Chem Unit 1- The Atom

Atomic Structure, Atom History/Theory, Half Life, Half Life

element

a fundamental substance made of only one kind of atom that cannot be broken down into simpler substances by normal chemical methods

protons

 found within the nucleus of an atom and carry a positive electric charge, written as +

neutron

located in the nucleus of an atom; they are electrically neutral, having no charge

proton’s job

• define atomic number and therefore element identity

• positive charge

neutron’s job

•  Stabilize the nucleus and prevent it from repelling protons due to proton positive charges.

• The total # of protons and neutrons in a nucleus determines its atomic mass.

electrons

• Electrons orbit the nucleus of an atom in energy levels or electron shells. They carry a negative electric charge, written as -1

electron’s job

• Responsible for chemical bonding and reactivity of elements.

• The # of electrons in an atom can vary, but it must equal the # of protons for the atom to be electrically neutral

atomic number

number of protons in the nucleus of an atom of that element.

mass number

total number of protons + neutrons in an atom is the mass number.

• The number of neutrons in an atom is the difference between the mass number and atomic number.

isotopes

atoms that have the same number of protons but different numbers of neutrons

ions

atoms that has an imbalance of proton and electron.

• more protons → positive

• more electron → negative

average atomic mass

weighted average mass of the atoms in a naturally occurring sample of the element.

= (isotope1 mass * % abundance1) + (isotope2 mass * % abundance2) → sum of all isotopes times their percent abundance

band of stability

• When # of protons and neutrons are plotted, stable atoms are found within the “band of stability”

radioactivity

spontaneous disintegration of an unstable nucleus

alpha particle

helium nucleus

• mass = 4

• atomic number = 2

beta partcile

positiron(beta plus) or electron(beta minus)

gamma ray

electromagnetic energy, no mass

when a nucleus disintegrates, it can emit:

alpha particles, beta particles, or gamma rays

after decay, a new remaining nucleus may be __

radioactive or stable

half life

time requried for half the atoms of a radioactive nuceotide to decay

Nucleon

 A term that means EITHER protons and neutrons

Nuclear(strong) force

• result of attractions between quarks, the sub-sub-atomic particles that make up protons and neutrons.

• responsible for holding protons and neutrons together in the nucleus. 

electric force

the forces between protons in the nucleus, which are repelled from each other.

MeV(Mega Electron volt)

measure energy of subatomic particles 

nuclear fission

split of large nucleus to make energy

• radioactive

ion charge

protons-electrons

nuclear fusion

small nuclei combine to get bigger, much more energy than fission

• not radioactive

valance electrons

electrons in outer shell

Dalton’s model(first)

proposed modern atomic theory

→

• atoms are circle

• indivisible

• can’t change identity

• atoms react by rearraging

• compounds form when atoms connect with other atoms in a fixed ratio

JJ Thompson’s model

plum pudding model

• discovered electrons → proved atoms are made of smaller parts

Rutherford’s model

discovered nucleus → showed atoms are mostly empty space with a small, dense, positvely charged nucleus at the center

bohr’s model

• orbital for electrons

• orbitals have a fixed energy level

→ electron energy was quantised, electrons could not occupy values of energy between the fixed energy levels

schrödinger(quantam mechanical model)

• electron cloud → electrons of different energy levels all just float around

• probability model → impossible to know exactly where one is, use overlapping probablity clouds

nucleus

small, dense, positvely charged center of the atom made of protons and neutrons

alpha decay

atom has too much mass, so it ejects alpha particle(helium nucleus)

mass = -4

atomic # = -2

beta(minus) decay

atom has too many neutrons, so it splits one into a proton and a high speed electron, which is released from the atom

mass = 0

atomic # = +1

gamma decay

atom has too much energy, so it is released in the form of a high speed electromagnetic wave

mass=0

atomic # = 0

electron capture

atom has too many protons, so a proton captures an inner electron and they combine into a neutron

positron emmision(beta plus)

too many protons, so a proton splits into a neutron and a high speed positron, which is ejected

penetration of nuclear decays

alpha

• very low

• few cm in air

• sheet of paper, dead outer layer of human skin

beta(-, +positron emmision)

• moderate

• a few meters in air

• thin sheet of aluminum, heavy clothing, plastic

gamma

• high

• hundreds of meters in air

• several inches of lead, several feet of concerte, thick packed earth

electron capture:

• high

• dense materials like lead or thick concrete to block x rays

stable ratio of proton : neutrons

1 : 1.5