Atomic Structure

Expectations

Modern Atomic Theory  All matter is composed of atoms  Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!  Atoms of an element have a characteristic average mass which is unique to that element.  Atoms of any one element differ in properties from atoms of another element  All matter is composed of atoms  Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!  Atoms of an element have a characteristic average mass which is unique to that element.  Atoms of any one element differ in properties from atoms of another element

The Atom The atom consists of two parts: 1. The nucleus which contains: 2. Orbiting electrons. protons neutrons

Protons Protons are positively charged, and are located in the nucleus –p –Same number as electrons

Electrons Negatively charged. They orbit around the nucleus. – e- –Same number as protons

Neutrons No charge (neutral). Found in the nucleus. Can be a different number of these than either protons or electrons –Note: This is important

Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle. Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Conclusions from the Study of the Electron  Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.  Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons  Electrons have so little mass that atoms must contain other particles that account for most of the mass  Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.  Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons  Electrons have so little mass that atoms must contain other particles that account for most of the mass

Thomson’s Atomic Model Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

Rutherford’s Gold Foil Experiment  Alpha (  ) particles are helium nuclei  Particles were fired at a thin sheet of gold foil  Particle hits on the detecting screen (film) are recorded

Rutherford’s Findings  The nucleus is small  The nucleus is dense  The nucleus is positively charged  Most of the particles passed right through  A few particles were deflected  VERY FEW were greatly deflected “Like howitzer shells bouncing off of tissue paper!” Conclusions:

Atomic Particles

Atomic Number Atomic number (Z) Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.

Mass Number Mass number Mass number is the number of protons and neutrons in the nucleus of an isotope. Mass # = p + + n Arsenic Phosphorus

Isotopes Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.

Atomic Masses Atomic mass Atomic mass is the average of all the naturally occurring isotopes of that element. Carbon =

Electrons and Bonding

Electrons For neutral atoms (just sitting around, not doing anything): # electrons = # protons Atomic number can tell you the # electrons WHEN NEUTRAL Atomic number can tell you the # electrons WHEN NEUTRAL + - Hydrogen Beryllium

Valence Electrons In outer most energy level Beryllium Responsible for chemical properties & how element reacts Responsible for chemical properties & how element reacts

How many valence electrons? Look at the group number at the top of each column. This only works for the tall stacks. If it’s double digits, look at the one’s position only

Groups The columns Groups

Groups All elements in a group (column) have the same number of valence electrons. + - Hydrogen Lithium They share common chemical properties; they react similarly. They share common chemical properties; they react similarly.

Try determining the valence… Determine the valence electrons for… –C (carbon) –H (Hydrogen) –Ne (neon)

Periods Periods The rows

The Periods Each successive row has one more energy level than the previous one. + - Hydrogen Beryllium Row #1: 1 energy level Row #2: 2 energy levels Set up in rows because certain properties repeat periodically. Set up in rows because certain properties repeat periodically.

Compound Substance formed when atoms of different elements combine chemically Properties of a compound are different than the properties of the elements that form it

Why do atoms chemically combine? To become STABLE For most atoms, this means obtaining 8 valence electrons Exception- the first energy level is full with only 2 electrons But to be stable, atoms lose their neutrality (they gain a positive or negative charge)

What do you mean they lose their neutrality? In its neutral state, Be looks like this Beryllium It’s NOT STABLE It’s NOT STABLE because it has only 2 because it has only 2 valence electrons. valence electrons. Finding 6 to fill the outer Finding 6 to fill the outer shell takes too much shell takes too much energy, SO it will ditch the energy, SO it will ditch the outer 2 leaving the full shell underneath. outer 2 leaving the full shell underneath.

Therefore… In giving away its two valence electrons, its new outer shell is full = STABLE Beryllium BUT now it has 4 protons (+) and only 2 electrons (- ) BUT now it has 4 protons (+) and only 2 electrons (- ) Has a +2 charge now = NOT NEUTRAL Has a +2 charge now = NOT NEUTRAL

So how do atoms chemically combine? They can SHARE electrons Or they can GIVE AWAY or TAKE electrons Only the valence electrons are involved

Covalent Bonds Form when atoms SHARE their valence electrons These electrons circulate between the valence shells of both atoms Usually occurs between two nonmetals Forms a molecule: a particle whose atoms are covalently bonded

Ionic Bonds Formed by atoms GIVING and TAKING valence electrons Each atom becomes a charged particle or ION Opposite charges attract, so the atoms stick together Usually forms between a metal and nonmetal –Metals give away electrons –Nonmetals take electrons