The Atom Vocabulary Element Compound Mixture Chemical Symbol

The Atom Vocabulary Element Compound Mixture Chemical Symbol Chemical Reaction (RXN) Law of Conservation of Mass Atom Proton Electron Neutron Atomic Mass Unit Isotope Ion Weighted average

Objectives 1. Differentiate between elements, compounds and mixtures. 2. Understand elements are represented by chemical symbols. 3. Define atom. 4. Distinguish between the subatomic particles in terms of relative charge and mass. 5. Draw a Bohr Model of an atom. Describe the structure of the atom, including the location of subatomic particles. 6. Define atom, isotope and ion. 7. Calculate the number of electrons, protons and neutrons in an atom. 8. Calculate the mass of an atom. 9. Explain and Calculate a weighted average.

Classifying Substances Elements cannot be broken down or changed into smaller substances by chemical means. –Elements are the simplest forms of matter that can exists in normal laboratory conditions.

Chemical Symbols Chemical symbols are a shorthand for the elements. A Chemical Symbol… -begins with a capitalized letter. The first letter MUST be capitalized to denote an element! -any additional letters are lowercase. -Some symbols do not make sense. This is because the symbol is derived from the Greek or Latin name for the element such as: aurum or gold has the symbol Au

History, Theory and Structure of the ATOM The smallest particle of an element is an atom. -Atoms contain 3 subatomic particles. -The development of our atomic model occurred throughout most of human history. Before learning history, we must first understand: • A model is a verbal, visual, or mathematical explanation of experimental data. • A theory is an explanation that has been repeatedly supported by many experiments –A theory states a broad principle of nature that has been supported over time by repeated testing. –Theories are successful if they can be used to make predictions that are true.

History, Theory and Structure of the ATOM A scientific law is a relationship in nature that is supported by many experiments, and no exceptions to these relationships are found. The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction, it is conserved.

History, Theory and Structure of the ATOM Greek Philosophers postulated what matter was composed of and took one of 2 positions: - Some believed matter was composed of such things as earth, water, air, and fire. - Others believed matter could be endlessly divided into smaller and smaller pieces.

History, Theory and Structure of the ATOM Two philosophers that had opposing views were: • Democritus (460– 370 B. C. ) – He was the first person to propose the idea that matter was not infinitely divisible, but made up of individual particles called atomos. • Aristotle (484– 322 B. C. ) – He disagreed with Democritus because he did not believe empty space could exist. Aristotle’s views went unchallenged for 2, 000 years until science developed methods to test the validity of his ideas.

History, Theory and Structure of the ATOM A man named John Dalton returned to Democritus’s belief of atoms. Dalton developed the “Atomic Theory” which explained conservation of mass in a reaction as the result of the combination, separation, or rearrangement of atoms.

History, Theory and Structure of the ATOM Dalton’s theory stated the following beliefs: – Matter is composed of extremely small particles: atoms – Atoms are indivisible and indestructible. – Atoms of a given element are identical in size, mass and chemical properties. – Atoms of a specific element are different from those of another element. – Different atoms combine in simple whole-number ratios to form compounds. – In a chemical reaction, atoms are separated, combined or rearranged.

History, Theory and Structure of the ATOM The cathode ray tube: −Within a ray tube, an electric charged is applied. A ray of radiation travels from the cathode to the anode. The ray is negative. −In 1886, Eugen Goldstein discovered that there was an opposing ray that contained a positive charge.

History, Theory and Structure of the ATOM While studying negative cathode rays, J. J. Thomson found: − the charge-mass ratio of the negative particles making up the rays. − the mass of this particle was much less than a hydrogen atom. − Thomson received the Nobel Prize in 1906 for identifying the first subatomic particle—the electron

History, Theory and Structure of the ATOM Using the information that matter is neutral, Thomson developed the “plum pudding model” -a uniform, positively charged sphere containing electrons.

History, Theory and Structure of the ATOM In 1911, while studying how positively charged alpha particles interact with solid matter, Ernest Rutherford discovered the nucleus. The Gold-foil Experiment: Hypothesis: By aiming the particles at a thin sheet of gold foil the paths of the alpha particles should be only slightly altered by a collision with an electron.

History, Theory and Structure of the ATOM The Gold-foil Experiment: Procedure: A lead block was used to emit alpha-particles towards a piece of gold-foil. A zinc-sulfide coated screen around the foil was used as a detector. Results: Most of the alpha particles went through the foil, HOWEVER a few of them bounced back at large angles! Conclusion: -atoms are mostly empty space -almost all of the atom’s positive charge and mass are in the center of an atom, the nucleus -Electrons are held in the atom by the positively charged nucleus

History, Theory and Structure of the ATOM The Gold-foil Experiment: Conclusion: -atoms are mostly empty space -almost all of the atom’s positive charge and mass are in the center of an atom, the nucleus -Electrons are held in the atom by the positively charged nucleus This led to the development of Rutherford’s Model: Rutherford refined the model to include positively charged particles in the nucleus called protons.

History, Theory and Structure of the ATOM In 1913, Niels Bohr introduced a new model, the Rutherford-Bohr model (Bohr Model). He added a quantum physical property. The electrons were now organized in energy levels. The new model: -contains a small positively charged nucleus -electrons are organized into energy levels

History, Theory and Structure of the ATOM James Chadwick received the Nobel Prize in 1935 for discovering the existence of neutrons, neutral particles in the nucleus which accounts for the remainder of an atom’s mass. This led to the development of the next model: − All atoms are made of three fundamental subatomic particles: the electron, the proton, and the neutron. − Atoms are spherically shaped. − Atoms are mostly empty space, and electrons travel around the nucleus held by an attraction to the positively charged nucleus.

History, Theory and Structure of the ATOM After more research in quantum mechanics, our current model, the quantum mechanical model, was developed. This can also be called a valence shell atom. In this model: -Electrons still exist in an e- cloud -the cloud is defined by electron probability shells -the electrons do not travel in a fixed path

Quantum Mechanical Model

Subatomic Particles in an Atom The atomic number is the number of protons (p+) in the nucleus and identifies the element. −Each element contains a unique positive charge in their nucleus. −All atoms of a particular element have the same number of protons and electrons (e-). −The number of neutrons (n 0) in the nucleus can differ. −The mass number is the sum of the protons and neutrons in the nucleus.

Subatomic Particles in an Atom Practice Problems: For each of the following calculate the number of protons, electrons and neutrons. 1. Lithium (Mass: 7) 2. Sodium (Mass: 23) 3. Phosphorus (Mass: 31) 4. Iron (Mass: 56)

Subatomic Particles in an Atom Isotopes are atoms with the same number of protons (p+) but different numbers of neutrons (n 0). − Isotopes containing more neutrons will have a greater mass − Isotopes of an element will share chemical behavior. Practice problem: Calculate the number of p+ and n 0 for Carbon (Mass 14) Carbon (Mass 12) When noting that the element used is an isotope, two different notations can be used: Carbon (Mass 13) Hyphen notation: Carbon-13 or Nuclear Symbol:

Subatomic Particles in an Atom

History, Theory and Structure of the ATOM How to draw a modified Bohr’s model: 1. Draw the nucleus with both the protons and neutrons. 2. Around the nucleus, draw electron orbits. 3. Fill the orbits according to the number of electrons allowed in each. 1. The number of electrons in each energy level can be determined by counting how many elements are in each row of the periodic table. 2. It turns our that the first 4 energy levels hold the respective numbers of electrons: 2, 8, 8, 18

Subatomic Particles in an Atom An ion is an atom that is charged. − This occurs by the addition or loss of an electron. − Ions will only gain or lose electrons… LEAVE THE PROTONS ALONE!!!!!!!!! A cation or positive ion is achieved by the loss of electrons – smaller and usually metals An anion or negative ion is achieved by gaining electrons – Larger and usually non-metals Practice Problems: Count the # of p+ & e− in each ion. Mg+2 p+ = _____ F− 1 p+ = _____ e− = ______

Mass of Atoms One atomic mass unit (amu) is defined as 1/12 th the mass of a carbon-12 atom. − One amu is nearly, but not exactly, equal to one proton or one neutron. − The atomic mass of an element is the weighted average mass of the isotopes of that element.