The Atom


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The Atom Theory

A breakdown of the atom
Nucleus

Nucleus

The small dense core of an atom consisting of neutrons and protons. Neutrons and protons have approximately the same mass.

Neutron

Neutron

A neutral particle located in the nucleus of an atom.

Proton

Proton

A positively charged particle located in the nucleus of an atom.

Electron

Electron

A negatively charged particle located in the volume of space surrounding the nucleus of an atom.

Charge

Charge

Electrons and protons have charge that is equal and opposite, because of this we can say the relative charge in atomic units is one.

A neutral atom has the same number of protons as electrons.

Particle Charge (Coulombs) Mass (kilograms) Relative Charge Mass (unified atomic mass units)
Proton \(+1.6\) x \(10^{-19}\,C\) \(1.673\) x \(10^{-27}\,kg\) +1 1.0073 \(\,u\)
Neutron No Charge \(1.675\) x \(10^{-27}\,kg\) 0 1.0087 \(\,u\)
Electron \(-1.6\) x \(10^{-19}\,C\) \(9.109\) x \(10^{-31}\,kg\) -1 \(5.4858\) x \(10^{-4}\,u\)

Unified atomic mass units (u):

\(\frac{1}{12}\) the mass of a carbon atom containing six protons and six neutrons.

Using this measurement, the mass of a proton and neutron is approximately \(1 \,u\).

The masses of all elements are defined using unified atomic mass units.

\(1\,u = 1.661\)x\(10^{-27}\,kg = \frac{1}{12}\) the mass of a carbon atom

atom notation

Z = number of protons = atomic number

A = number of protons + number of neutrons = mass number

Element

One or more atoms of the same type.

Chemical Symbol (X)

A one or two letter abbreviation assigned to each element.

Atomic Number (Z)

The number of protons in the nucleus of an atom. The type of atom/element is determined by the number of protons.

Mass Number (A)

The sum of the number of neutrons and protons in the nucleus of an atom.

The number of protons defines the type of atom or element!

Isotopes are atoms of the same type that have the same number of protons and varying numbers of neutrons.

 
 
isotopes

We know:

  • The number of protons determines the type of atom or element.
  • Neutrons and protons comprise the nucleus of an atom and have the same approximate mass.

Isotopes:

  • The number of protons determines the type of atom or element.
  • Isotopes of elements are distinguished by the mass number (A).

Isotopes Example:

Atoms of an element that have a different number of neutrons.

silicium periodic table element
Writing Isotope Notation examples



28-14=14 neutrons


29-14=15 neutrons


30-14=16 neutrons

Isotope Notation

Including the atomic number is redundant when representing isotopes. It is common to use the following notation: X-A.
For example: Si-28, Si-29, Si-30

We know:

  • Isotopes are atoms of the same type that have a different number of neutrons.
  • The mass of an atom is dictated by the mass of protons and neutrons because the mass of an electron is negligible in comparison.

Atomic Mass:

  • Is the weighted average of isotopic masses based on the natural abundance of each isotope.
  • The natural abundance is the relative amount of each isotope found in a natural sample of any element. Natural abundance is relatively constant and unique for each element.

The atomic mass of an element as seen on the periodic table is a weighted average of isotopic masses of that element.

Atomic Mass Example:

copper sample example

This Copper (Cu) sample will contain approximately 30.83 % of Cu-65 and 69.17 % of Cu-63

Atomic Mass Calculations:

Atomic mass for an element can be calculated with the following formula:

$$\sum_{n} \text{(fraction of isotope n) x (mass of isotope n)}$$

In simple terms this formula is telling us that atomic mass can be calculated by:

1) Multiplying each isotopic mass of an element by its relative abundance.

2) Adding the resulting masses of the isotopes together.

An atomic mass calculation is included in the example section!

A measurement similar to a pair, dozen or score. A pair is 2 objects, a dozen is 12 objects and a score is 20 objects.

1 mole \(= 6.022\)x\(10^{23}\) objects = Avogadro's number

crowd of people

One mole of people is the Earth’s population multiplied by \(9\)x\(10^{13}\).

crowd of people

One mole of bricks is equivalent to \(2\)x\(10^{14}\) Great Walls of China.

crowd of people

One mole of sand is the entire Sahara Desert!

Defining the Mole:

The mole is defined using the C-12 isotope.

One mole = number of atoms in exactly 12 g C-12.

12 g of C-12 = 1 mole of C-12 atoms = exactly \(6.02\)x\(10^{23}\) C-12 atoms

The mass of one mole of C-12 atoms is 12 g

carbon on a scale

1 mole of carbon (C) = \(6.02\)x\(10^{23}\) atoms

Molar Mass:

Using C-12 as a standard for the mole gives a relationship between mass, number of atoms, and unified atomic mass units.

The mole can be used to count atoms based on their mass.

The molar mass of an element is the mass of one mole of each element.

carbon on a scale

1 mole of carbon (C) \(6.02\)x\(10^{23}\) atoms

sulfur on a scale

1 mole of sulfur (S) \(6.02\)x\(10^{23}\) atoms

gold on a scale

1 mole of gold (Au) \(6.02\)x\(10^{23}\) atoms

Molar Mass of an Element:

The molar mass of an element is the mass of one mole of each element.

Molar mass = grams mole = g mol = the mass of the 6.02 x 10 23 atoms of each element.

Molar mass is numerically equivalent to an element’s mass in unified atomic mass units u .

Molar Mass \(\frac{\text{g}}{\text{mol}}\) is numerically equivalent to atomic mass \(u\)

carbon square from the periodic table

Metals, Non-Metals and Metalloids

Periodic Table

Periodic Trends

Metals:

Generally good conductors of heat and electricity

Tend to lose electrons in a chemical reaction

Non-Metals:

Generally poor conductors of heat and electricity

Tend to gain electrons in a chemical reaction

Metalloids:

Have properties of both metals and non-metals.


Main Group Elements and Transition Metals

Periodic Table

Periodic Trends

Main Group:

Elements that have predictable properties

Transition Metals:

Elements that have unpredictable properties.

Lanthanoids and Actinoids:

Elements that are placed separately for a more compact periodic table. They have similar properties to the elements lanthanum and actinium.


Groups and Periods of the Periodic Table

Periodic Table

Periodic Trends

Each column in the periodic table is called a group and numbered  1-18

Each row in the periodic table is called a row and numbered 1-7

The elements in a group have similar properties.

For example: The noble gases in group 18 are all relatively unreactive.

Common Main Group Ions:

Ions on the Periodic Table

We know:

  • The number of protons determines the type of atom or element.
  • Neutrons and protons comprise the nucleus of an atom, and have the same approximate mass.

Ions:

  • One or more atoms that has gained or lost an electron(s).
  • Ions are charged particles.
  • The periodic table can help predict which ions will form.

Ions and Groups:

  • A main group non-metal will generally gain electrons.
  • A main group metal will generally lose electrons.

Ions are charged particles that have gained or lost an electron(s).

Cation (+):

One or more atoms that has lost an electron(s).

A main group METAL will LOSE electrons to form a CATION with the same number of electrons as the nearest noble gas.

Anion (-):

One or more atoms that has gained an electron(s).

A main group NONMETAL will GAIN electrons to form an ANION with the same number of electrons as the nearest noble gas.

Ion Notation:

Ions are represented as the chemical symbol of the ion with the charge of the ion noted as a superscript to the right.

\(\text{X}^{\text{charge}}\)

Lithium ion \(=\text{Li}^{\text{+}}\), Oxygen ion \(=\text{O}^{2-}\)

Examples

Problem Set

The Atom Problem Set

Below are two documents. One is practice problems, the second is the same problems with solutions.
They can be downloaded and changed to suit your needs.

Click or Tap the images below to view.

Practice Problems

The Atom problem set

Problem Solutions

The Atom problem set solutions

Quiz

Atomic Structure


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Atomic Structure Theory

Bohr Model of an atom

Bohr Model of an Atom

An electron's position described as a trajectory.

Quantum Mechanical Model of an atom

Quantum Mechanical Model of an Atom

An electron's position described as a probability distribution.

Quantum Mechanical Model of an atom

Schrodinger - Quantum Mechanical Models

Quantum Mechanical Model of an Atom. An electron's position described as a probability distribution.


Orbital:

The likely position of an electron in an atom described by a probability distribution map.

Graph of probability distribution

Quantum Numbers = Electron's "Address"

Principal Quantum Number (n)

Orbital’s size and energy level of an electron.

A whole number 1,2,3,4…..

n increases with an electron’s energy and distance from the nucleus.

Each level is called a shell.

Angular Momentum Quantum Number (l)

The shape of an orbital.

0,1,2,3,…,n-1

Each value for l is a subshell

Each number value of l is assigned a letter s,p,d,f,g….

Magnetic Quantum Number (\(m_{l}\))

Orientation of orbital in 3D space.

Value= integer values from –l…l.

There are 2(l)+1 values of \(m_{l}\) for each value of l.

Orbital Shapes = Angular Momentum Number (l)

Values of (l) angular momentum
Number Letter
0 s
1 p
2 d
3 f
4 g

Letter values continue according to the alphabet after f, with the exception of J.

s,p,d,f,g,h,i,k,….. Sober Physicists Don’t Find Giraffes Hiding In Kitchens


s Orbital:

l=0, \(m_{l} = 0\)

One allowed value for \(m_{l}\) = one type of s orbital for each value of n.

S orbitals

p Orbitals:

l=0, \(m_{l} = -1, 0, 1\)

Three allowed values for \(m_{l}\) = three types of p orbitals for n values greater than and equal to two.

P orbitals

d Orbitals:

l=0, \(m_{l} = -2, -1, 0, 1, 2\)

Five allowed values for \(m_{l}\) = five types of d orbitals for n greater than and equal to three.

D orbitals

Spin is intrinsic to all electrons, it is quantized, with only two orientations; spin up or spin down.

Value \(=+\frac{1}{2}\) OR \(-\frac{1}{2}\)

spin quantum number diagram

No two electrons in an atom can have the same four quantum numbers, however, Any three quantum numbers can be the same.

This implies: One orbital may contain two electrons of opposite spin orientation!

In a one electron system the subshells are degenerate; meaning s,p,d,f,.., all have the same energy level in each shell. However, in a multi-electron atom the subshells have different energy levels.  

Single electron atom

Single electron atom

Multi-electron atom

Multi-electron atom

The Aufbau Principle:

That means that we want to know about a system in which the lower energy orbitals fill before the higher energy orbitals. This idea is referred to as the Aufbau principle, and the general order in which electrons fill orbitals in an atom to achieve the ground state configuration is: 1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,5p,6s,4f,5d,6p,7s,5f,6d,7p

Aufbau Principle diagram

Electrons will singly occupy degenerate orbitals until they are half full before pairing up with other electrons. Electrons in half full orbitals will have the same spin.

Hund’s Rule diagram

A notation that represents the orbitals occupied by electrons in an atom.

Write the value of n, followed by the number of electrons allowed in each orbital until all electrons in the atom are assigned to an orbital.

Example Carbon: 6 electrons = \(1s^{2}2s^{2}2p^{2}\)

Core Electrons

Electrons in a filled inner shell.

Valence Electrons

Electrons in the outermost shell (highest n value).

Example Carbon:
6 electrons = \(\stackrel{1s^{2}}{Core}\) \(\stackrel{2s^{2}2p^{2}}{Valence}\)

Short-hand Electronic Configuration

An electronic configuration using a noble gas to represent the filled shells and only writing out the valence electrons.

Example Carbon:
6 electrons = \(\stackrel{He}{Core}\) \(\stackrel{2s^{2}2p^{2}}{Valence}\)

A diagram that represents the filled orbitals in an atom by denoting the electron as an arrow.

Each subshell is represented as a line or a circle, electrons are represented as arrows pointing up or down according to the electron's spin.

Example Carbon: 6 electrons = \(1s^{2}2s^{2}2p^{2}\)

Example Orbital Diagram

Examples

Problem Set

Atomic Structure Problem Set

Below are two documents. One is practice problems, the second is the same problems with solutions.
They can be downloaded and changed to suit your needs.

Click or tap the images below to view.

Practice Problems

Atomic Structure problem set

Problem Solutions

Atomic Structure problem set solutions

Quiz