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 (n 0 ): neutrally charged subatomic particles

What’s in the Electron Cloud?  The 3 rd 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 = = 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  n o = 12  e - = 11

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

Determine the number of subatomic particles in the following:  Cl has a mass # of 35 and an atomic # of 17  p + = ____,n o = ____, e - = ____  K has a mass # of 39 and an atomic # of 19  P + = ___,n o = ___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- H 2 O2 Hydrogen 1 Oxygen

J.J. Thomson  J.J. Thomson was a British scientist who found that atoms contain negatively charged particles in  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.