Unit 2 Atomic Structure Read Chapter 4

Matter: Something that occupies space and has mass. Atom: The basic unit of matter.

Greek philosopher Democritus believed all matter was made of extremely small particles. (About 400 B.C.) He used the greek word Atomos, which means “uncut” or “indivisible”, to describe atoms.

John Dalton’s Theory – late 1700’s All elements are made of atoms Atoms of the same element have the same mass, and different elements have different masses Compounds have atoms from more than one element Atoms in particular compounds always combine in the same ratios

Joseph John (J.J.) Thomson Cathode tube experiment

Thomson provided the first evidence that atoms were made of smaller particles. Since an atom was neutral and he discovered negative particles he reasoned in had positive particles as well – Plum Pudding Model

J.J. Thomson’s Plum Pudding model Dalton’s solid sphere model J.J. Thomson’s Plum Pudding model

Ernest Rutherford Gold Foil Experiment - 1909

He learned that protons have a positive charge and are located in the nucleus

Rutherford Atomic Model

The atom is made up of 3 subatomic particles: Proton Neutron Electron

The nucleus (Protons & Neutrons) account for 99% of the atom’s mass. Particle Charge Mass Location Proton 1 Nucleus Neutron Electron -1 Outside The nucleus (Protons & Neutrons) account for 99% of the atom’s mass.

There are 92 naturally occurring elements. The # of protons = the # of electrons This makes atoms neutral in charge Element: A pure substance made up of entirely one type of atom which cannot be broken down by chemical processes into smaller substances. There are 92 naturally occurring elements.

4 elements which make up 96% of living matter are: Carbon Hydrogen Oxygen Nitrogen

Each element has a one or two letter symbol. (Ex. Na – Sodium) The Atomic number indicates how many protons are found in that element. The Atomic mass indicates the average weight of the element.

Niels Bohr Bohr agreed with Rutherford’s model of the nucleus but he focused on the electrons The electrons moved at a constant speed in fixed orbits (energy levels) around the nucleus

Today we use the electron cloud model Bohr was correct in assigning energy levels but incorrect that they moved like planets around the sun. Today we use the electron cloud model

This model describes the most likely locations for electrons as they travel around the nucleus

Energy levels and orbitals are NOT the same thing! Energy level is the range of distance electrons are from the nucleus Orbital – a region of space around the nucleus where an electron is likely to be found Video Clip

Each orbital can contain 2 electrons at the most. Except for the first energy level, an energy level may have more than one orbital The maximum number of electrons in an energy level is twice that of the number of orbitals

Electron Configuration is the arrangement of electrons in an atom The most stable configuration is the one in which the electrons are in the orbitals with the lowest possible energies

An electron can move from one energy level to another as it gains or loses energy

When all electrons have the lowest possible energies the atom is said to be in its ground state When an electron absorbs enough energy to move to an orbital with a higher energy it is referred to as an excited state

When excited electrons move to lower energy levels they may emit some energy as light

Don’t mess with Protons! The number of electrons and neutrons within an element may change but not the number of protons. Don’t mess with Protons!

Isotopes: Atoms of the same element that have a different number of neutrons.

Isotopes are named according to their weight (Ex. Carbon 14)

Uses of Radioactive Isotopes Determine the ages of rocks and fossils 2. Treat cancer and kill bacteria which cause food to spoil 3. Used as “tracers” for medical purposes

This image, taken from medical scan of a 47-year old woman, shows leukemia present in the bone marrow before treatment (left) and after.

Ions: Positively or negatively charged atoms. Ions form when electrons are given or taken away from another atom

Loses an electron = positive charge This is called a cation B B 5 (+) 5 (+) 5 (0) 5 (0)

B B Gains an electron = negative charge This is called an anion 5 (+) 5 (0) 5 (0)

Chlorine Ion with 1 extra electron = Cl¯ Chlorine Ion with 2 extra electrons = Cl2- Sodium Ion with 1 less electron = Na+ Sodium Ion with 2 less electrons = Na2+