12. what is the difference between an atom and an element?

13. why is the periodic table called the \periodic table of the elements\ instead of the \periodic table of atoms\?

14. what basic information is found in each box of the periodic table?

15. which particles contribute most to an atom’s mass, and why?

16. what is an isotope?

17. why is the atomic mass listed on the periodic table usually not a whole number?

18. why is hydrogen considered an exception when comparing numbers of protons and neutrons?

19. what are energy levels, and where are they located?

20. how many energy levels does an atom in the third period have?

21. what are valence electrons, and why are they important?

22. why do elements in the same group have similar chemical properties?

23. what is a covalent bond?

24. what two conditions must be met for atoms to form a covalent bond?

Question

12. what is the difference between an atom and an element?

13. why is the periodic table called the \periodic table of the elements\ instead of the \periodic table of atoms\?

14. what basic information is found in each box of the periodic table?

15. which particles contribute most to an atom’s mass, and why?

16. what is an isotope?

17. why is the atomic mass listed on the periodic table usually not a whole number?

18. why is hydrogen considered an exception when comparing numbers of protons and neutrons?

19. what are energy levels, and where are they located?

20. how many energy levels does an atom in the third period have?

21. what are valence electrons, and why are they important?

22. why do elements in the same group have similar chemical properties?

23. what is a covalent bond?

24. what two conditions must be met for atoms to form a covalent bond?

Accepted Answer

### Question 12:
# Brief Explanations:
An atom is the smallest unit of matter with its element's properties. An element is a pure substance with unique atoms (same proton number). So an atom is a particle, an element is a substance made of same - type atoms.
# Answer:
An atom is the smallest particle of matter that retains the chemical properties of an element. An element is a pure substance made up of only one type of atom (all atoms of an element have the same number of protons). For example, a single carbon atom is an atom, while the substance "carbon" (which can be made up of many carbon atoms) is an element.

### Question 13:
# Brief Explanations:
The periodic table organizes elements. An element is a substance with unique atoms (same protons). Atoms of an element can have isotopes (different neutrons), so the table is about elements (based on proton number, defining the element) rather than atoms (which can vary in neutrons).
# Answer:
The periodic table is organized based on the properties of elements. An element is defined by the number of protons in its atoms (atomic number). While atoms of an element can have different numbers of neutrons (isotopes), the periodic table groups substances (elements) with the same number of protons. So it's about elements (which are defined by their proton - number - based identity) rather than individual atoms (which can have variations in neutrons for the same element).

### Question 14:
# Brief Explanations:
Each box in the periodic table has the element's symbol, atomic number (protons), atomic mass (average of isotopes' masses), and often the element's name. Some tables may also have electron configuration info.
# Answer:
In each box of the periodic table, the following basic information is typically found:
- The chemical symbol of the element (e.g., $\text{H}$ for hydrogen).
- The atomic number (the number of protons in the nucleus of an atom of the element, e.g., 1 for hydrogen).
- The atomic mass (the average mass of the isotopes of the element, usually given in atomic mass units, e.g., approximately 1.008 for hydrogen).
- The name of the element (e.g., "Hydrogen" for $\text{H}$).

### Question 15:
# Brief Explanations:
Protons and neutrons contribute most to an atom's mass. Electrons have negligible mass (about 1/1836 of a proton's mass). Protons and neutrons are in the nucleus, so nucleus (with protons and neutrons) has almost all the atom's mass.
# Answer:
Protons and neutrons contribute most to an atom’s mass. Electrons have a very small mass (about $\frac{1}{1836}$ the mass of a proton or neutron) and contribute almost nothing to the atom's mass. Protons and neutrons are located in the nucleus of the atom, so the nucleus contains almost all of the atom's mass.

### Question 16:
# Brief Explanations:
Isotopes are atoms of the same element (same protons) with different numbers of neutrons. This means they have the same atomic number but different mass numbers (since mass number = protons + neutrons).
# Answer:
Isotopes are atoms of the same element (they have the same number of protons, so the same atomic number) that have different numbers of neutrons. As a result, isotopes of an element have the same chemical properties (due to same electron configuration from same proton number) but different physical properties (due to different mass numbers). For example, carbon - 12 ($^ {12}\text{C}$) and carbon - 14 ($^{14}\text{C}$) are isotopes of carbon (both have 6 protons, but carbon - 12 has 6 neutrons and carbon - 14 has 8 neutrons).

### Question 17:
# Brief Explanations:
Atomic mass on the periodic table is the average of the masses of an element's isotopes, weighted by their natural abundances. Since isotopes have different masses and are present in different proportions, the average is usually not a whole number.
# Answer:
The atomic mass listed on the periodic table is a weighted average of the masses of all the naturally occurring isotopes of the element. The weighting is based on the relative abundance of each isotope. For example, chlorine has two main isotopes, $\text{Cl - 35}$ (abundance about 75.77%) and $\text{Cl - 37}$ (abundance about 24.23%). The atomic mass is calculated as $(0.7577\times35)+(0.2423\times37)\approx35.45$, which is not a whole number.

### Question 18:
# Brief Explanations:
Hydrogen has 1 proton. Most elements have neutrons $\geq$ protons (or at least 1 neutron when protons = 1). But hydrogen - 1 (most common isotope) has 1 proton and 0 neutrons, so it's an exception as it has no neutrons while having 1 proton.
# Answer:
The most common isotope of hydrogen (hydrogen - 1) has 1 proton and 0 neutrons. For most other elements, when the number of protons is 1 or more, the number of neutrons is at least equal to (or greater than) the number of protons (or at least 1 neutron for 1 proton). But hydrogen - 1 has no neutrons, making it an exception when comparing the number of protons and neutrons.

### Question 19:
# Brief Explanations:
Energy levels are regions around the nucleus where electrons are likely to be found. They are located outside the nucleus, and electrons occupy these levels (also called electron shells) with different energies (closer to nucleus = lower energy).
# Answer:
Energy levels (also called electron shells) are regions of space around the nucleus of an atom where electrons are likely to be found. They are located outside the atomic nucleus. Electrons in different energy levels have different amounts of energy, with energy levels closer to the nucleus having lower energy.

### Question 20:
# Brief Explanations:
The period number of an element in the periodic table is equal to the number of energy levels (electron shells) its atoms have. An atom in the third period has 3 energy levels (electron shells) occupied by electrons.
# Answer:
An atom in the third period of the periodic table has 3 energy levels (electron shells). The period number of an element in the periodic table corresponds to the number of energy levels (electron shells) that the atoms of that element have.

### Question 21:
# Brief Explanations:
Valence electrons are electrons in the outermost energy level of an atom. They are important because they determine how an atom will react chemically (whether it will gain, lose, or share electrons to achieve a stable electron configuration, like a full outer shell).
# Answer:
Valence electrons are the electrons in the outermost energy level (valence shell) of an atom. They are important because they determine the chemical properties of an atom, such as its reactivity. Atoms tend to gain, lose, or share valence electrons to achieve a stable electron configuration (usually a full outer shell, like the noble gases). For example, sodium (with 1 valence electron) tends to lose that electron to achieve stability, while chlorine (with 7 valence electrons) tends to gain 1 electron.

### Question 22:
# Brief Explanations:
Elements in the same group (vertical column) of the periodic table have the same number of valence electrons. Since valence electrons determine chemical reactivity (how atoms bond/react), elements with the same number of valence electrons have similar chemical properties.
# Answer:
Elements in the same group of the periodic table have the same number of valence electrons. As valence electrons determine the chemical behavior of an atom (how it will form bonds, react with other substances, etc.), elements with the same number of valence electrons have similar chemical properties. For example, all elements in Group 1 (alkali metals) have 1 valence electron and are highly reactive metals that tend to lose that 1 electron in chemical reactions.

### Question 23:
# Brief Explanations:
A covalent bond is a chemical bond formed when two atoms share one or more pairs of electrons. This sharing occurs so that both atoms can achieve a more stable electron configuration (usually a full outer shell).
# Answer:
A covalent bond is a type of chemical bond that involves the sharing of electron pairs between atoms. The shared electrons are attracted to the nuclei of both atoms, and this sharing allows the atoms to achieve a more stable electron configuration (often a full outer electron shell, similar to the electron configuration of noble gases). For example, in a hydrogen molecule ($\text{H}_2$), two hydrogen atoms share their single electrons to form a covalent bond, and each hydrogen atom then has a full outer shell (2 electrons, like helium).

### Question 24:
# Brief Explanations:
For atoms to form a covalent bond, first, both atoms should be non - metals (or atoms with high electronegativity that tend to share rather than transfer electrons). Second, both atoms should need to gain electrons to achieve a stable electron configuration (so sharing electrons allows them to get closer to a full outer shell without fully transferring electrons, which would be more like an ionic bond).
# Answer:
Two conditions for atoms to form a covalent bond are:
1. The atoms involved are typically non - metal atoms (or atoms with relatively high electronegativity that tend to share electrons rather than transfer them completely).
2. Both atoms need to gain electrons (or achieve a more stable electron configuration) and sharing electrons is a more favorable way to do so than transferring electrons (which would be characteristic of an ionic bond). For example, two oxygen atoms (non - metals) each need 2 electrons to fill their outer shells, so they share 2 pairs of electrons to form a double covalent bond in $\text{O}_2$.

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