 # The Structure of an Atom

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The Structure of an Atom
4.2

The Element Song

Three Types of Subatomic Particles
Protons – Positive Charge (+1) Neutrons – Neutral (no charge, 0) Electrons – Negative Charge (-1) Locations Protons & Neutrons: Nucleus of Atom Electrons: Found in the space outside the nucleus

Mass of Subatomic Particles
When we measure the mass of protons, neutrons and electrons, we consider RELATIVE masses That means we compare the mass of protons and neutrons to the mass of electrons Masses Proton = 1 amu (or Atomic Mass Units) Neutron = 1 amu Electron = 0 amu

What makes each atom different?
John Dalton predicted that atoms of any element are different from the atoms of all other elements Atoms of any given element always have the same number of PROTONS Atoms of different elements have different numbers of protons

Atomic Number Equals the number of protons in an atom
Found on the Periodic Table of Elements

Number of Electrons Each positive charge in an atom is balanced by a negative charge because atoms are neutral What does neutral mean? The atomic numbers gives the number of protons AND the number of electrons in a neutral atom of an element

Practice How many protons does Sulfur have?
How many electrons does Sulfur have?

How many protons does hydrogen have? How many electrons does oxygen have? How many protons does Magnesium have? How many electrons does Neon have? How many electrons does Lithium have?

Mass Number To find the number of neutrons in an atom, you have to do some simple math… The mass number of an atom is the sum of the number of protons and neutrons in an atom Protons have a mass of one, neutrons have a mass of one If we add them together we can find the total mass Formula for Finding # of Neutrons Number of Neutrons = Mass Number – Atomic Number

Practice A carbon atom has a mass number of 14 Steps
How many protons does it have? How many neutrons does it have? Steps Look on the Periodic Table Elements to find the Atomic Number of Carbon Subtract the number of protons from the mass number. This is the number of neutrons.

Finding Mass Number If an atom has 2 protons and 2 neutrons…
What is the name of the element? How many electrons does it have in a neutral atom? What is the mass number? If an atom has 11 protons and 12 neutrons…

Isotopes Atoms of the same element that have different numbers of neutrons, and, therefore, different mass numbers For example How many protons does Oxygen have? Sometimes Oxygen has 9 neutrons, sometimes it has 10 neutrons What is the mass number of an atom of oxygen that has 9 neutrons? What is the mass number of an atom of oxygen that has 10 neutrons?

Identifying Isotopes Element Name Oxygen - 18 Mass Number

Practice Nitrogen – 14 Neon – 20 How many protons?
What is the mass number? How many neutrons? How many electrons? Neon – 20

Modern Atomic Theory 4.3

Bohr Model Looks almost like a diagram of a solar system with planets orbiting around the Sun Describes the arrangement of electrons in atoms

Energy Levels or Shells (Bohr Model)
Electrons move in fixed orbits around the nucleus Each electron has a certain amount of energy that determines the energy level that it is in Electrons can NOT exist between energy levels

Electron Cloud Model This model was an improvement on Bohr’s model
Bohr assumed electrons moved like planets in a solar system, but this is actually not the case Electron Cloud Visual model of the most likely locations for electrons in an atom Describes the POSSIBLE locations of electrons around the nucleus At any moment in time we cannot pinpoint the location of an electron

Atomic Orbitals A region of space around the nucleus where an electron is most likely to be found Each energy level contains a certain number of atomic orbitals Each orbital can contain a MAXIMUM of 2 electrons Each Energy has a certain number of atomic orbitals

Energy Levels, Orbitals & Electrons Maximum Number of Electrons
Number of Orbitals Maximum Number of Electrons 1 2 4 8 3 9 18 16 32

Creating a Model of an Atom
We can use the energy level and atomic orbital information in order to create a model of an atom Each energy level can contain a certain number of electrons.

Steps for Making Models of Atoms

Step 1 Find the number of electrons in a neutral atom of the element

Step 2 Draw concentric circles to represent the energy levels

Step 3 Fill up the energy levels from the inside out with the correct number of electrons. REMEMBER: Only 2 electrons will fit in the first level (or circle), 8 in the second, 18 in the third, etc.

Practice Create atomic models for the following elements: Helium Argon
Magnesium Carbon