Scientists of the Atom & Atomic Structure

Scientists of Atoms Greeks –Democritus: Matter is composed of small units called atoms Nothing is smaller than an atom Atoms of a solid were round and spikey, Atoms of a liquid were round and smooth –Aristotle: Atoms are forever divided into smaller parts

Dalton –All elements are composed of atoms. –All atoms of the same element have the same mass. Atoms of different elements have different masses. –Compounds contain atoms of more than 1 element. –In a particular compound, atoms of different elements always combine in the same way.

Thomson Experiment –Like charges repel, opposite charges attract –Negative charges attract positive charges and vice versa. –Beam of charged particles sent through a tube with a positive disk and a negative disk. The beam bent toward the positively charged disk. Discovery –Atoms are made up of subatomic particles that have charges. –Plum Pudding model: negative charges are spread out in a positively charged mass

Rutherford –Experiment Alpha particles = positively charged Some went through, a few bounced back

Rutherford –Discovery An Atom’s positive charges are concentrated in center of atom (called the nucleus).

Bohr Bohr Model –Focused on electrons –Move in fixed orbits around nucleus

Electron Cloud Model –Nucleus has Protons and Neutrons –Electrons are moving at high speeds around nucleus –Electrons do not orbit in fixed distances around nucleus, however exists in a cloud (where electrons probably are). –Cloud is denser at those locations where the probability of finding an electron is high. –Airplane propeller analogy.

Parts of an Atom Nucleus - inside –Made of Protons (positive charge) Neutrons (neutral)

Parts of an Atom Electron Cloud - outside –Made of Electrons (negative charge) Same number of protons and electrons! (Same number of positives as negatives!!)

If the protons & electrons are the same in every atom the same element…. …then what is the charge of a typical atom? 2 protons & 2 electrons +2 + (-2) = 0 NEUTRAL!! Atoms are typically neutral or they have no charge!

Atomic Number and Mass Number Atomic Number = number of protons in an atom (PAN) Mass Number = number of protons and neutrons (massive)

How many Protons? Electrons? Neutrons?

Atomic Mass Notice how it has decimals? It is an AVERAGE!!!! There are different atoms of the same element…we call these ISOTOPES.

Isotopes Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes.

The Nucleus is the Heaviest Part… The Neutrons and Protons are VERY HEAVY!! –They are made of even smaller part called QUARKS. The Electrons are so small they are almost WEIGHTLESS!! The “Rings” don’t actually exist, they are just drawn in the picture to illustrate the path of the orbits.

Orbits in the Electron Cloud Electrons spin and circulate around the nucleus. Electrons orbit the nucleus in “rings” or energy levels. Notice how the electrons orbit the nucleus in many different dimensions. Remember, the atom is 3D!

Energy Levels or “Rings” For our purposes (very simplified), there are four rings around the nucleus. The closest ring holds 2 electrons. The second ring holds 8 electrons. The third ring holds 8 electrons. The fourth ring holds 2 electrons.

Different areas of the electron cloud have different levels, each representing a different amount of Energy. (Think of levels in a parking garage). There are up to SEVEN levels where electrons can exist, depending on how much energy they have. How many rows are in the Periodic Table?

Periodic Table

Energy levels closest to the nucleus are filled with lower energy electrons. The farther away the levels get from the nucleus, the higher the energy of the electrons in that level. Electrons always fill up the lower energy levels first, just like cars in a parking garage fill in the spaces closest to the ground floor, then spread farther up as the spots get filled.

The energy levels are sometimes also called orbitals. The chemical properties of atoms depend on how many electrons are in the outermost or “valence” energy level.

Think of it this way: an atom “likes” to have its outermost level filled with electrons. The Octet Rule states that atoms are most stable when their valence shell contains eight electrons.

How does the location of an element in the periodic table tell us where its electrons of its atoms are located? Which atoms will react similarly, those in the same row, or those in the same column?

Something to Think About… What is the difference between an atom and a cell? Cells are the smallest LIVING things. Atoms are the smallest part of EVERYTHING and they are not alive!