models for atoms · electrons are negatively charged subatomic particles. symbol is e-. (j.j....
TRANSCRIPT
Models for Atoms Chapter 3
•The atom-building blocks of the
elements
• Atoms are the basis of chemistry and the basis of everything in the universe. Remember all matter is composed of atoms!
• Atom – smallest particle of an element that still retains the properties of that element
• the atom is a microscopic structure. It is similar to the cell in biology. Like the cell the atom can be broken down into it simplest parts as well.
Atomic Structure • Atoms can be broken down into electrons, protons,
and neutrons.
Electrons are negatively charged subatomic particles. Symbol is e- . (J.J. Thompson)
Protons are positively charged subatomic particles. (Rutherford)
Neutrons are subatomic particles with no charge. (James Chadwick)
An atom refresher
• An atom has three parts:
• Proton = positive
• Neutron = no charge
• Electron = negative
• The proton & neutron are found in the center of the atom, a place called the nucleus.
• The electrons orbit the nucleus.
HELIUM ATOM
+ N
N
+
- -
proton
electron neutron
Shell
Protons and Neutrons are found in the nucleus,
electrons are found in shells outside the nucleus
Empedoles (~450 BC) -
• All matter was made of four elements - Earth, Air, Fire and Water
What scientists believed about the elements
A quick review
Democritus (~400 BC) – Matter was made of tiny particles that cannot be broken down further (called atoms)
What scientists believed about the elements A quick review
The same elementcan make up
different kinds ofsubstance.
Aristotle (~ 350 BC) –
A return to the “4-element” model
• What scientist believed about the elements
A quick review
Alchemists (500 – 1600 AD) –
Still believed in the “4 – element” model!
Tried to turn cheap metal into gold
• What scientists believed about the elements
•A quick review
Boyle (1650 AD) –
• A return to the atom theory…
• Believed in the existence of elements
• What scientists believed about the elements
•A quick review
Dalton (1808 AD)
Developed the atomic model for matter:
- All matter is made up of atoms
- Each element has it’s own atom and atomic mass
- Compounds can be created when atoms from different elements come together to form molecules
- Atoms cannot be created, destroyed, or subdivided in chemical changes
Faraday (1800’s) –
Discovered that an atom can gain an electric charge and form an ion.
• Modified Dalton’s model….
- Matter must contain positive and negative charges
- Atoms can combine to form molecules because of electrical attraction
Thompson (1904) –
Discovered electrons!!!!
• Developed the “raisin bun” model
- Dough → positive charge
- Raisins → negative charge
- Overall, atoms have a neutral charge
- Dalton’s model is now incorrect because atoms are divisible (but only through a nuclear reaction
• Rutherford (1911) –
Experimented using gold foil and discovered that atoms are
1. not solid
2. most of the atom is made up of space
3. have a small center called a nucleus
• Ernest Rutherford, through experimentation,
discovered the nucleus.
• The nucleus is the heaviest part of the atom; it’s
comprised of both protons and neutrons. Thus the
nucleus has a positive charge. The electrons
occupy the outside of the nucleus (the majority of
volume of the nucleus)
• Different elements have different numbers of
subatomic particles thus giving different elements
different characteristics/properties
Basics of the Atom
Particle Charge Location in
the Atom Mass
a.m.u.: unit used to measure mass of
atoms
proton
neutron
electron
1+
0
1–
in nucleus
in nucleus
orbiting
nucleus
~1
a.m.u. ~1
a.m.u. ~0 a.m.u.
Review Protons • Protons are very important because they
help determine the identity of an element
• They are inside the nucleus
• They have a positive charge +
• Atom Identity – the number of protons an
atom has will determine its identity.
Ex:
8 protons = oxygen
19 protons = potassium
Review Neutrons
• Found inside the nucleus
• Have a neutral charge
• Have a relative mass of 1 atomic unit
• Important for creating isotopes (coming
soon!)
Review electrons
• Found outside the nucleus
• Forms an orbital cloud (the location
where an electron might be found)
• Has a negative charge –
• Different elements have different
numbers of subatomic particles thus
giving different elements different
characteristics/properties
• So how do we know how many
subatomic particles an element has?
• Grab your periodic tables and let’s find
out....
Created by G.Baker www.thesciencequeen.net
8
O Oxygen
16
Atomic Number:
Number of protons and it is also
the number of electrons in an
atom of an element.
.
Elements Name
Atomic Mass/Weight:
Number of protons +
neutrons.
Chemical Symbol of
element
For each element, indicate the:
1. Mass number
2. atomic number
3. # of protons
4. # of electrons
a. sulfur b. Calcium c. Iron
•Charged Atoms (ions) • Why is it dangerous to work with electrical appliances around water?
Tap water and rain water (not pure) contain charged atoms that can move in a solution and carry electric current.
• These charged atoms are called ions (atoms that have become charged by gaining or losing one or more electrons).
• They have a charge because the number of electrons is NOT equal to the number of protons
Neils Bohr (1913) –
• Electrons move around the nucleus in paths called orbitals
• Each electron in an orbit has a definite amount of energy ; further away from nucleus = greater energy
• Electrons cannot exist between orbits, but they can move up or down from one orbit to another
• Electrons are more stable when they are at lower energy (closer to the nucleus)
What do we believe now
The first “modern” model of the atom
Excited state – when electrons jump to a
higher orbit due to being energized by
light, heat or electricity.
Ground state – When electrons fall to
lower orbits and a more stable state
Created by G.Baker www.thesciencequeen.net
•Bohr Model
• The Bohr Model shows all of the particles in the atom.
• In the center = write the # of protons and neutrons in the nucleus, using “p” and “n”
• In a circle around the nucleus are the electrons. Represent electrons with dots.
Created by G.Baker www.thesciencequeen.net
•Electrons have special rules….
• You can’t just shove all of the electrons into the first orbit of an electron.
• Electrons live in something called shells or energy levels.
• Only so many can be in any certain shell.
• The order of filling of electrons for the first 4 orbits: 2, 8, 8, 18
ATOMIC STRUCTURE
Electrons are arranged in Energy Levels or
Shells around the nucleus of an atom.
• first shell a maximum of 2 electrons
• second shell a maximum of 8 electrons
• third shell a maximum of 8 electrons
•So let’s try it….
• How to draw a Lithium atom
• First, look at the Periodic Table
• Second, determine the number of protons (Look @ the atomic number)
• Then determine the number of neutrons (Atomic mass – atomic number)
• Then determine the number of electrons (Look @ the atomic number)
3
Li
Lithium
7
Created by G.Baker www.thesciencequeen.net
3
Li
Lithium
7
Protons = 3
Neutrons = 4
(7-3=4)
Electrons = 3
2 in the 1st shell, 1 in the 2nd shell
ELECTRONIC CONFIGURATION
With electronic configuration elements are represented
numerically by the number of electrons in their shells
and number of shells. For example;
N
Nitrogen
7
14 2 in 1st shell
5 in 2nd shell
configuration = 2 , 5
2 + 5 = 7
ELECTRONIC CONFIGURATION
Write the electronic configuration for the following elements;
Ca O
Cl Si
Na 20
40
11
23
8
17
16
35
14
28
B 11
5
a) b) c)
d) e) f)
2,8,8,2 2,8,1
2,8,7 2,8,4 2,3
2,6
Isotopes and Radioisotopes
• Isotope – any of two or more forms of
an element, each having the same
number of protons but having different
mass due to a number of neutrons.
• Isotopes of the same element have the
same physical properties and the same
chemical properties
• Hydrogen has three common isotopes:
Chlorine has two common isotopes:
• Cl- 35 (atomic number 17 and mass number 35; 17 protons plus 18 neutrons)
• Cl – 37 (atomic number 17 and mass number 37; 17 protons plus 20 neutrons)
• There are three isotopes for Carbon. Let’s draw the isotopes for Carbon using Bohr diagrams:
• Remember all that has changed is the neutrons and thus the atomic
mass. (The number beside Carbon is the new atomic mass).
Carbon – 12, Carbon- 13 and Carbon -14
Now write the standard atomic structure for these:
Carbon – 12 Carbon- 13 Carbon -14
• Some isotopes are unstable, or radioactive, which means that the nucleus has a tendency to break apart and eject very-high-energy particles into its surroundings.
• The huge amount of energy these particles have can be both dangerous and useful. An atom with an unstable nucleus is called a radioisotope.
• Decay – splitting apart of unstable nuclei to produce radioactive particles
• Around the year 1900, three different kinds of radioactivity were identified:
• alpha ( ) particles were found to be helium nuclei, containing 2 protons and 2 neutrons
• beta ( ) particles are high energy electrons
• gamma rays (Ƴ )- radiation is high – energy electromagnetic radiation with no mass.
• Radioisotopes can damage living tissue; they may alter the DNA, which affects how cells divide.
• This can cause serious diseases including cancer and birth defects.
• However, radioisotopes can be very useful and are useful and are used in smoke detectors and also to diagnose medical problems such as thyroid disease.