Atomic Structure www.lab-initio.com
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
The Atom The atom consists of two parts: 1. The nucleus which contains: protons neutrons 2. Orbiting electrons.
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
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 • 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: • The nucleus is small • The nucleus is dense • The nucleus is positively charged
Atomic Number Atomic number (Z)of an element is the number of protons in the nucleus of each atom of that element.
Mass Number Mass number is the number of protons and neutrons in the nucleus of an isotope. Mass # = p+ + n0 18 8 8 18 Arsenic 75 33 75 Phosphorus 15 31 16
Isotopes Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.
Atomic Masses Atomic massis the average of all the naturally occurring isotopes of that element. Carbon = 12.011
For neutral atoms (just sitting around, not doing anything): # electrons = # protons - - - - - 0 0 0 0 0 + + + + + Electrons Hydrogen • Atomic number can tell you the # electrons WHEN NEUTRAL Beryllium
- - - - 0 0 0 0 0 + + + + Valence Electrons • In outer most energy level • Responsible for chemical properties & how element reacts Beryllium
Look at the group number at the top of each column. How many valence electrons? 1 8 8 8 8 8 8 8 8 8 8 8 8 8 • This only works for the tall stacks. If it’s double digits, look at the one’s position only.
Groups The columns Groups Groups Groups
All elements in a group (column) have the same number of valence electrons. - - - - 0 0 0 0 Hydrogen + + + + Lithium Groups • They share common chemical properties; they react similarly.
Try determining the valence… • Determine the valence electrons for… • C (carbon) • H (Hydrogen) • Ne (neon)
Periods The rows Periods Periods Periods
Each successive row has one more energy level than the previous one. - - - - - 0 0 0 0 0 Hydrogen + + + + + Beryllium Row #1: 1 energy level The Periods • Set up in rows because certain properties repeat periodically. Row #2: 2 energy levels
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)
In its neutral state, Be looks like this. - - - - 0 0 0 0 0 + + + + Beryllium What do you mean they lose their neutrality? • It’s NOT STABLE because it has only 2 valence electrons. • Finding 6 to fill the outer shell takes too much energy, SO it will ditch the outer 2 leaving the full shell underneath.
In giving away its two valence electrons, its new outer shell is full = STABLE - - - - - - - - - - 0 0 0 0 0 0 0 0 0 0 + + + + + + + + + + + + + + + + + + + + Beryllium Therefore… • BUT now it has 4 protons (+) and only 2 electrons (-) • 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