Chem unit 2

Dalton’s Atomic Theory (1808)

NOT the law we use today. (updated laws in Modern atomic theory (below)) 

1. All matter is composed of atoms

2. Atom of a given element are identical in size, mass, and other properties 

3. The ratio of whole number of atoms in a compound are always whole numbers 

4. Atoms cannot be created nor destroyed in a chemical reaction 

Modern Atomic Theory 

• Modern atomic theory updated Dalton's 1st and 2nd law.

1. Discovery of subatomic particles: subatomic particles make up atoms 

2. Discovery of isotopes: atoms of the same element can have different number of neutrons     witch then have different mass numbers

Law of conservation of Mass

• Mass is neither created nor destroyed during chemical or physical reactions 

• Total mass of reactants = total mass of products 

Law of Definite properties 

• Regardless of the amount of a compound is composed of the same element in the same proportions by mass 

• Ex: table salt (NaCl) always consisted of 39.34% by  mass of the element sodium and 60.66% by mass of the element chlorine. 

- so a 50g sample of salt contains (50)(0.3934) = 19.67g of NA (50)(0.6066) = 30.33g of Cl 

- a 100g sample of NaCl contains 

The Electron 

• In 1897 J.J Thomson used a cathode ray tube to deduce the presence of negatively charged particles. 

• Cathode ray tubes pass electricity (electrons) through a gas that is contained at a very low pressure 

The Nucleus (proton)

• rutherfords ‘s Gold foil experiment (proton) 

• In  1911, scientist bombarded a thin, gold foil with fast moving alpha particles (positively charged particles)

• Assumed that the mass charge were uniformly distributed throughout the atoms of the gold 

Rutherford Observations 

• Most of the particles pass right through 

• A few particles were deflected 

• A very few were greatly defected 

Conclusions of the nucleus

1. Small 

2. Dense

3. Positively charge 

Atomic Number

Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element 

• Identifies element 

• Also identifies number of electrons for a neutral atom

Mass Number 

• Mass number (A) is the number of protons and neutrons in the nucleus of an isotope. (mass = protons but neutrons) 

Average Atomic Mass 

• Multiply atomic mass with abundance

• Then add it together 

• Unit = AMU 

• Example Below: 

Isotope 

• Elements occur in nature as mixtures of isotopes (nuclides) 

• Not all isotopes are stable 

• Isotopes are atoms of the same element that differ in the number of neutrons so they have different masses 

Ions 

- Results when a neutral atom gains or loses electrons 

Two kinds 

1. Cation 

• Has a positive charge

• Loses electrons 

2. Anion 

• Has negative charge  

• it gains electrons 

- A charge only affects the number of electrons and not the number of protons or the number of neutrons 

Nuclear chemistry 

• The atom is called a nuclide (which reminds us that we are only focused on what is happening in the nucleus!)

• Inside nucleus are nucleons 

1. Nucleons 

• The particles in the nucleus 

• The collective name for protons and neutrons

Nuclear reaction 

• A reaction that only changes nucleus of an atoms 

• Compared to a “typical” chemical reaction that involved the electron the electron of an atom 

Fission vs. Fusion 

1. Fission - breaking or splitting one thing into parts

2. Fusion- joining parts into a whole

Radioactive Half life 

• The time required for half of the atoms of a radioactive isotope (also called nuclide) of a given sample to decay. 

• Never end with a sample of zero; it always decreases in size. 

Radioactive Decay 

• Radioactive decay is the spontaneous disintegration of an unstable nucleus into a slightly light nucleus accompanied by 

- emission of particles (alpha, beta)

- electromagnetic radiation (gamma)

- the more unstable an isotope is the faster it decays and the shorter the half life. 

Decay types 

1. Beta 

• When the nucleus contains too many neutrons it goes into beta decay 

• A neutron transforms into a proton and beta Decay particle known as beta decay 

• Are negative attracts to positive 

2. Alpha 

• Nuclei with atomic number greater than 83 Naturally stabilizes by losing protons and neutrons 

• Alpha particle is equivalent in mass and charge to a helium atom 

• Are positive attracts to negative 

3. Gamma  

• Usually occurs immediately following other types of decay 

• Does NOT involve change in identity 

• Are high energy electromagnetic waves emitted from a  nucleus as it goes from an excited state to a ground level energy state

• neutral / no change 

Scientists 

Particle types 

1. Radioactive element 

• All the isotopes for the element are unstable and will undergo radioactive decay 

2. Transuranium element 

• Element with more than 92 protons in their nuclei. 

• Any element past uranium on the periodic table 

3. Synthetic element 

• Was created by scientist in a lab 

• Not naturally occuring on earth