Correct answer: 98.6%

Why?:

Percent error is a relative measure of the deviation of an experimentally obtained value from the true value. This can also serve as a standard for comparison if an experiment can be valid or not. In a measurement, a negative percent error indicates that the experimental value is lower than the true value. A positive percent error indicates that the experimental value is higher than the true value. The difference from the true value is the magnitude of the percent error.

Correct answer and Why:

The transitions from higher-energy to lower-energy levels by electrons are of different energies for different elements. This means that the emission spectrum of an element is a fingerprint that identifies its presence. By confirming the presence of the spectral lines characteristic of an element, the identity of an element can be confirmed.

A star produces considerable amounts of energy. This energy is exciting the electrons of the elements on the surface of the star to higher energy levels. These electrons are in their excited states and are unstable. For them to be stable, they would have to release the extra amount of energy and go back to their ground states. With this transition, the energy released will be in the form of light. Light is composed of different wavelengths corresponding to the energies in an atom. With the different wavelengths of the light produced, scientists will have a way of identifying the kinds of elements present based on the spectra that will be produced.

Answering this question:

Always remember that a spectrum an element produces is unique to that element. This means that an unknown element can be identified by comparing its spectrum with that of a known element. Recall also that atoms emit light because they have released energy. Light is a form of energy. This only happens if electrons go down from a higher energy level to its ground state. But electrons will only be in a higher energy level if they have absorbed energy. Since a star is home to many reactions that produce great amounts of energy, the elements on its surface can definitely absorb energy and get their electrons excited.

So the spectra of the elements on the surface of a star can be used to determine and confirm their identities.

Correct answer: Hydrogen's bright-line spectrum is produced when an excited electron of a hydrogen atom transfers from a higher energy level to a lower energy level.

Why?:

The electric current that is passed through the hydrogen excites electrons. An electron is initially in its lowest energy level. This electron is said to be in its ground state. The electron absorbs energy from the electric current. This electron then climbs up to a higher energy level. After sometime, it will go back to its ground state. On its way to the ground state, it releases energy. This energy is manifested in the form of light. This light corresponds to different wavelengths that also correspond to different energy values. And as we know, different colors of light are characteristic of the energy of the light.

Answering this question:

Recall that light is a form of energy.  When light is produced, energy is released. This released energy is the light. Energy can only be released if the energy of an energy-containing body will decrease. In an atom, energy release can only occur when an electron goes down from a higher energy level to a lower energy level.

So the line spectrum of a hydrogen atom is due to the transition of electrons from a higher energy level to its ground state.

Correct answer: The atom is in an excited state because a 2^nd energy level electron has jumped to the 3^rd energy level.

Why?:

An atom is said to be in an excited state if an electron jumps from one energy level to a higher energy level. After some time, the electron will go back to its ground state and release energy. The 1^st energy level contains only one orbital, the 1s orbital. It can hold 2 electrons. The 2^nd energy level contains four orbitals, the 2s orbital and three 2p orbitals. All of them can hold 8 electrons total. As stated in this problem, the 1^st energy level has 2 electrons. The 1s orbital is filled with 2 electrons. The 2^nd energy level contains only 6 electrons. The 3^rd energy level has 1 electron. The 2^nd energy level is supposed to be filled with 8 electrons first before the 3^rd energy level can be assigned an electron. In this case here, since the 2^nd energy level has only 6 electrons and the 3^rd already has 1 electron, an electron has jumped from the 2^nd energy level to the 3^rd energy level. That is the reason why this atom is in an excited state. If it is not in an excited state, the electron configuration is supposed to be 2-7 only and not 2-6-1.

Answering this question:

Come up with the electron configuration of the atom. It has an atomic number of 9. This also means that there are 9 electrons to distribute because the atomic number is equal to the number of electrons in a neutral atom. If it would be found out that an electron has jumped to a higher energy orbital, then this atom is in an excited state.

So this atom is in an excited state because one of the 2^nd energy level electrons has jumped to the 3^rd energy level.

Correct answer: 10 neutrons

Why?:

An atom contains neutrons, electrons and protons. An electron has negligible mass as proven by the alpha scattering experiment of Rutherford. A proton has a mass of approximately 1 amu. A neutron also has a mass of approximately 1 amu. With these facts, the mass of an atom is only contributed by the masses of the protons and neutrons because electrons have negligible masses. Since a neutron and a proton each have a mass of 1 amu, the mass number of an atom can be obtained by adding the number of protons and the number of neutrons. Therefore, the number of neutrons can be obtained by subtracting from the mass number the number of protons.

Answering this question:

Simply subtract the number of protons from the mass number.

Mass number = number of protons + number of neutrons

Number of neutrons = mass number - number of protons

Number of neutrons = 19 - 9 = 10 neutrons

So the correct answer is 10 neutrons.

Correct answer: (2) B

Why?:

The wave-mechanical model of an atom describes protons as being located in the nucleus and electrons as being dispersed in orbitals. An orbital is a region in an atom where an electron can be found most of the time. Orbitals are outside the nucleus.

Answering this question:

The table only presents the location of the protons and electrons. We know that the protons are located in the nucleus. This is what's giving the nucleus the positive charge. The electrons, even in the older models of an atom such as that of Bohr, electrons reside outside the nucleus. They are going around the nucleus. In the wave-mechanical model of an atom, the position and momentum of an electron cannot be determined specifically at the same time. This is the Uncertainty Principle. The electrons are located in regions called orbitals, where they are frequently located. In the table, only row B gives the correct description of the locations of the electrons and protons under the wave-mechanical model.

So the correct answer is (2).

Correct answer: (4) one more proton

Why?:

An atom is identified by its atomic number. It is also the number of protons in the nucleus. In a neutral atom, the number of protons must be balanced by the number of electrons to have a net charge of 0. Thus, the number of protons is equal to the number of electrons. The mass number is the number of protons and neutrons combined together. To get the number of neutrons, simply deduct the number of protons from the mass number.

Sulfur-32 has an atomic number of 16 and a mass number of 32. Therefore it has 16 electrons and 16 protons. It has 16 neutrons as well. Phosphorus-31 has an atomic number of 15 and a mass number of 31. It has 15 electrons and 15 protons. Deducting the number of protons from the mass number, it has 16 neutrons.

They both have the same number of neutrons. They have different number of protons, thus they have different atomic numbers and number of electrons. Sulfur contains 1 more proton, 1 more electron, 1 more atomic number, and 1 more mass number compared to phosphorus.

Answering this question:

You can create a simple table showing the number of the different subatomic particles and other characteristics of the two atoms. It could be like this.

With this, the difference could be easily seen. The choices talk about the number of protons and the number of neutrons. Since they both have the same number of neutrons, (1) and
(3) can be eliminated. Referring back to the table, sulfur contains 1 more protons than phosphorus.

So the correct answer is (4).

Correct answer: (4) 18

Why?:

Ions with negative charges are atoms added with electrons. The magnitude of the charge indicates how many electrons an atom has gained. Ions with a positive charge are taken away with electrons. The magnitude of the positive charge is also indicative of how many electrons they have lost. Atoms gain or lose electrons to gain stability. Most atoms prefer to have 8 valence electrons for them to be stable. This is called the octet rule. Sulfur atom has 16 electrons, the same as its atomic number. It has an electron configuration of 1s², 2s², 2p⁶, 3s², 3p⁴. The outermost energy level of the valence shell contains 6 valence electrons. For a sulfur atom to be more stable, it has to fill its valence shell with a complete set of 8 electrons. So it needs two more electrons. When this happens, a sulfur atom will be isoelectronic with the stable noble gas argon.

Answering this question:

Remember that an electron is negatively charged. If an atom gains electrons, it becomes more negative. Thus, it acquires a negative charge. When atoms lose electrons, they become less negative. Another way of saying it is that they become more positive. Thus, they carry positive charges.

The question talks about sulfur. A sulfur atom has an atomic number of 16. Since in a neutral atom the number of electrons is the same as the atomic number, sulfur atom contains 16 electrons. If it has a charge of -2, then it was added with 2 more electrons. Simply add 2 to 16 and you will end up with the correct total number of electrons in a sulfur ion.

So the correct answer is (4).

Correct Answer: (3) The Cd atom loses two electrons and its radius increases.

Why?:

Electrons occupy energy levels. These energy levels determine the size of an atom or ion. The greater the number of electrons, the bigger would be the size of an atom or ion.

If an atom gains electrons, it would acquire a negative charge because electrons are negatively charged. The number following the charge would be indicative of the number of electrons gained. Since there is addition of electrons, this means that the atom will become bigger as it has to accommodate the additional electrons. Furthermore, the additional electrons will be pushed away by the already existing electrons due to electron-electron repulsion. This means that that the atom becomes bigger as electrons are pushed outwards.

If an atom loses electrons, it would lose negativity. So it develops a positive charge. The number after the charge is indicative of the number of electrons lost. Since electrons are lost, it would make the atom go smaller as there would be fewer electrons to occupy energy levels. The fewer the energy levels, the smaller the atom or ion.

Answering this question:

Always remember that the greater the number of electrons in an ion or atom, the bigger is its size since it has to occupy energy levels. Those energy levels determine the size of the ion or atom. Cations are smaller than their parent atoms and anions are bigger than their parent atoms. If an ion has a negative charge, it gained electrons. Electrons are negative so atoms added with electrons develop negative charges. If an ion has a positive charge, it has lost electrons. It has lost negativity. The number following the positive or negative charge is the number of electrons lost or gained.

So the correct answer is (3).

Why?:

Isotopes are elements having the same atomic number but different atomic masses. Atomic number is the number of protons in an element. This means that isotopes will not differ in their number of protons since they have the same atomic number. And since an atom is neutral, the number of protons is equal to the number of electrons. Otherwise, it will be charged and is called an ion. Mass number is the number of protons and the number of neutrons combined together. Since isotopes differ only in their mass number, it could only be the number of protons and the number of neutrons that could be different from each other. But then again, the number of protons does not change. It is the "identification card" of an element. And isotopes are of the same kind of element.

Answering this question:

Always remember that the number of protons in an atom is constant. It does not change even in isotopic forms. The number of electrons would only differ in ions. Cations are taken away with electrons and anions are added with electrons. They are charged particles. In isotopes, the only difference would be in the mass number. The mass number is the sum of the number of protons and the number of neutrons. And since the number of protons would always be constant in an element, it will only be the number of neutrons that is different in isotopes.

So the correct answer is (2).