Atomic Structure Objectives: -Democritus’s theory -Dalton’s theory -Bohr’s Model vs. Modern Model of the Atom Chapter 3- Section 1

Democritus Ancient Greek philosopher – 4 th century B.C Suggested that the universe was made of invisible units called atoms His theory explained some observations but could not provide enough evidence to convince people that atoms really existed

John Dalton 19 th century -Dalton proposed a modern atomic model based on experimentation not on pure reason. Every element is made of tiny, unique particles called atoms Atoms of the same element are identical/ exactly alike Atoms of different elements combine in constant ratios to form compounds. Dalton’s theory was widely accepted because there was much evidence to support it

1808 John Dalton Dalton’s Model of the Atom: – Uniform Shape – Uniform Density – Indivisible So what did Dalton’s model look like? A simple design that promoted future research into atomic theory…

The Atom Small, dense center called the nucleus Nucleus is positively charged and made up of protons and neutrons Nucleus is surrounded by tiny negatively charged particles called electrons

What’s in an Atom? Atoms are made up of 3 subatomic particles: Protons- positively charged particle, found in the nucleus Neutrons – neutral particle (no charge), found in the nucleus Electrons- tiny negatively charged particle, moving outside the nucleus

Subatomic Particles ParticleChargeMass (kg)Location in the atom Proton x In the nucleus Neutron01.67 x In the nucleus Electron9.11 x Moving around outside the nucleus

Atoms have NO overall charge Atoms are not charged even though they are made up of charged particles Atoms have an equal number of protons (positive charge) and electrons (neg. charge) whose charges exactly cancel. Ex: Helium (He) – 2 protons, 2 neutrons, 2 electrons = 0 total charge

Models of the Atom All previous models contributed to modern model of the atom New discoveries required consideration of new models Like most scientific models and theories, the model of the atom has been revised many times

Bohr’s Model In 1913, Bohr suggested that electrons in an atom move in set paths around the nucleus much like planets orbit the sun

Modern Theory Electrons behave more like waves on a vibrating string Electrons exist in orbitals (areas of probability) Demo– just like turning blades of a fan, the exact position, speed, direction and location of electrons in an atom cannot be determined

Modern Model of the Atom Small, dense nucleus consisting of protons and neutrons Electrons exist in orbitals within each energy level Space between energy levels decreases at higher energy (higher energy levels are closer together)

Valence Electrons Valence electrons- electrons in the outermost shell of an atom and determine the atom’s chemical properties Valence electrons are the electrons in an atom that participate in chemical bonding. Every atom has between one and eight valence electrons.

How many valence electrons?? Br Al K Mg Cl S N

Orbitals Orbitals- a region in an atom where there is a high probability of finding an electron Electrons are not in neat fixed orbits like Bohr proposed but in orbitals. – 3-D region around the nucleus that indicates the probable location (90%) of an electron.

Orbitals An orbital is the actual region of space occupied by an electron in a particular energy level. There are many different types of orbitals and each have their own fundamental shape. They are designated by letters s, p, d,and f.

So how many orbitals are there and what do they look like??

s-subshell Consists of one s orbital Has a spherical shape It can hold 2 electrons Every energy level has an s-orbital How many electrons can occupy an s- subshell? 1 orbital x 2 electrons = 2

p-subshell Consists of 3 p orbitals Shaped like a dumbbell Each orientation can hold two electrons Energy levels 2 and higher have 3 p orbitals How many electrons may occupy the p-subshell ? 3 orbitals x 2 electrons each= 6

d- subshell Consists of 5 d orbitals Each orientation can hold two electrons Energy levels 3 and higher have 5 d orbitals How many electrons may occupy a d- subshell? 5 orbitals x 2 electrons per orbital= 10

f- subshell Consists of 7 f orbitals Each of these orientations can hold up to 2 electrons Energy levels 4 and higher have 7 f orbitals How many electrons may occupy an f-subshell ? 7 orbitals x 2 electrons per orbital= 14

Check it out! jNgq16jEY#t=86

Sublevel Summary Sublevel Number of Orbitals it contains Totals number of electrons s12 p36 d510 f714 (2 x number of orbitals)

Electron Configuration!! What is it? Electron configuration of an atom is a notation that describes the a)energy levels b)orbitals (s, p, d or f) c)total number of the electrons in each orbital. ab c

When you are giving directions to your house, you need to include several pieces of information…. What town, name of street, number of house, etc. The same is true for giving the “address” of an electron in an atom of an element.

What information do we need in order to write the address (electron configuration) of electrons in an element? 1) Which energy level(s) does it occupy? 2) Shapes of the orbitals – considering different orientations 3) How many total electrons??

An electron’s “address” will be different (specific) for each element….. WHY?

How do we use this info? The periodic table is organized based on electron configuration.

Diagonal rule: the order of filling

HOW TO: write electron configurations Step 1) Look up the atomic number of the element to determine the number of electrons Step 2) Consult the periodic table (or arrow diagram) to determine the order in which the sublevels should be filled Step 3) Write the configuration, filling in up to two electrons in each “s” subshell, up to 6 in “p”, up to 10 in “d”, and up to 14 in “f” Step 4) When you think you are finished, add up the exponents (superscripts) to see if you have the correct number of electrons