University of La Salette, Inc. – High
School Department
Module in
Malvar, Santiago City
PHYSICAL SCIENCE
GRADE 12 ABM and
HUMSS
Lesson 2: STRUCTURE OF THE ATOM
Particles smaller than the atom were discovered. The main
subatomic particles are the protons, neutrons, and electrons.
Content Standard
THE NUCLEUS
You will demonstrate an
understanding of the formation
the elements during the Big
Bang and during stellar evolution.
Performance Standard
You
Illustrations of helium nucleus and carbon nucleus
In the center of each atom is the nucleus. Within the nucleus
there are two kinds of particles. Positively-charged particles called
protons and particles with no charges called neutrons. The protons
give the nucleus a positive charge. For example, a helium atom has 2
protons and 2 neutrons. It would have a net charge of +2. A carbon
atom has 6 protons and 6 neutrons and a net charge of +6.
ELECTRONS
Electrons are one of three main types of particles that make up
atoms. Unlike protons and neutrons, which consist of smaller, simpler
particles, electrons are fundamental particles that do not consist of
smaller particles. They are a type of fundamental particle called
leptons. All leptons have an electric charge of −1−1 or 00. Electrons
are extremely small. The mass of an electron is only about 1/2000 the
mass of a proton or neutron, so electrons contribute virtually nothing
to the total mass of an atom. Electrons have an electric charge
of −1−1, which is equal but opposite to the charge of a proton, which
is +1+1. All atoms have the same number of electrons as protons, so
the positive and negative charges "cancel out", making atoms
electrically neutral.
will be able to make a
creative representation of
the historical development of the
atom or the the chemical element
in a timeline.
Most Essential Learning
Competencies
At the end of the lesson, the
students are expected to:
1. give evidence for and
describe the formation of
heavier elements during
star formation and
evolution; and
2. explain how the concept
of atomic number led to
the synthesis of new
elements in the
laboratory.
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PROTONS
A proton is one of three main particles that make up the atom. Protons are found in the nucleus of
the atom. This is a tiny, dense region at the center of the atom. Protons have a positive electrical charge of
one (+1)(+1) and
a
mass
of
1
atomic
mass
unit (amu)(amu),
which
is
about 1.67×10−271.67×10−27 kilograms. Together with neutrons, they make up virtually all of the mass of
an atom.
NEUTRONS
Atoms of all elements—except for most atoms of hydrogen—have neutrons in their nucleus.
Unlike protons and electrons, which are electrically charged, neutrons have no charge—they are
electrically neutral. The zero stands for "zero charge". The mass of a neutron is slightly greater than
the mass of a proton, which is 1 atomic mass unit (amu)(amu). (An atomic mass unit equals
about 1.67×10−271.67×10−27 kilograms.) A neutron also has about the same diameter as a proton,
or 1.7×10−151.7×10−15 meters. The neutron is neutral. In other words, it has no charge
whatsoever and is therefore neither attracted to nor repelled from other objects. Neutrons are in
every atom (with one exception), and they are bound together with other neutrons and protons in
the atomic nucleus.
The Structure of an Atom
HOW THE CONCEPT OF ATOMIC NUMBER LED TO THE SYNTHESIS OF NEW
ELEMENTS IN THE LABORATORY?
Loosely speaking, the existence or construction of a periodic table of elements creates an ordering
of the elements, and so they can be numbered in order.
Dmitri Mendeleev claimed that he arranged his first periodic tables (first published on March 6, 1869)
in order of atomic weight. However, in consideration of the elements' observed chemical properties, he
changed the order slightly and placed tellurium (atomic weight 127.6) ahead of iodine (atomic weight 126.9).
This placement is consistent with the modern practice of ordering the elements by proton number, Z, but that
number was not known or suspected at the time.
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In 1911, Ernest Rutherford gave a model of the atom in which a central nucleus held most of the
atom's mass and a positive charge which, in units of the electron's charge, was to be approximately equal to
half of the atom's atomic weight, expressed in numbers of hydrogen atoms. This central charge would thus
be approximately half the atomic weight (though it was almost 25% different from the atomic number of
gold (Z = 79, A = 197), the single element from which Rutherford made his guess). Nevertheless, in spite of
Rutherford's estimation that gold had a central charge of about 100 (but was element Z = 79 on the periodic
table), a month after Rutherford's paper appeared, Antonius van den Broek first formally suggested that the
central charge and number of electrons in an atom was exactly equal to its place in the periodic table (also
known as element number, atomic number, and symbolized Z). This proved eventually to be the case.
The experimental position improved dramatically after research by Henry Moseley in 1913. Moseley,
after discussions with Bohr who was at the same lab (and who had used Van den Broek's hypothesis in
his Bohr model of the atom), decided to test Van den Broek's and Bohr's hypothesis directly, by seeing
if spectral lines emitted from excited atoms fitted the Bohr theory's postulation that the frequency of the
spectral lines be proportional to the square of Z.
Russian chemist Dmitri
Mendeleev, creator of the
periodic table.
Ernest Rutherford
Henry Moseley
All atoms can be identified through the number of protons in their nucleus and the number of electrons
that move around the nucleus. In an atom, the number of protons is equal to the number of electrons. This
number is the atomic number of the atom. On the other hand, the atomic mass is equal to the total number of
protons and neutrons. The mass number, therefore, is given by the following equation:
mass number (A) = number of proton (Z) + number of neutrons
The accepted way of denoting the atomic number
and mass number of an atom of an element is as follows:
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Atoms of the same element are not all identical because most elements have two or more isotopes.
Isotopes are atoms with the same atomic number (Z) but different mass number (A). This is due to their
different number of neutrons. An example is hydrogen. The most abundant hydrogen is protium with 1 proton
and no neutron. Deuterium is hydrogen with 1 proton and 1 neutron, so its mass number is 2. The third
isotope, tritium, has 1 proton and 2 neutrons with mass number of 3. See how to denote the isotopes of
hydrogen in figure below.
Isotopes of hydrogen
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