Atomic Structure Chapter 4, Section 1

Atomic History In the fourth century BC, the Greek philosopher Democritus suggested that the universe was made of invisible units called atoms. The word atom is derived from the Greek work meaning unable to be divided. The problem: Democritus was unable to provide the evidence needed to convince people that atoms really existed.

Atomic History, Cont. In 1808, an English school teacher named John Dalton proposed that atoms could not be divided and that all atoms of a given element were exactly alike. Dalton’s theory is considered the foundation for the modern atomic theory. Dalton’s theory was developed with scientific basis and was accepted by others.

Atomic History, Cont. In , Rutherford ran experiments to determine the structure of an atom. When positively charged particles are fired into gold foil, most pass straight through while a few are violently deflected. This implies a dense, positively charged central region containing most of the atomic mass and that the atom is mostly space.

The Gold Foil Experiment Rutherford expected the alpha particles to go straight through the gold foil. Instead, some of the alpha particles were deflected, implying a central positively charged region (nucleus).

Atomic History, Cont. In 1913, the Danish scientist Niels Bohr suggested that electrons in an atom move in set paths (energy levels) around the nucleus much like the planets orbit the sun. Electrons can only be in certain energy levels and must gain energy to move to a higher energy level or lose energy to move to a lower energy level.

Atomic History, cont. In the 1920’s deBroglie & Shrodinger showed that the “solar system” model of the atom was incorrect. Instead, electrons orbit the nucleus in orbitals. This is called quantum mechanics.

The Atomic Model PROTONS –Charge is positive –Found in the nucleus NEUTRONS –Charge is neutral –Found in the nucleus ELECTRONS –Charge is negative –Found in the energy levels (electron cloud).

Anything Smaller? Protons & Neutrons are each made of 3 quarks. What is a quark? Don’t worry about it…it’s past the scope of this class – just know that they exist. ?

The Atomic Model, cont. There is an equal number of protons and electrons in an atom. This causes every atom to be neutral: Charge of two protons:+2 Charge of two neurons: 0 Charge of two electrons: -2 Total charge of atom: 0

Atomic Model, cont. Each energy level can hold only a certain number of electrons. –1 st energy level can hold up to 2 electrons – this level must be completely filled before putting electrons in the next level –2 nd level can hold up to 8 electrons (also must fill this level before filling next level)

Location of Electrons – The Problem with Bohr’s Model An electron’s exact location cannot be determined. Imagine the moving blades of a fan – If you were asked where any one of the blades was located at a certain instant, you would not be able to give an exact answer – the blades are moving too quickly! It is the same with electrons –the best a scientist can do is calculate the chance of finding an electron in a certain place within an atom To visually show the likelihood of finding an electron, shading is used. The darker the shading, the better the chance of finding an electron. This shaded region is called an electron cloud.

Orbitals Electrons exist in energy levels, but the regions in an atom where electrons are likely to be found are called orbitals. There are four different kinds of orbitals.

s-orbital Simplest orbital Shaped like a sphere. Can hold up to 2 electrons

p-orbital Shaped like a dumbbell. Can be oriented three different ways – x, y, and z Each orientation can hold 2 electrons for a total of 6 electrons in each energy level First found in the 2 nd energy level.

Click to See Shading of Each Orbital

d-orbital, f-orbital More complex – past the scope of this class There are 5 possible d-orbitals There are 7 possible f-orbitals