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Topic 1: Elements and the Periodic Table
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Topic
Elements and
the Periodic
Table
1.1
Elements and
the Periodic
Table
1.2
Essential Knowledge
All matter is made from about 100 different chemical elements. The Periodic Table of the Elements
shows all of the known elements, arranged by increasing atomic number. Each element has a symbol. The
symbol for many of the elements is one capital letter. In two-letter symbols for elements, the first letter is
always an upper case letter, the second one a lower case.
The smallest particle of an element is an atom. Some common elements that are gases are composed of
diatomic molecules containing two atoms of the same element. Example: hydrogen H2(g) and oxygen
O2(g).
Atoms are made of three types of subatomic particles: protons, neutrons and electrons. Each atom has
a nucleus in the center, made of protons and neutrons packed tightly together. An electron cloud surrounds
the atomic nucleus.
The atomic number for an element is the same as the number of protons. All atoms of the same element
have the same number of protons. A proton has a positive charge and a relative mass of one.
The number of electrons is the same as the number of protons in a neutral atom. An electron has a
negative charge and a relative mass of zero.
A neutron has no charge and a relative mass of one.
Elements and
the Periodic
Table
1.3
There are only certain regions in the electron cloud where electrons are likely to be found. These regions are
called energy levels. The lowest energy level is closest to the nucleus; the highest energy level is farthest
away from the nucleus. Electrons will occupy the lowest available energy level(s) before they fill in higher
levels.
The outermost electrons in an atom are called valence electrons. The period (row) number on the periodic
table corresponds to the highest energy level occupied by the valence electrons in an element.
Elements in the same group (column) on the periodic table have the same number of valence electrons. All
of the group 1 elements have one valence electron and group two elements have two. Group 13 elements
have three valence electrons, group 14 elements have 4, group 15 have 5 and so on through group 18
elements, which have eight valence electrons.
Elements and
the Periodic
Table
1.4
An ion is an atom that has a charge because it has gained or lost electrons. Positive ions (cations) have
lost electrons; negative ions (anions) have gained electrons. The amount of charge is equal to the number
of electrons lost or gained.
The principal energy levels (n) around the nucleus of an atom identify the specific regions (distances
from the nucleus) where electrons are likely to be found. Principal energy levels are identified by n=1, 2, 3
. . . with n=1 closest to the nucleus. As the value of n increases, so does the distance from the nucleus.
Using the periodic table, the period (row) where an element is found indicates the number of occupied
energy levels for that element. The energy level of the valence electrons corresponds to the period
number (row) where the element is found.
SOL Review: Topic 1
Each principal energy level is divided into sublevels (s,p,d and f). In a given energy level, the s sublevel
holds up to 2 electrons, and always fills before the p sublevel, which can hold up to 6 electrons.
Electron configurations indicate the filling order of all of the electrons in an atom. The coefficients
represent the principal energy level, the letters represent the sublevels and the superscripts represent the
number of electrons in the sublevel.
Elements and
the Periodic
Table
1.5
Elements and
the Periodic
Table
1.6
Going down a group on the Periodic Table, each element has one more principal energy level filled with
electrons than the element above it, so the outer electrons are farther away from the nucleus. This means
the size of the atoms increases going down a group. Therefore the atomic radius increases going down
a group.
Going from left to right across a period of the Periodic Table the valence electrons are all in the same
principal energy level, but the number of protons in the nucleus increases from one element to the next.
This means that the nucleus becomes more positively charged and attracts the electrons more strongly.
Therefore, the atomic radius decreases going from left to right across a period.
Electronegativity is the ability of an atom in a bond to attract electrons. The electronegativity of an
element can be judged from its position on the periodic table.
The electronegativity increases across a period of the periodic table. (The atomic radius decreases,
which means that the valence electrons are held more tightly by the nucleus.).
The electronegativity decreases down a group (The valence electrons are further away from and more
loosely held by the nucleus).
Elements and
the Periodic
Table
1.7
Ionization energy is the energy needed to remove a valence electron from an atom.
Ionization energy increases going from left to right across a period of the periodic table because
the atomic radius decreases, which means that the valence electrons are held more tightly by the nucleus.
Ionization energy decreases going down a group because the valence electrons are further away from
and more loosely held by the nucleus.
Elements and
the Periodic
Table
Metals react by losing electrons (oxidation). Group 1 metals (called the alkali metals) are the
most reactive metals, because they have only one valence electron to lose. Activity of metals decreases
going across each period to groups 2 and 13.
1.8
Groups 3 through 12 contain the transition elements. They are all metals, but less reactive than those in
groups 1 and 2. Their oxidation states cannot be easily predicted, and so are given in the names. (Example:
iron (III) means Fe3+)
The nonmetal elements gain electrons (reduction) when they react. Group 17 metals (called the
halogens) are the most active nonmetals, because they have only one electron to gain to get a stable
valence shell with eight electrons. Activity decreases in groups 16 and then 15.
Elements in group 18 (called the noble gases) have all their electron shells filled with electrons and so
have little chemical reactivity.
Elements and
the Periodic
Table
Reactivity of metal atoms increases going down a group because metals react by losing valence
electrons, and as the atomic radii increase going down a group, the valence electrons are held more loosely.
1.9
Reactivity of non-metal atoms decreases going down a group because non-metals react by gaining
valence electrons, and as the atomic radii increase going down a group, there is less pull on the valence
electrons.
Elements and
the Periodic
Table
Atomic radius increases going from top to bottom down a group because, the valence electrons
are added at higher principal energy levels and are farther away from the nucleus. This means that both
ionization energy and electronegativity decrease going down a group.
1.10
Atomic radius decreases going from left to right across a period because valence electrons are
added at the same principal energy level and protons are added to the nucleus, increasing the pull of the
nucleus on the electrons.
SOL Review: Topic 1
Review Questions:
1.
What is the smallest particle of an element? ___________________________
2.
Which elements exist as diatomic molecules? Write their formulas here:
_______________________________________________________________
3.
What is the relative mass and charge on a proton? _______ neutron________,
electron ________
4.
Electrons tend to fill up (lower/higher) energy levels before (lower/higher) energy levels.
5.
What is the atomic number of Carbon? _____________ How many protons, neutrons and electrons does C14 have? __________________________________________
6.
What is the difference between atomic mass and mass number? ________________
____________________________________________________________________
7.
What are valence electrons? _______________________________________
8.
What is a “group” on the periodic table and what do elements in a group have in common?
________________________________________________________
9.
What is an ion? __________________ A cation has _____________electrons and is ______ charged, while
and anion has ____________electrons and is _____________ charged.
10. Principal energy levels correspond to ____________ or _________ on the periodic table.
11. Lithium is in the _________ principal energy level, while Bromine is in the ______ principal energy level.
This means that Bromine’s valence electrons are (farther/closer) to the nucleus than Lithium’s valence
electrons.
12. The electron configuration 1s22s22p63s23p4 is the configuration of ____________ in its ground state. The
coeffiencient “3” indicates the highest occupied ________________. The letters s and p indicate
_______________________. The superscripts indicate the ____________________ in each sublevel.
13. What happens to atomic radii as one moves across a row or period? _____________
14. What happens to atomic radii as one moves down a column or family? ____________
15. What is electronegativity? _______________________________________________
16. What is ionization energy? _____________________________________________
17. What happens to both electronegativity and ionization energy as one moves across a row or period?
__________________________________________________________
18. What happens to both electronegativity and ionization energy as one move down a column or family?
___________________________________________________
19. Why does atomic radii decrease as one moves across a period (example Lithium is much bigger than
Fluorine)? ______________________________________________________
20. Why does atomic radii increase as one moves down a family (example: Lithium is much smaller than
Francium) ____________________________________________________
SOL Review: Topic 1
SOL Type questions with Answers
1. The number 32 in phosphorus-32 refers to the
a. Number of protons in the atom
b. Number of neutrons in the atom
c. Total number of protons and neutrons in the atom
d. Total number of electrons and protons in the atom
2. What is the electron configuration of a sodium atom?
a. 1s22s22p1
b. 1s22s22p4
c. 1s22s22p63s1
d. 1s22s22p63s23p5
3. An atom has only two stable isotopes. One stable isotope has a mass number of 10 and the
other has a mass number of 11. Which of the following could be the atomic weight
(sometimes called atomic mass) of the element?
a. 9.5
b. 10.8
c. 11.7
d. 12.4
4. Which of the following describes how difficult it is to remove a valence electron from the
atom of a particular element?
a. Electronegativity
b. Ionization energy
c. Electron affinity
d. Atomic radius
5. Which of the following elements has the greatest electronegativity?
a. Al
b. Si c. P
d. S
6. Cobalt (Co) is
a. An inert gas
b. an alkaline earth metal
c. a transition metal
d. a halogen
7. Which of the following best describes the element iodine?
a. Iodine is a halogen and a nonmetal
b. Iodine is a halogen and a metal
c. Iodine is a noble gas and a nonmetal
d. Iodine is a noble gas and a metal
8. Which of the following atoms has valence electrons in its 2p subshell?
a. Li
b. Na
c. O
d. P
9. An argon atom is not very reactive because
a. It has a complete valence shell of electrons
b. It has a large number of protons
c. It has a relatively small mass number
d. It has an equal number of protons and neutrons
10. Which of the following statement is true of an electron?
a. An electron is positively charged and smaller than a proton
b. An electron is negatively charged and larger than a proton
c. An electron is positively charged and larger than a proton
d. An electron is negatively charged and smaller than a proton.
Answers: 1. c 2. c 3. b 4. b 5. d 6. c 7. a 8. c 9. a 10. d
SOL Review: Topic 1