atoms. His discovery completed the modern atomic model. It was the hardest to detect because it had no charge. This electrically neutral particle was called a neutron.