Atomic Structure

n Atom: the smallest particle of an element that retains the chemical identity of that element

n Nucleus: very small, very dense, positively charged center of the atom containing neutrons and protons

u 99.9% of mass

u Small : bee in a football stadium

Atomic Structure

n Atomic Number = # of protons (p⁺)

u determines what element

n For neutral atoms: # p⁺ = e^- (electrons)

n Chemical properties are determined by number of electrons in outer shell

n Mass number = # p⁺ + # n

Fundamental Subatomic Particles

Bohr models

n p⁺ & n in nucleus

n e^- in energy levels around nucleus

n 3 energy levels

-1^st has up to 2e^-

-2^nd has up to 8e^-

-3^rd has up to 8e^-

Forces within the Atom

n Electromagnetic Forces: keep electrons in orbit around nucleus

u Particles with like charges = repulsion

u Particles with opposite charge = attraction

Why don’t protons in nucleus explode from this repulsive force?

n STRONG force: Opposes electromagnetic force

u “Glues” protons together in nucleus

u Works only when protons close together

u Greatest force, but small range

Forces Continued

n Weak Force: Responsible for radioactive decay

u A neutron in nucleus turns into a proton and an electron

n Gravity: the weakest force

u Force of attraction exerted between all objects in nature

u Keeps planets around sun, makes apple fall to ground

Ions

n Ions: charged particle that is formed when an atom or group of atoms loses or gains electrons

Charge of ion = # of p⁺ - # e^-

Ex: An atom of calcium (Atomic # 20) loses 2 e^-

Charge of ion = 20 – 18 = +2 => Ca ²⁺

Isotopes

n Isotopes: atoms that have the same number of protons but different number of neutrons

u Most elements exist in nature as a mixture of isotopes

u Isotopes have essentially identical chemical properties, but have different masses

n Identify isotopes by mass number

Mass number = # of protons + # of neutrons

Example: Aluminum-27

Practice

• What is the atomic number of Krypton (Kr)?

• Write the symbol for the ion that has 16 protons and 18 electrons.

• Write the symbol for the atom that has 12 protons and 12 electrons.