What is the structure of all negative atomic group include؟
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All negative atomic groups (polyatomic anions) include: Central atom: Usually a nonmetal. Surrounding atoms: Often oxygen (in oxyanions). Extra electrons: Giving the group an overall negative charge.
Negative atomic groups, also known as **anions** or **polyatomic anions**, are groups of atoms covalently bonded together that carry a net negative electrical charge. Here's a breakdown of their structure:
**General Structure Components:**
* **Central Atom:** This is usually the least electronegative atom in the group (excluding hydrogen, which almost never forms a central atom in anions). It is the "hub" to which other atoms are bonded.
* **Ligands/Terminal Atoms:** These are the atoms that are bonded directly to the central atom. Common ligands are oxygen, halogens (fluorine, chlorine, bromine, iodine), hydrogen, and sometimes sulfur or nitrogen.
* **Covalent Bonds:** The atoms within the polyatomic ion are held together by covalent bonds, where electrons are shared between the atoms. The number of covalent bonds depends on the valence (bonding capacity) of each atom.
* **Lone Pairs:** Both the central atom and the ligands can have lone pairs of electrons (non-bonding electrons) which influence the shape and reactivity of the ion.
* **Negative Charge:** This is the crucial feature! The entire group has a net negative charge, meaning there are more electrons than protons in the combined atoms. This charge is delocalized across the entire structure.
**Factors Determining the Structure:**
* **Valence/Bonding Capacity:** Each atom's ability to form bonds is key. Consider the typical valence of common elements:
* Hydrogen: 1
* Oxygen: 2
* Nitrogen: 3
* Carbon: 4
* Halogens: 1
* Sulfur: 2, 4, or 6
* Phosphorus: 3 or 5
* **Octet Rule (and exceptions):** Most atoms (especially those in the second period, like carbon, nitrogen, and oxygen) strive to have 8 electrons in their valence shell (an octet). Hydrogen aims for 2 (a duet). Elements in the third period and beyond (like sulfur, phosphorus, chlorine) can sometimes exceed the octet rule, forming expanded octets.
* **Electronegativity:** Electronegativity influences bond polarity and which atom will be central.
* **Resonance:** Many polyatomic ions exhibit resonance, meaning that the actual structure is a hybrid of multiple Lewis structures. The true structure is more stable than any single Lewis structure suggests. Resonance delocalizes the charge and the electron density, increasing stability.
**Examples and Their Structures:**
Let's look at some common examples:
* **Hydroxide (OH
-):**
* Central atom: Oxygen
* Ligand: Hydrogen
* Structure: H-O
- (linear)
* Charge: -1
* **Nitrate (NO
3-):**
* Central atom: Nitrogen
* Ligands: Three Oxygen atoms
* Structure: A central N atom single-bonded to one O with a negative charge on that O, and double-bonded to the other two O's. This structure has three resonance forms, where the double bond is distributed amongst the three oxygen atoms. Trigonal planar geometry.
* Charge: -1
* **Sulfate (SO
42-):**
* Central atom: Sulfur
* Ligands: Four Oxygen atoms
* Structure: A central S atom double bonded to two O's and single bonded to two O's with a negative charge on each of the single-bonded O's. This can also be drawn with all single bonds and expanded octet, with a -0.5 charge on each O. Tetrahedral geometry.
* Charge: -2
* **Phosphate (PO
43-):**
* Central atom: Phosphorus
* Ligands: Four Oxygen atoms
* Structure: A central P atom double-bonded to one O and single-bonded to the other three O's (each with a negative charge). Can also be drawn with single bonds only and an expanded octet on P, with each O having -0.75 charge. Tetrahedral geometry.
* Charge: -3
* **Carbonate (CO
32-):**
* Central atom: Carbon
* Ligands: Three Oxygen atoms
* Structure: A central C atom double-bonded to one O, and single-bonded to two O's (each with a negative charge). Three resonance structures. Trigonal planar geometry.
* Charge: -2
* **Cyanide (CN
-):**
* Central atom: Carbon
* Ligand: Nitrogen
* Structure: C≡N
- (triple bond between C and N). Linear geometry.
* Charge: -1
**Steps for Determining Structure (Simplified):**
1. **Count Valence Electrons:** Add up the valence electrons of all atoms in the ion, and add the number of electrons corresponding to the negative charge.
2. **Identify Central Atom:** The least electronegative atom (excluding hydrogen) is usually central.
3. **Connect Atoms:** Use single bonds to connect ligands to the central atom.
4. **Complete Octets (or duets):** Distribute the remaining electrons as lone pairs to satisfy the octet rule (or duet for hydrogen). Start with the most electronegative atoms.
5. **Assign Formal Charges:** Formal charge = (valence electrons) - (non-bonding electrons) - (1/2 bonding electrons). Minimize formal charges.
6. **Draw Resonance Structures:** If possible, create resonance structures by moving lone pairs and multiple bonds.
7. **Determine Geometry:** Use VSEPR theory to predict the geometry around the central atom based on the number of bonding and non-bonding electron pairs.
**Key Takeaways:**
* Polyatomic anions are held together by covalent bonds.
* They have a net negative charge.
* Their structure is determined by valence, the octet rule, electronegativity, and the possibility of resonance.
* VSEPR theory can be used to predict their shape.
This explanation should give you a solid understanding of the structure of negative atomic groups. Keep in mind that drawing Lewis structures and understanding formal charges is crucial for predicting the most likely and stable structure.
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