HESI A2 CHEMISTRY STUDY
GUIDE
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,Atom
The atom is the smallest unit of matter. Every atom contains one nucleus at least one electron.
The nucleus contains at least one positively charged proton and an equal number of neutrally
charged neutrons. Every atom also has at least one negatively charged electron bound to the
nucleus. When the number of protons (the atomic number) is equal to the number of electrons,
the atom is neutral. When the number of protons and number of electrons differ, the result is an
ion.
The way electrons are distributed around an atom or molecule is known as an electron
configuration. Electrons are distributed among “shells” which may be thought of as energy
levels. The configuration that corresponds to the lowest energy level is called ground state.
Every other configuration is considered an excited state. Electrons always fill the lower energy
levels first.
To write an electron configuration:
Locate the atomic number
If the atom being studied is charged, this should be taken into account. Cations
(positively charged) will have fewer electrons in the magnitude of the charge. Anions
(negatively charged) will have extra electrons in the magnitude of the charge.
Understand the order of orbitals:
o S – 1 single orbital, holds 2 electrons
o P – 3 orbitals, holds 6 electrons
o D – 5 orbitals, holds 10 electrons
o F – 7 orbitals, holds 14 electrons
Fill the orbitals in the following order, according to the number of electrons of the atom
being studied.
o 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, 8s.
The number of electrons is written in superscript to the right of the orbital letter. For
example, the configuration for neutral neon would be written as 1s2 2s2 2p6.
Periodic Table
The periodic table contains the most important information about all known elements. An
element is a substance that is composed purely from one type of atom.
The periodic table is broken into four main groups.
Main group elements: active metals in columns (also called groups) 1 and 2 as
well as the metals, semimetals and nonmetals in columns 13-18.
Transition metals: the elements that serve as a transition between the two main
groups
Lanthanides and Actinides: known as the inner transition metals because their
atomic numbers are actually located between the first and second elements in
the last two rows (also called periods) of transition metals.
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Electronegativity is an important concept in chemistry and can be charted predictably across the
periodic table. Electronegativity is an atom’s measure of attraction for electrons.
Electronegativity increases from bottom to top within groups
Electronegativity increases from left to right across periods
Additional important information about each element can be gathered from the periodic table:
The number of protons in an atom will always determine what element it is from. The number of
protons will always be the same for all atoms from the same element. The number of protons is
called the atomic number.
The number of electrons, in a neutral atom, is equal to the number of protons. If the atom is an
ion, the number of electrons will vary.
The number of neutrons in an atom can also vary. If an atom has a different number of neutrons,
it is called an isotope.
The mass number is equal to the number of protons plus the number of neutrons. In order to
calculate the number of neutrons, subtract the atomic mass minus the atomic number.
The relative atomic mass, also called the atomic weight, is an average of the atomic masses of all
the different isotopes in a sample.
Example:
Hydrogen’s chemical symbol is H. Its atomic number is 1 and atomic weight is
1.00794. This shows that there are some isotopes of hydrogen. A neutral
hydrogen atom would have a mass of 1.
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Atomic Number
Each element has its own atomic number, which is based on the number of protons in an atom. The number
of protons will always remain the same for an element. In a neutral atom, the number of protonsis equal
to the number of electrons. The number of electrons can change based on various reactions that can occur
involving them.
Example: All atoms with 2 protons are Helium.
Subatomic particles
An individual unit of an element is called an atom. Atoms cannot be broken into smaller pieces while still
maintaining the properties of that element. Atoms get their properties from their subatomic particles.
Atoms consist of two regions: the nucleus and
the electron cloud. The nucleus is in the center
of the atom and contains positively charged
protons and uncharged neutrons. This is what
makes up the mass of the atom. The electron
cloud is a much larger region of the atom;
however, the mass is negligible.This region is
where electrons are found. Electrons are
negatively charged. It is the attraction between
the positively charged protons and the
negatively charged electrons that holds the
atom together.
Most atoms have all three of these subatomic
particles, except hydrogen, which doesn’t have
any neutrons.
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GUIDE
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© 2017 All Rights Reserved. www.ReadyForTheHESI.org
,Atom
The atom is the smallest unit of matter. Every atom contains one nucleus at least one electron.
The nucleus contains at least one positively charged proton and an equal number of neutrally
charged neutrons. Every atom also has at least one negatively charged electron bound to the
nucleus. When the number of protons (the atomic number) is equal to the number of electrons,
the atom is neutral. When the number of protons and number of electrons differ, the result is an
ion.
The way electrons are distributed around an atom or molecule is known as an electron
configuration. Electrons are distributed among “shells” which may be thought of as energy
levels. The configuration that corresponds to the lowest energy level is called ground state.
Every other configuration is considered an excited state. Electrons always fill the lower energy
levels first.
To write an electron configuration:
Locate the atomic number
If the atom being studied is charged, this should be taken into account. Cations
(positively charged) will have fewer electrons in the magnitude of the charge. Anions
(negatively charged) will have extra electrons in the magnitude of the charge.
Understand the order of orbitals:
o S – 1 single orbital, holds 2 electrons
o P – 3 orbitals, holds 6 electrons
o D – 5 orbitals, holds 10 electrons
o F – 7 orbitals, holds 14 electrons
Fill the orbitals in the following order, according to the number of electrons of the atom
being studied.
o 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, 8s.
The number of electrons is written in superscript to the right of the orbital letter. For
example, the configuration for neutral neon would be written as 1s2 2s2 2p6.
Periodic Table
The periodic table contains the most important information about all known elements. An
element is a substance that is composed purely from one type of atom.
The periodic table is broken into four main groups.
Main group elements: active metals in columns (also called groups) 1 and 2 as
well as the metals, semimetals and nonmetals in columns 13-18.
Transition metals: the elements that serve as a transition between the two main
groups
Lanthanides and Actinides: known as the inner transition metals because their
atomic numbers are actually located between the first and second elements in
the last two rows (also called periods) of transition metals.
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Electronegativity is an important concept in chemistry and can be charted predictably across the
periodic table. Electronegativity is an atom’s measure of attraction for electrons.
Electronegativity increases from bottom to top within groups
Electronegativity increases from left to right across periods
Additional important information about each element can be gathered from the periodic table:
The number of protons in an atom will always determine what element it is from. The number of
protons will always be the same for all atoms from the same element. The number of protons is
called the atomic number.
The number of electrons, in a neutral atom, is equal to the number of protons. If the atom is an
ion, the number of electrons will vary.
The number of neutrons in an atom can also vary. If an atom has a different number of neutrons,
it is called an isotope.
The mass number is equal to the number of protons plus the number of neutrons. In order to
calculate the number of neutrons, subtract the atomic mass minus the atomic number.
The relative atomic mass, also called the atomic weight, is an average of the atomic masses of all
the different isotopes in a sample.
Example:
Hydrogen’s chemical symbol is H. Its atomic number is 1 and atomic weight is
1.00794. This shows that there are some isotopes of hydrogen. A neutral
hydrogen atom would have a mass of 1.
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Atomic Number
Each element has its own atomic number, which is based on the number of protons in an atom. The number
of protons will always remain the same for an element. In a neutral atom, the number of protonsis equal
to the number of electrons. The number of electrons can change based on various reactions that can occur
involving them.
Example: All atoms with 2 protons are Helium.
Subatomic particles
An individual unit of an element is called an atom. Atoms cannot be broken into smaller pieces while still
maintaining the properties of that element. Atoms get their properties from their subatomic particles.
Atoms consist of two regions: the nucleus and
the electron cloud. The nucleus is in the center
of the atom and contains positively charged
protons and uncharged neutrons. This is what
makes up the mass of the atom. The electron
cloud is a much larger region of the atom;
however, the mass is negligible.This region is
where electrons are found. Electrons are
negatively charged. It is the attraction between
the positively charged protons and the
negatively charged electrons that holds the
atom together.
Most atoms have all three of these subatomic
particles, except hydrogen, which doesn’t have
any neutrons.
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