the structure of the atom
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The Structure of the Atom. Chapter 4.2. Bell Ringer. The ancient Greek philosopher Democritus coined what word for a tiny piece of matter that cannot be divided? Element Electron Atom Molecule - PowerPoint PPT PresentationTRANSCRIPT
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The Structure of the AtomChapter 4.2
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Bell Ringer1. The ancient Greek philosopher Democritus coined what word for a tiny piece of
matter that cannot be divided?a) Elementb) Electronc) Atomd) Molecule
2. Dalton’s theory (~1800; based on behavior of gasses) included all but one of the following points. Which is not from Dalton?
a) All elements are composed of atoms.b) Most of an atom’s mass is in its nucleus.c) Compounds contain atoms of more than one element.d) In a specific compound, atoms of different elements always combine in the same
way.3. J. J. Thomson’s gas tube electrical experiments (~1897) provided first evidence of:
a) Atomsb) Nucleusc) Subatomic particlesd) Elements
4. Rutherford’s gold foil experiment (~1914) provided evidence for one of these:a) Negative and positive charges are spread evenly throughout an atom.b) Alpha particles have a positive charge.c) Gold is not as dense as previously thought.d) There is a dense, positively charged mass in the center of an atom.
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Properties of Subatomic Particles
J.J. Thomson was the first to detect subatomic particles (later named electrons)
Electron: › Negatively charged subatomic particle.› Found in the space outside the nucleus.› Very light mass (~1/2000 of Hydrogen
atom)› Charge of 1-.
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Thomson’s ExperimentsIn his experiments, J. J. Thomson used a sealed tube containing a very small amount of gas.
Thomson’s Model of the Atom
Sealed tube filled with gas at low pressure
Glowing beam
Metal disk Source of electric current
Metal disk
Metal disk Source of electric current
Positive plate
Negative plate
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Properties of Subatomic Particles
Rutherford found positive charge varies among the elements. › Based on experiments with gold and other
elements. Proton:
› Positively charged subatomic particle› Found in the nucleus of an atom. › Charge of 1+. › Each nucleus contains at least one proton.
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The Gold Foil ExperimentRutherford’s Atomic Theory
Beam of alpha particles
Source of alpha particles
Slit
Deflected particle
Undeflected particle
Screen
Gold atoms
Alpha particles
Alpha particles
Nucleus
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Properties of Subatomic ParticlesJames Chadwick 1932 - the English physicist
carried out an experiment to show that neutrons exist.› Radioactive decay produced particles about
the mass of a proton but without any charge.› Proved the particles produced were neutral
because a charged object did not deflect their paths.
3) Neutron:› Neutral (no charge) subatomic particle › Found in the nucleus of an atom.› Mass ~ equal to that of a proton.
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Everything we know about subatomic particles is based on how the particles behave in experiments.
› Scientists still do not have an instrument that can show the inside of an atom.
Comparing Subatomic Particles
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Comparing Subatomic Particles
Mass• Protons and neutrons have almost the same
mass.Mass of ~2000 electrons = the mass of one
proton. Charge
• An electron charge (-1) is equal in size but the opposite sign of a proton charge (+1).
• Neutrons have no charge.Location in the atom
• Nucleus - Protons and neutrons • Outside the nucleus - electrons
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Subatomic Particles - Protons Atoms of different elements have different
numbers of protons. Atomic Number = Number of protons in
an atom of that element.
For any given element: All atoms have same atomic number.
› Hydrogen has 1 proton; atomic number = 1. › Helium has 2 two protons; atomic number = 2› Etc.
Each element has a unique atomic number.
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Atomic Number and Mass NumberEach element has a different atomic number.
›A The atomic number of sulfur (S) is 16.›B The atomic number of iron (Fe) is 26.›C The atomic number of silver (Ag) is 47.
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Atomic Number and Mass Number Atoms are neutral when:
• Each positive charge in an atom is balanced by a negative charge. i.e. # of protons = # of electrons Each proton has an electron to offset its charge
Examples:• Hydrogen atomic number = 1
Each neutral hydrogen atom has 1 proton & 1 electron.
• Sulfur atomic number = 16 Each neutral sulfur atom has 16 protons & 16
electrons.
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Atomic Number and Mass Number Mass Number = Sum of protons +
neutrons. To find # of neutrons (when mass # &
atomic # are known): # of neutrons = mass # - atomic #
Example: Aluminum with mass # = 27 Mass # = 27 Atomic # = 13. 27 - 13 protons = 14 neutrons
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Atomic Number and Mass Number
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Charged Atoms An atom becomes charged when it
looses or gains an electron (e-).› Positive Charge = loss of e-’s
H becomes H+1
› Negative Charge = gaining of e-’s H becomes H-1
Electrons are the only subatomic particle that can be gained or lost.
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Isotopes Isotopes = same element; different #
neutrons
Isotopes of an element:1)Same # of protons
• i.e. same atomic number2)Different # of neutrons
• Different atomic mass because of the neutrons
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Isotopes Isotopes are referred to by mass
number› For example, oxygen has 3 isotopes:
“oxygen 16”, “oxygen 17”, and “oxygen 18”
All three oxygen isotopes have same reactions
› Examples: All combine with hydrogen to form water All combine with iron to form rust.
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Isotopes Most elements: Physical and chemical
properties of all isotopes are very similar.
Hydrogen is an exception. ›Hydrogen-1 no neutrons (most H is hydrogen-1) ›Hydrogen-2 has one neutron›Hydrogen-3 has two neutrons.
Because a hydrogen-1 atom has only one proton, adding a neutron doubles its mass.
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Heavy Water - hydrogen-2 atoms & oxygen-16
Hydrogen-2 atoms twice the mass of Hydrogen-1› Properties of heavy water are different from the
properties of ordinary water.
Isotopes
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Assessment Questions
1) Of the subatomic particles that form the atom, the one with the smallest mass is:
a) Neutron. b) Electronc) Protond) Nucleus
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Assessment Questions
2) Of the subatomic particles that form the atom, the one with the positive charge is:
a) Neutron. b) Electronc) Protond) Nucleus
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Assessment Questions
3) Of the subatomic particles that form the atom, the one with the neutral charge is:
a) Neutron. b) Electronc) Protond) Nucleus
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Assessment Questions
4) Of the subatomic particles that form the atom, which ones are found in the nucleus?
a) Neutron. b) Electronc) Protond) None
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Assessment Questions
5) In which way do isotopes of an element differ? a. number of electrons in the atomb. number of protons in the atomc. number of neutrons in the atomd.net charge of the atom