Atoms and Elements

Structure of an Atom

Charges•Electron – negative•Proton – positive•Neutron - neutral

Location in Atom•Electrons – in orbits•Protons & Neutrons – in nucleus

Orbits

Nucleus

Elements

• Pure chemical substances made up of one type of atom

• Building blocks of matter

• Over 115 elements known - and more being discovered

Elements – Symbol / Name

Symbol: one or two letter abbreviation

Examples: Latin

C Carbon Au Gold-aurum

O Oxygen Lb Lead-plumbum

Mg Magnesium K Potassium-kalium

Br Bromine Na Sodium-natrium

Elements – Atomic Number

DEFINES the element

• ALL atoms of the same element have the same number of protons.

• All neutral atoms have no overall (net) charge, so … have the same number of electrons as protons

• BUT… they can have different numbers of neutrons These are called isotopes of carbon

Atoms are neutral… so have same # electrons (-) as protons (+)

Elements – Atomic Mass

• MASS NUMBER = mass of all particles in one atom (protons, neutrons and electrons)

• Measured in “atomic mass units” (amu)

Calculated from Mass Number

Elements – Mass Number• MASS NUMBER = mass of all particles in one

atom (protons, neutrons and electrons)• Mass of 1 proton = 1 amu

Mass of 1 neutron = 1 amuMass of 1 electron = really, really tiny!

• SO… mass number = # protons + # neutrons

Carbon - 12•6 protons x 1 amu/proton = 6 amu•6 neutrons x 1 amu/neutron = 6 amu•6 electrons x really tiny mass = 0 amu

TOTAL = 6 amu + 6 amu + 0 amu = 12 amu

Elements – Mass Number

Helium - 4

Atomic Number =

Mass Number =

Nitrogen - 14

Atomic Number =

Mass Number =

2 (protons)

4 amu (p + n)

7 (protons)

14 amu (p + n)

Pause for….

Practice and Review

Isotopes• Different versions of the same element• Same number of protons and electrons• Different number of neutrons• SO…. Mass number (# protons + # neutrons)

is different• Identified by writing mass number after

element nameex: Carbon-12, Carbon-13

Isotopes… • Explain why the atomic mass for Lithium

(atomic number 3; with 3 protons) is 6.94 ! !

Lithium-6 - 3 protons, 3 neutrons - mass number = 6 - 7.5% of all lithium

Lithium-7 - 3 protons, 4 neutrons - mass number = 7 - 92.5 % of all lithium

NOTE: Mass number is used to name isotope

Isotopes and Atomic Mass

• Why the atomic mass for Lithium (atomic number 3; with 3 protons) = 6.94…

• Calculation: Weighted Average (info. only):– 7.5 % Lithium-6 at 6 amu– 92.5 % Lithium-7 at 7 amu

• (0.075 x 6 amu) + (0.925 x 7 amu) = (0.45 amu) + (6.48 amu) = 6.93 amu

Elements – Atomic Mass

• The atomic mass of an element is calculated by doing a weighted average of the mass numbers of all naturally existing isotopes.

• If you round the atomic mass of an element to the nearest whole number, you get the mass number of the most common isotope.

7.0

Elements – Symbol / Name

Atomic Mass• “Average” for all carbon• Calculated from mass

numbers of isotopes• On the periodic table

Mass Number• Calculated for each

individual isotope• Used to name the

isotope• Averaged together to

represent all carbonAll have same # protons

Same # protons, different # neutrons

Elements - SUMMARY

Structure of an Atom

• Outside the nucleuselectron– Electron

• Atomic mass 0• Charge (-)

nucleus(containing protonsand neutrons)

Structure of an Atom

• Electrons are arranged in orbits– Orbit 1 holds

2 electrons– Orbit 2 holds

8 electrons– Orbit 3 holds

8 electrons…

Structure of an Atom

• Nucleus – the center of an atom contains…– Protons

• Atomic mass 1• Charge (+)

– Neutrons• Atomic mass 1• Charge none

Symbols

• Either 1 or 2 letters symbols– First letter Capital, second lower case– Some based on Latin nameExample LatinC Carbon Au Gold-aurumO Oxygen Lb Lead-plumbumMg Magnesium K Potassium-kaliumBr Bromine Na Sodium-

natrium

The Periodic Table

• Elements are arranged by size and chemical properties.

• Rows – Period

• Size

• Columns – Group

• Properties

Properties

• Properties are related to atomics structure.• The number of electrons in the outer most

orbit (valence electrons) determine how an atom will behave in chemical reactions.