Chemistry notes unit 3

### Atomic Structure

Atomic Number  

Atomic Number on the Periodic Table: 

All atoms of an element have the same number of protons.

Learning Check  

State the number of protons for atoms of each of the following:  

A. Nitrogen: 7  

B. Sulfur: 16  

C. Barium: 56

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Number of Electrons  

An ATOM is neutral.  

The net charge is zero.  

Number of protons = Number of electrons.  

Atomic number = Number of protons.

All atoms of an element have the same number of protons and electrons.

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Atomic Mass  

Atomic mass is the weighted average mass of all the atomic masses of the isotopes of that atom.

Atomic Mass on the Periodic Table  

Ave. Atomic mass, symbol, and atomic number are all things you can see on the periodic table. However, there are other versions of the elements. This part talks about how else you may see something represented.

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Mass Number  

Counts the number of protons and neutrons in an atom  

or is determined by the whole number closest to the average atomic mass.  

Use this when the exact number of neutrons is not known.

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Atomic Symbols  

Sometimes you will see something like:  

\[mass number\]  

      Na (sodium-23)  

   \[atomic number\]

This tells you the version of the element. The 23 is the number of protons and neutrons added together. Mostly, we use the rounded average atomic mass since we are not always given the version. Pay attention when this is important.

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Isotopes  

All elements have the same number of protons, but can have different numbers of neutrons.  

Atoms of the same element (same atomic number) with different mass numbers are isotopes.

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IONS  

Atoms with a charge (positive or negative).  

Atoms gain or lose electrons:

Example:  

Atom → #p: 11, #e: 11 → Symbol of atom: Na  

Ion → #p: 11, #e: 10 → Symbol of ion: Na⁺  

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Changes in Subatomic Particles  

- If protons change: new element.  

- If neutrons change: new isotope.  

- If electrons change: new ion.

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Average Atomic Mass  

Atomic mass is the weighted average mass of all the atomic masses of the isotopes of that atom.  

It accounts for all the different forms of atoms of an element and how frequent they are.

To find the Average Atomic Mass (generally referred to as the atomic mass of an element), use:  

1. The mass of the isotope.  

2. Multiply by the % frequency (divided by 100).  

3. Add up all the results for the isotopes of that element.

3.6

Joseph Priestley (1774)

Priestley was an English chemist known for his discovery of oxygen and other gases. His work laid foundational concepts for modern chemistry and was pivotal in the development of the pneumatic theory.

Antoine Lavoisier (1778)

Lavoisier is often referred to as the "father of modern chemistry" for his role in establishing the law of conservation of mass. He helped create a systematic chemical nomenclature and disproved the phlogiston theory.

Alessandro Volta (1799)

An Italian physicist, Volta is best known for inventing the voltaic pile, the first chemical battery, which provided a steady source of current. His work laid the groundwork for the field of electrochemistry.

John Dalton (1803)

Dalton was an English chemist known for developing atomic theory and the concept of the atomic weight of elements. His research provided a framework for understanding chemical reactions and compounds.

Humphrey Davy (1807)

Davy was an English chemist recognized for his discoveries of several alkali and alkaline earth metals through electrolysis. He also invented the Davy lamp, a safety lamp for miners.

Dmitri Mendeleev (1869)

Mendeleev was a Russian chemist who formulated the periodic law and created the periodic table of elements, arranging them by atomic mass. His predictions of undiscovered elements based on gaps in his table were remarkably accurate.

J.J. Thomson (1897)

Thomson was a British physicist credited with discovering the electron and proposing the "plum pudding" model of the atom. His work on cathode rays led to significant advances in atomic theory.

Marie Curie (1898)

Curie was a Polish-born physicist and chemist who conducted pioneering research on radioactivity, discovering the elements polonium and radium. She was the first woman to win a Nobel Prize and remains the only person to win in two different scientific fields (Physics and Chemistry).

Robert A. Millikan (1909)

An American experimental physicist, Millikan is best known for measuring the charge of the electron through his oil drop experiment. He also won the Nobel Prize in Physics in 1923 for his work on the photoelectric effect.

Ernest Rutherford (1910)

Rutherford, a New Zealand-born physicist, is often referred to as the father of nuclear physics for his discovery of the nucleus and his gold foil experiment that revealed the structure of the atom. He won the Nobel Prize in Chemistry in 1908.

Henry Moseley (1913)

Moseley was an English physicist whose work on X-ray spectra led to the redefinition of atomic numbers, establishing the modern periodic table. His contributions significantly advanced the understanding of atomic structure before his untimely death in World War I.

James Chadwick (1932)

Chadwick was an English physicist who discovered the neutron, providing crucial insight into atomic structure. He was awarded the Nobel Prize in Physics in 1935 for this groundbreaking work.

Plum Pudding

The plum pudding model is a historical concept of atomic structure proposed by J.J. Thomson in 1904. In this model, the atom is envisioned as a sphere of positive charge with negatively charged electrons embedded within it, resembling "plums" in a "pudding." This model was an early attempt to explain the distribution of electrical charge within the atom before the discovery of the nucleus, which eventually led to the development of the more accurate Rutherford model of the atom.